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'[PIC] Dying PICs'
2005\10\18@034426 by Pic

flavicon
face
I have a project I set aside a few months ago in a "mostly done" state.  I
finally got back back to it today planning to fiish it up.

When I powered it, it seemed completely dead.  After 30 mins of
troubleshooting, I decided to see if I could read the PIC (16F628) in my
PicStart plus (using MPLAB IDE 7.21).  It showed up in a weird state, the
config bits wrong, including code protect set to all (I don't use code
protect during development).

I tried blanking the device which doesn't generate any message then when I
try to program it, MPLAB says:
"Program Memory or EEPROM Data is protected. Reprogram
the entire part.  If "Auto select memory areas and range"
on the Memory Ranges tab of the programmer settings
is enabled, it must be disabeled  before proceeding."

Even with the auto select feature disabled.

I decided the chip might have been damaged, so I got out a new 16F628,
programmed it and put it in the project.  It worked fine for about 20
seconds, then the PIC got so hot it would burn me if I touched it for more
than a fraction of a second and the project stopped working.  When I put the
PIC back in the picstart it behaves exactly as the original chip that wasn't
working.

When I put a damaged pic back in, the project is totally dead, but the pic
stays cold.

Any ideas what could be going wrong here?  It was working fine when I
shelved it a few months ago.  I spent a lot of time looking for loose
connections that might be shorting, and the only possibility is that I
grounded pin 16.  This is RA7/OSC1/CLKIN.  My oscillator is set for
INTRC/IO.

I've fixed the loose connection that might have caused the short, but I
really don't want to blow another chip, especially since I only have one
F628 left.

Jason


2005\10\18@041906 by Jinx

face picon face
> When I put a damaged pic back in, the project is totally dead,
> but the pic stays cold

That's because it got as munted as it could be before you took
it out. Probably it latched, drawing a lot of power and heating
up. When you turned the power off to remove it, that fixed the
latching but the damage had been done

> Any ideas what could be going wrong here ?

Is the power on the correct way ? (It happens)

Are there any floating inputs, by accident or oversight ?

Is there any other damage ? eg moisture, corrosion

Is there any possibility of voltages below 0V getting on to
pins ? I've sent a couple to PIC heaven with sub-0V spikes.
They also got very very hot before they, before they...sniff

> I've fixed the loose connection that might have caused the short,
> but I really don't want to blow another chip, especially since I
> only have one F628 left

Might not be a short - something could be open, leaving a
pin vulnerable to damage. I'd suggest checking the board
very thoroughly (which you probably have done already
but check it again anyway, and methodically). Then put
power on it, without the PIC in, and measure around. To
state the obvious, there is a logical reason why this is
happening

Maybe someone should pop around and take your last
PIC into protective custody.................

2005\10\18@043025 by Jason

flavicon
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From: "Jinx" <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz>
Sent: Tuesday, October 18, 2005 1:19 AM


>> Any ideas what could be going wrong here ?
>
> Is the power on the correct way ? (It happens)

Yep, first thing I checked :)

> Are there any floating inputs, by accident or oversight ?

There is one floating input.  The wire that shorted pin 16 to ground was
supposed to be tying pin 17 (RA0, configured as an input) to ground through
a 1k resistor.  The way the wire broke lose left pin 17 floating for sure,
and might have grounded pin 16.

Leaving an input floating can fry the chip?

> Is there any other damage ? eg moisture, corrosion

There doesn't seem to be on the original chip, and the second one is brand
new.

> Is there any possibility of voltages below 0V getting on to
> pins ? I've sent a couple to PIC heaven with sub-0V spikes.
> They also got very very hot before they, before they...sniff

I'm using an unregulated DC wall wart...so I don't think so, but I don't
know 100%.

> Maybe someone should pop around and take your last
> PIC into protective custody.................

I guess my next order to glitchbuster will be a lot sooner than I thought :(

Jason


2005\10\18@045208 by Jinx

face picon face
> > Are there any floating inputs, by accident or oversight ?
>
> There is one floating input.............

> Leaving an input floating can fry the chip?

http://www.piclist.com/techref/logic/xtrapins.htm

The author says (about a floating input) -

"The power loss due to rapid switching of a tri-stated input pin
may be significant. Disclamer: I do not have hard data on this,
but I believe I'm correct. Rapid switching will not fry the chip"

That is unproven AFAIK, but maybe Microchip have documentation.
In either case, as you know, best not to let pins float

> > Is there any other damage ? eg moisture, corrosion
>
> There doesn't seem to be on the original chip, and the second one
> is brand new

I was thinking more of the board

> > Is there any possibility of voltages below 0V getting on to
> > pins ? I've sent a couple to PIC heaven with sub-0V spikes.
> > They also got very very hot before they, before they...sniff
>
> I'm using an unregulated DC wall wart...so I don't think so, but
> I don't know 100%

Noise (eg ripple) coming from the PSU "probably" won't do
any harm, but spikes, especially sub-0V, coming from other
sources into pins can fatally damage a PIC. The reason
is that the internal diode conducts and the pin's circuitry burns
out. The cure for this is to protect the internal diode with an
external Schottky diode, which will conduct before the internal
one and so shunt the spike away

Now, that's assuming the problem you have is noise. What
other components are there ? motor ? relay ? solenoid ? or
maybe even lighting ?

2005\10\18@051712 by Jason

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face
From: "Jinx" <.....joecolquittKILLspamspam@spam@clear.net.nz>
Sent: Tuesday, October 18, 2005 1:52 AM


{Quote hidden}

I know it's best not to let pins float, but this was due to a wire that
broke loose.  So far it's the only thing I can see being wrong, but I can't
see how it can damage the chip, and you seem to be saying it shouldn't too.

>> > Is there any other damage ? eg moisture, corrosion
>
> I was thinking more of the board

There's no damage I can see beyond the wire and resistor broken loose (it
was a temporary modification and I was finally going to redo it more cleanly
nad neatly with the permanent parts).

There's no visible corrosion, etc.

> Now, that's assuming the problem you have is noise. What
> other components are there ? motor ? relay ? solenoid ? or
> maybe even lighting ?

There's about 1 amp of LED's being driven off the PSU controlled through
transistors from the PIC.  There are diodes between the transistor and the
power rails.  It might be causing a fair bit of ripple, but I don't think a
negative voltage on any pin is possible.

Jason


2005\10\18@052146 by Dmitriy Kiryashov

picon face
Hi Jinx.

> > > Are there any floating inputs, by accident or oversight ?
> > There is one floating input.............
> > Leaving an input floating can fry the chip?
> www.piclist.com/techref/logic/xtrapins.htm
> The author says (about a floating input) -
> "The power loss due to rapid switching of a tri-stated input pin
> may be significant. Disclamer: I do not have hard data on this,
> but I believe I'm correct. Rapid switching will not fry the chip"

I believe that rumors about switching tri stated pin will cause chip
damage are complete nonsense. Floating pins are proved to cause
extensive current sonsumption ( in battery application power saving
modes ) but not to extend frying the chip out.

I rather check with scope if electrolytic capacitor in power adapter
has dried out and power adapter is out of specs or pulsating above
the +5 power specs. Output pin shortages to +5 power or ground is
seconds thing to check.


> Noise (eg ripple) coming from the PSU "probably" won't do
> any harm, but spikes, especially sub-0V, coming from other
> sources into pins can fatally damage a PIC. The reason
> is that the internal diode conducts and the pin's circuitry burns
> out. The cure for this is to protect the internal diode with an
> external Schottky diode, which will conduct before the internal
> one and so shunt the spike away

I would recomend to place small resistor i.e. 200 OHm is series
with PIC inputs connected to outside 0 to 5V signals to limit ingoing
current to prevent that possibility in most situations ( for higher
possible input voltages resistor should be increased ) After all if
ESD is an issue ( all mystical deaths when winter time is approaching )
ESD protection elements should be used on the board entrances.


WBR Dmitry.

2005\10\18@053021 by Jinx

face picon face
> I would recomend to place small resistor i.e. 200 OHm is
> series with PIC inputs connected to outside 0 to 5V signals
> to limit ingoing current

That is recommended by MC in their circuit for taking /MCLR
low with a reset switch. Jason, do you use a reset switch and,
if so, does it take /MCLR direct to 0V ?

2005\10\18@054359 by Jason

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face
From: "Dmitriy Kiryashov" <vze27bymspamKILLspamverizon.net>
Sent: Monday, October 17, 2005 6:19 PM


> I rather check with scope if electrolytic capacitor in power adapter
> has dried out and power adapter is out of specs or pulsating above
> the +5 power specs. Output pin shortages to +5 power or ground is
> seconds thing to check.

Since it's a sealed wall wart, I can't get at the cap inside it.  It reads
around 7.3 volts no-load (it's rated at 5v, 2 amp).

One of the first thing I checked was output pin shortages to ground.  I did
find that pin 16 (RA7/OSC1/CLKIN) might have been grounded.

Actually it's a bit more complicated on closer consideration.  All of port A
is set for digital input.  I'm using RA0 in my project with the rest left
floating (bad design I know, but It's a retrofit on a pre-existing board
which limited my options).  This did work fine during about 48 hours of
continuous on-time a few months ago.

It was originally based on a F84 with an external RC circuit which is still
physically connected to the OSC1 pin.  The F628 I'm using is configured for
INTRC/IO, and I'm not doing anything with the other port A pins in software.

> I would recomend to place small resistor i.e. 200 OHm is series
> with PIC inputs connected to outside 0 to 5V signals to limit ingoing
> current to prevent that possibility in most situations ( for higher
> possible input voltages resistor should be increased )

This will be quite hard to do as it's on a pre-made PCB.  How likely is this
to help?

> After all if
> ESD is an issue ( all mystical deaths when winter time is approaching )
> ESD protection elements should be used on the board entrances.

I did think ESD was a strong possibility for the first blown chip.  I was
extra careful in my handling of the second one.

Jason


2005\10\18@060020 by Jason

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face
MCLR is going through 100k to Vcc.  There is no reset switch.

I just tried powering it again to take some voltage measurements with the
first blown chip.  It got super hot again, worse than before since I wasn't
feeling it and I didn't notice until I started smelling it.

I know for sure that the clkin pin was not shorted to anything this time
though.

If 2 outputs from the driver transistors get shorted, it should not cause a
problem for the pic; just the transistors will get hot, right? (I'm trying
to narrow down where to look)

Jason

{Original Message removed}

2005\10\18@060340 by Jan-Erik Soderholm

face picon face
Jason wrote :

> it's a sealed wall wart,...  It reads
> around 7.3 volts no-load (it's rated at 5v, 2 amp).

How do you regulate/limit that down to 5V ?
What voltage do you have with no LEDs on ?

Jan-Erik.



2005\10\18@061934 by Jason

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face
From: "Jan-Erik Soderholm" <.....jan-erik.soderholmKILLspamspam.....telia.com>
Sent: Tuesday, October 18, 2005 3:03 AM

>> it's a sealed wall wart,...  It reads
>> around 7.3 volts no-load (it's rated at 5v, 2 amp).
>
> How do you regulate/limit that down to 5V ?

I don't :)

> What voltage do you have with no LEDs on ?

Around 6.2V, but that's measuring now with the problem.

2005\10\18@062459 by Dmitriy Kiryashov

picon face
Measure +5 supply with _scope_ right on the PIC Vcc pin.
( no pulses above specififed in 628 datasheet supply voltage )

It is reasonable to use 7805 regulator if you didnt do so yet.

Question :
Anybody heard of PIC heating up not being properly reset
or when no clock is applied ?



Jason wrote:
{Quote hidden}

> {Original Message removed}

2005\10\18@062953 by Dmitriy Kiryashov

picon face
I afraid you damaging the PIC like that.
Use voltage regulator to supply the PIC.


Jason wrote:
{Quote hidden}

2005\10\18@062955 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu]
>Sent: 18 October 2005 11:20
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Dying PICs
>
>
>From: "Jan-Erik Soderholm" <KILLspamjan-erik.soderholmKILLspamspamtelia.com>
>Sent: Tuesday, October 18, 2005 3:03 AM
>
>>> it's a sealed wall wart,...  It reads
>>> around 7.3 volts no-load (it's rated at 5v, 2 amp).
>>
>> How do you regulate/limit that down to 5V ?
>
>I don't :)

And there you have your problem, or at least one of them.  Have a look in the PIC's datasheet under absolute maximum ratings.  The 16F628 has an absolute maximum voltage on Vdd wrt Vss of 6.5 volts.  This specification does not even mean the PIC will work properly at 6.5, it's just the maximum voltage you can apply without permanent damage occuring.  The maximum operating voltage is 5.5 volts.

Regards

Mike

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2005\10\18@063021 by Jan-Erik Soderholm

face picon face
Jason wrote :

> > How do you regulate/limit that down to 5V ?
>
> I don't :)
>
> > What voltage do you have with no LEDs on ?
>
> Around 6.2V, but that's measuring now with the problem.

And what is the allowed voltage range according
to the data sheet for the PIC you are using ?

Jan-Erik.



2005\10\18@063917 by Dominic Stratten

picon face
That’s probably your problem then - if you try running a PIC on a voltage
that’s too high you'll burn it out.

I think around 5.5 volts is the maximum for the 628 (6.5 volts absolute
max).

You're probably frying the PIC with an unregulated supply - get a 5 volt
regulator in there and see if it helps.        

{Original Message removed}

2005\10\18@063925 by Jinx

face picon face
> >> around 7.3 volts no-load (it's rated at 5v, 2 amp).
> >
> > How do you regulate/limit that down to 5V ?
>
> I don't :)

I think you mean "Oh no ! I don't ;-(((((((("

Absolute maximum rating for a 628 is 6.5V. You're asking
for trouble running it over 5.5V. The problem is that your
PIC isn't taking anywhere near 2A (admittedly you're doing
your very best to make it do so ;-) ) and so the voltage isn't
going to drop significantly below 7.3V with a temporarily
healthy PIC

The LEDs can be run on the 7.3V, although the more you
have on, the dimmer they'll get, but the PIC must be down
at 5V-ish. The margin between 7.3 or 6.2 is too low for a
78L05, you'll have to use a low drop-out (LDO - there are
a few common ones around like the LP29xx) regulator, or,
as so much power is being used by the LEDs anyway, a 5V1
zener. The zener's series resistor, together with a reservoir
cap on the PIC side, say a few 100uF, should give you a
smooth stable PIC 5.1V

Your PIC's aren't dying, they're being murdered. You, you,
you .... murderer you ;-)

2005\10\18@064745 by Jason

flavicon
face
From: "Michael Rigby-Jones" <RemoveMEMichael.Rigby-JonesTakeThisOuTspambookham.com>
Sent: Tuesday, October 18, 2005 3:29 AM

> And there you have your problem, or at least one of them.  Have a look in
> the PIC's datasheet under absolute maximum ratings.  The 16F628 has an
> absolute maximum voltage on Vdd wrt Vss of 6.5 volts.  This specification
> does not even mean the PIC will work properly at 6.5, it's just the
> maximum voltage you can apply without permanent damage occuring.  The
> maximum operating voltage is 5.5 volts.

It did work for before I set it aside for a few months.

It's not my design.  It's a commercial LED light show display I'm modifying
because I don't like the factory designed show.

All I've done is replace the original CF745 that had the code I don't like
with a 16F628 with my code.


2005\10\18@070056 by Sean Schouten

face picon face
Use a simple 7805 voltage regulator; it should fix your problem and save pvt
Pic!

:-)

2005\10\18@070222 by Michael Rigby-Jones

picon face


{Quote hidden}

Perhaps the PSU has died then?  If there is no onboard regulation then the PSU should be of the regulated type.  Either way 7.3volts is enough to damage a PIC.

Regards

Mike

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information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
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No part of this message can be considered a request for goods or
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2005\10\18@071000 by Jinx

face picon face
> All I've done is replace the original CF745 that had the
> code I don't like with a 16F628 with my code.

The 16C745 has a max Vdd of 7.5V, so it wouldn't have
died at 7.3V. It has a working Vmax of 5.25V, less than
the F628. Pretty poor design to expect it to work properly
unless the majority of LEDs were on to get Vdd down

2005\10\18@071616 by Sean Schouten

face picon face
Ah... The C745 can take 7.5 volts vdd in contraty to the opposing and lower
5.5 volts of the F628. That's your problem then; You where careless
selecting the right pic for the right job!

Sean



On 10/18/05, Jason <picEraseMEspam.....canadaspeaks.com> wrote:
{Quote hidden}

>

2005\10\18@071727 by Sean Schouten
face picon face
On 10/18/05, Jinx <RemoveMEjoecolquittEraseMEspamEraseMEclear.net.nz> wrote:
>
> > All I've done is replace the original CF745 that had the
> > code I don't like with a 16F628 with my code.
>
> The 16C745 has a max Vdd of 7.5V, so it wouldn't have
> died at 7.3V. It has a working Vmax of 5.25V, less than
> the F628. Pretty poor design to expect it to work properly
> unless the majority of LEDs were on to get Vdd down
>
>

2005\10\18@073138 by Jason

flavicon
face
From: "Jinx" <RemoveMEjoecolquittspam_OUTspamKILLspamclear.net.nz>
Sent: Tuesday, October 18, 2005 4:09 AM

> The 16C745 has a max Vdd of 7.5V, so it wouldn't have
> died at 7.3V. It has a working Vmax of 5.25V, less than
> the F628. Pretty poor design to expect it to work properly
> unless the majority of LEDs were on to get Vdd down

Then would you recommend I try with an F84 (that also says absolute max of
7.5V in the datasheet) which I happen to have on hand?

I'm obviosuly not going to be able to get a CF745.  The 16C54 and 16F54
which would be the direct replacements aren't as common.



2005\10\18@073803 by Jan-Erik Soderholm

face picon face
Jason wrote :

> Then would you recommend I try with an F84 (that also says
> absolute max of 7.5V in the datasheet) which I happen to
> have on hand?

Note that the "abs max", are the limit where it will break
down, you can not be sure it *runs* at that voltage.

Isn't it much much easier just to fix the supply ?

Jan-Erik.



2005\10\18@074703 by olin piclist

face picon face
pic@canadaspeaks.com wrote:
> I decided the chip might have been damaged, so I got out a new 16F628,
> programmed it and put it in the project.  It worked fine for about 20
> seconds, then the PIC got so hot it would burn me if I touched it for
> more than a fraction of a second and the project stopped working.

This should have been a strong clue.  Obviously something about your circuit
is blowing up PICs.  That has to be fixed before anything else.

> When
> I put the PIC back in the picstart it behaves exactly as the original
> chip that wasn't working.

Big surprise, since fried PICs can have all sorts of strange behavior.

> When I put a damaged pic back in, the project is totally dead, but the
> pic stays cold.

Note that fuses work the same way.  They get really hot, once.

> Any ideas what could be going wrong here?

Your circuit is frying PICs.  Turn on the circuit without a PIC in there and
carefully look at the voltage on all the pins.  You'll probably find
something very wrong.  Also make really sure you're not plugging the PIC in
backwards.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\10\18@075535 by Jinx

face picon face
> Then would you recommend I try with an F84 (that also
> says absolute max of  7.5V in the datasheet) which I
> happen to have on hand ?

No. You're too close to the edge. It would be much better
to give the PIC the 5V it should have. It will take just a
zener and a resistor. Either that or look for a PIC with
"Stunt" written on it

Are the transistors low-side ? ie, are their emitters to 0V
and their collectors to the LEDs ? If so then a 5V PIC o/p
will drive them on just as hard as at 7.3V, you'll not see
any difference in LED brightness

To be honest, I'd be looking for a more stable supply for the
LEDs too. How are they arranged ? 2 or 3 in series or just
one per transistor ? How about resistors ?

2005\10\18@080619 by olin piclist

face picon face
Jason wrote:
>> What voltage do you have with no LEDs on ?
>
> Around 6.2V, but that's measuring now with the problem.

Argh!  So you think the maximum voltage rating doesn't apply to you?  You
run the PICs well outside their specified range, and then you wonder why
they fry!!!!?  What part of "maximum" can't you understand?


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\10\18@202343 by Chen Xiao Fan

face
flavicon
face
Try it and you will learn more about 7805. ;-)

A 7805 will not work with input voltage as low as 6.2V.
An LDO like LP2980-5.0 will.

Regards,
Xiaofan

-----Original Message-----
From: Sean Schouten
Sent: Tuesday, October 18, 2005 7:01 PM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] Dying PICs

Use a simple 7805 voltage regulator; it should fix your
problem and save pvt Pic!

:-)

2005\10\18@203226 by Chen Xiao Fan

face
flavicon
face
Jinx: you do not know what is a CF745 and I guess
not many people know that part. It is not a 16C745!
16C745 will be too high cost for the vendors using
CF745.

CF745 is a special part by Microchip specifically for the
Chinese market to combat the low cost Taiwanese clones.
It is the same as 16C54 without the testing part to
lower the cost. Apparently Jason knows this.

It is not advertised by Microchip since it should not
goes to the retail market. The idea is to let the
vendors using the part do the tests.

Regards,
Xiaofan

{Original Message removed}

2005\10\18@222002 by Jinx

face picon face

> Jinx: you do not know what is a CF745 and I guess
> not many people know that part. It is not a 16C745!
> 16C745 will be too high cost for the vendors using
> CF745.

So what are the specs for the CF745 ? Apparently it
is tough enough to run on 7.3V

2005\10\18@225215 by Chen Xiao Fan

face
flavicon
face
I think it is still out of spec for the CF745 a.k.a.
untested 16C54. The 16C parts are made in old process
(0.7um CMOS or even older) and seems to be more robust
than the 16F parts (0.7um, 0.5um, 0.4um CMOS or newer).

Regards,
Xiaofan

{Original Message removed}

2005\10\19@025812 by William Chops Westfield

face picon face
>
> So what are the specs for the CF745 ? Apparently it
> is tough enough to run on 7.3V
>
You didn't accidentally switch from a regulated wall wart to an
unregulated
version, did you?  Easy enough to do if you read the specs printed on
the
warts and actually believe them...

BillW

2005\10\19@034204 by Jason

flavicon
face
From: "Chen Xiao Fan" <RemoveMExiaofanTakeThisOuTspamspamsg.pepperl-fuchs.com>
Sent: Tuesday, October 18, 2005 5:32 PM


> CF745 is a special part by Microchip specifically for the
> Chinese market to combat the low cost Taiwanese clones.
> It is the same as 16C54 without the testing part to
> lower the cost. Apparently Jason knows this.

I know this thanks to you :)

When I first started working on this project I posted here asking what the
CF745 was.


From: "William Chops Westfield" <EraseMEwestfwspamspamspamBeGonemac.com>
Sent: Tuesday, October 18, 2005 11:58 PM


> You didn't accidentally switch from a regulated wall wart to an
> unregulated version, did you?  Easy enough to do if you read the specs
> printed
> on the warts and actually believe them...

Nope, it's the same wall wart, the project has just been sitting on a box on
a shelf for about 3 months.  Nothing at all should have changed from when it
was working, but clearly something did.


For the people who are saying I should be using a regulated supply.  You're
right, but I don't think that's the problem here.  The project worked in a
48 hour constant running test with the 16F628 before I shelved it
temporarily.  Now it doesn't work.

So while I should regulate the power supply I don't think it will fix the
problem, I'm pretty sure something else is blowing the chips.

The second one that blew yesterday lasted about 20 seconds and blew, and
while running it's got enough LEDs lit to bring the power supply voltage
down.

I have a few untouched versions of the project, so maybe my next step should
be to start from scratch with a new one.  It's just so hard to desolder the
18-pin chip.


2005\10\19@051508 by Richard Prosser

picon face
If it's been 6 months since you last used it you may have had some
change in your
AC supply. An increase  here due to a change in supply or operting
conditions will increase the voltage out of an unregulated wall-wart.
It could be as simple as operating at a different time of day.
The 7.3V you are measuring is also probably an average - there may be
peaks and/or transients well above this - again this can change with
season, time and weather.
I'd start with making sure the supply voltage is close to 5.0 and then
spend more time searching in the unlikely event that this doesn't fix
the problem.

Once you do desolder the chip - sloder a socket in its place. That way
you can almost guarantee you won't ever have to change the chip again.
Richard P


On 19/10/05, Jason <RemoveMEpicKILLspamspamcanadaspeaks.com> wrote:
{Quote hidden}

> -

2005\10\19@102933 by Neil Baylis

picon face
Are you measuring 7.3V RMS, (average)? If so, you'll regularly be
getting peaks of 10 V or so, with line noise on top of that.

-->"For the people who are saying I should be using a regulated supply.  You're
right, but I don't think that's the problem here.  The project worked in a
48 hour constant running test with the 16F628 before I shelved it
temporarily.  Now it doesn't work."<--

That's not evidence of anything. Could be that the act of switching it
off damaged the wall wart, or damaged some component on the board.
Then it just waits in the closet 6 months before you find out. Could
be that the act of getting it out of the closet damaged something with
a static zap.

In any case.. it's all speculation. You may never find out why it
failed. But what you need to do now is troubleshoot using standard
procedures. The standardest procedure of all is.. use a power supply
that is unimpeachable.

And yes, start with a new board, with a socket for the PIC. Are there
many other components? Start by adding just the PIC and nothing else,
and test with the UPS (== Unimpeachable Power Supply ;-). If that
works, (doesn't burn) you can add more components, etc.

2005\10\19@113918 by Robert A LaBudde

flavicon
face
At 04:52 AM 10/18/2005, you wrote:
> > > Are there any floating inputs, by accident or oversight ?
> >
> > There is one floating input.............
>
> > Leaving an input floating can fry the chip?

1. Check the power connections and voltage and polarity again. DC, not AC?
Within chip limits? Ground and +Vdd correct? Right PIC pins? Add a blocking
diode or bridge and a current limiting resistor for continuing testing, so
you don't blow more chips. Even you "think" you've checking power, you may
be blind to your own assumptions. Clear your mind of preconceptions and
check in detail.

2. You might consider putting in an unprogrammed PIC and seeing if it gets
hot. If so, stop it within the first few seconds and pull the power. The
chip should survive this. If it gets hot, you have a problem with applied
power polarity and pins.

3. If the unprogrammed chip does not get hot, and the programmed one does,
then you have a problem with logic and design. E.g., you ground a pin, but
then declare it output in the program and pull it high. This will short the
output and ruin the chip.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: KILLspamralspamBeGonespamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

2005\10\19@115931 by Robert A LaBudde

flavicon
face
At 08:23 PM 10/18/2005, Xiaofan wrote:
>Try it and you will learn more about 7805. ;-)
>
>A 7805 will not work with input voltage as low as 6.2V.
>An LDO like LP2980-5.0 will.

Actually, the real input voltage was 7+ VDC from the unloaded wal-wart.

The 6.2 V was with the maximum load drawn from a blown PIC device.

The 78L05 should work fine with this application. The wal-wart is rated for
2 A, so the current draw should not appreciably drop the voltage.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: EraseMEralspamEraseMElcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

2005\10\19@160400 by Jason

flavicon
face
From: "Richard Prosser" <@spam@rhprosser@spam@spamspam_OUTgmail.com>
Sent: Wednesday, October 19, 2005 2:15 AM


> Once you do desolder the chip - sloder a socket in its place. That way
> you can almost guarantee you won't ever have to change the chip again.
> Richard P

This one is a given.  I have never soldered a chip to a board, I always use
a socket.


From: "Neil Baylis" <spamBeGoneneil.baylisspamKILLspamgmail.com>
Sent: Wednesday, October 19, 2005 7:29 AM


> Are you measuring 7.3V RMS, (average)? If so, you'll regularly be
> getting peaks of 10 V or so, with line noise on top of that.

I'm getting 7.3V DC.   It's a DC power suppy.  It's a DMM, so it samples,
but the 7.3 is steady.

> And yes, start with a new board, with a socket for the PIC. Are there
> many other components? Start by adding just the PIC and nothing else,
> and test with the UPS (== Unimpeachable Power Supply ;-). If that
> works, (doesn't burn) you can add more components, etc.

Besides the PIC, the only other components are the 8 output driver circuits
(transistor, resistor, diode), the LEDs, a cap across the power supply, a
resistor on MCLR to Vcc, and an RC oscillator circuit (which I ignore in
INTRC/IO mode).  These are all part of the original factory build.  It also
has a resistor from one of the previously unused/not-connected IO lines to
Ground and pushbutton to Vcc that I added.

From: "Robert A LaBudde" <.....ralspam_OUTspamlcfltd.com>
Sent: Wednesday, October 19, 2005 8:59 AM


> The 6.2 V was with the maximum load drawn from a blown PIC device.
>
> The 78L05 should work fine with this application. The wal-wart is rated
> for
> 2 A, so the current draw should not appreciably drop the voltage.

I doubt the  blown pic was drawing "maximum" load... with no LEDs lit.

Under normal operation before I opened the control box, that 2 amp wall wart
got very hot, as did the output transistors.  There's 160 LEDs in the
project, and I get the impression it draws pretty close to (if not over) 2
amps.  I actually picked up a few of these quite cheaply as surplus, and I
believe it was because the power supply and transistors are underrated.

Part of the changes I've made are to PWM the outputs to reduce power so it
won't run so hot.  I still think it's pulling an average of around an amp.
The output will be well below 6.2 V under normal load, but I didn't measure
it when the thing was working.

Besides that, I've always been told I need at least an extra 3 volts for a
7800 series regulator, so I should have at least 8 volts under load.


2005\10\19@164750 by Neil Baylis

picon face
-->"I'm getting 7.3V DC. It's a DC power suppy. It's a DMM, so it samples,
but the 7.3 is steady."<--

Have you measured it on the AC range of your DMM?

You need to know how much ripple is there, as well as the average, DC
voltage.

With the ripple, you could easily see peaks that will damage the PIC if the
DC component is 7.3. Ripple increases with load, and it sounds as though
your wall wart is way overloaded.

The wall wart getting hot is a bad sign. It's OK if the output transistors
get hot.

Instead of a 7805, you could use an LT1085 or similar LDO regulator. Also,
I've got a feeling you're going to need a different power supply.

Neil

--
http://www.pixpopuli.com

2005\10\19@191408 by Gerhard Fiedler

picon face
Neil Baylis wrote:

> Instead of a 7805, you could use an LT1085 or similar LDO regulator. Also,
> I've got a feeling you're going to need a different power supply.

Yes, and you only have to have the PIC on a regulated supply. You could
even, just for tests, build a Vcc regulator into a stacked socket for the
PIC.

Gerhard

2005\10\19@214443 by William Chops Westfield

face picon face

>> I've got a feeling you're going to need a different power supply.
>>
In the short term, do you have any sort of regulated 5V supply that
you can use for testing?  A better wall-wart, lab-style supply, or
even a connector into your PC's supply?  This would let you find out
pretty quickly if the power supply really IS the problem...

I can't help but notice that the suggestions to throw in a 7805 don't
match well with the fact the current supply is spec'ed at 2 amps :-)
A lot of LEDs end up using a lot of current.
(Perhaps your problem is even along the lines that your modified code
doesn't cause the LED drivers to consume enough current to keep the
unregulated supply down to a reasonable voltage...)

You mentioned you have several units  - do ALL the power supplies put
out 7+V?  It's potentially possible that the particular supply was
damaged
somehow...

BillW

2005\10\19@220945 by David Van Horn

picon face
> I can't help but notice that the suggestions to throw in a 7805 don't
> match well with the fact the current supply is spec'ed at 2 amps :-)
> A lot of LEDs end up using a lot of current.
> (Perhaps your problem is even along the lines that your modified code
> doesn't cause the LED drivers to consume enough current to keep the
> unregulated supply down to a reasonable voltage...)

If it was me, I'd run the LEDs off cheap unregulated power, and run the
processor and logic off the regulated power.




2005\10\19@231734 by Jason

flavicon
face
From: "William Chops Westfield" <TakeThisOuTwestfw.....spamTakeThisOuTmac.com>
Sent: Wednesday, October 19, 2005 6:44 PM


> In the short term, do you have any sort of regulated 5V supply that
> you can use for testing?  A better wall-wart, lab-style supply, or
> even a connector into your PC's supply?  This would let you find out
> pretty quickly if the power supply really IS the problem...

I can use my PC power supply, but I was thinking 4 NiMH AA's would work too.
They should be around 4.8-5v, and able to handle 2 amps.

> I can't help but notice that the suggestions to throw in a 7805 don't
> match well with the fact the current supply is spec'ed at 2 amps :-)

Yeah I noticed that too.  I hadn't looked up the specs of the 7805's yet but
I figured at least I was looking at a TO-220 with a hefty heatsink.  A few
volts a a couple of amps is a big amount to turn into heat.

> A lot of LEDs end up using a lot of current.
> (Perhaps your problem is even along the lines that your modified code
> doesn't cause the LED drivers to consume enough current to keep the
> unregulated supply down to a reasonable voltage...)

That's an interesting idea, it's quite possibly the problem if the chip is
frying due to overvoltage.


> You mentioned you have several units  - do ALL the power supplies put
> out 7+V?  It's potentially possible that the particular supply was damaged
> somehow...

The other units are still sealed in their retail packaging, I haven't
measured their power supplies.


2005\10\20@183455 by Robert A LaBudde

flavicon
face
At 11:17 PM 10/19/2005, Jason wrote:
>>I can't help but notice that the suggestions to throw in a 7805 don't
>>match well with the fact the current supply is spec'ed at 2 amps :-)
>
>Yeah I noticed that too.  I hadn't looked up the specs of the 7805's yet
>but I figured at least I was looking at a TO-220 with a hefty heatsink.  A
>few volts a a couple of amps is a big amount to turn into heat.

This is one of the things that doesn't make sense in your story.

No one would drive 2 A through 200 LED's controlled by pass transistors
through a linear regulator unless absolute brightness control were
necessary, and even then they'd wire the transistors as current regulators
instead. Otherwise, the LEDs and transistors don't need highly regulated
power. The PIC does. A 78L05 on the PIC power supply should be more than
sufficient. Running all the current through a 78xx regulator would just
waste 30% of the power that could have been used to light LEDs instead.

Driving a PIC beyond its absolute maximum voltage will cause breakdown and
shorting within the chip, which could be why the voltage drops to 6.2 V
from 7.3 V, and the LEDs (which are through resistors) don't light. It
would also, of course, fry the PIC.

Once you've determined you are violating a key endurance specification, why
not fix that problem and then see what happens? Why is any more discussion
necessary?

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: TakeThisOuTralKILLspamspamspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

2005\10\21@104236 by Wouter van Ooijen

face picon face
> > I would recomend to place small resistor i.e. 200 OHm is
> > series with PIC inputs connected to outside 0 to 5V signals
> > to limit ingoing current
>
> That is recommended by MC in their circuit for taking /MCLR
> low with a reset switch. Jason, do you use a reset switch and,
> if so, does it take /MCLR direct to 0V ?

The correct question is "is there a >100nF cap directly on the /MCLR
pin?".

The failure mode for which this series resistor is the cure is the Vdd
dropping below /MCLR, which could cause the whole circuit to be powered
from the cap via /MCLR, which is not a mode the PIC die is designed for.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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