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PICList Thread
'Power Supply'
1998\03\13@184351 by Gavin Jackson

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Hi there

I'm looking for a DC-DC converter that can
step 24V down to 5V. It's for a PIC I have
onboard a boat. It only needs to be able
to supply 5mA max and have as few
external components as possible.
I have a zener, but that's just using
too much batter power.

I've been looking at the MAXIM IC's but I
can't find any that will do, although I'm
sure I've seen one before.

Please suggest any that you may used
before.

Regards

Gavin
--------------------------
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www.geocities.com/TheTropics/Cabana/2625
--------------------------

1998\03\13@192322 by Brian Schousek

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Gavin-
   If this is for a one-off you might consider a monolithic DC-DC converter
such as a Power Convertibles HPR118. This is spec'd to drop 24V to 5V and is
all in a nice convenient, potted package suitable for breadboarding. Just
supply 24V and out comes 5V. It is not, if I remember correctly, regulated
however. And you might run into some minimum load requirements (page 2 of my
datasheet shows 8 mA.) But once you start looking at this option you might
find others along the same lines from different companies. It will save you
a lot of effort in magnetics design. Not cheap though!
   Another idea- National Semiconductor 'simple switcher' family. I've got
a 'pitch pack' from them which includes the entire supply (one 8 pin SO, two
caps, a low profile inductor, and a diode) all on a convenient PCboard. Part
# is LM2594M-5.0.
   Yet another idea- For such a low current, you might be able to get away
with a hefty charge pump IC. Maxim has a selection I believe.

   For all the ideas: be sure and pay good attention to power supply
decoupling whenever you *intentionally* start imposing high frequency noise
on your supply lines.

Brian

{Original Message removed}

1998\03\13@201452 by Bob Blick

face
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Your requirement is only 5 milliamps of 5 volts from 24 volts? Does it
need isolation? It's not going to be cheap or easy to find a DC-DC
converter that is efficient at that low current.

If you don't need isolation, use a low current linear regulator, like the
LM2936Z-5. US$1.29 and virtually no quiescent current. Put a transzorb or
zener of the right voltage across the inputs, though, along with a series
resistor. I imagine load dumps in the boat's electrical system will make
spikes over the rated input voltage of the regulator.

-bob

1998\03\13@205039 by Michael S. Hagberg

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sounds like a simple 78L05 with some good filtering (make SURE you use a
diode to prevent the boat from sucking all the power from your pic when the
engine starts)

michael

{Original Message removed}

1998\03\14@004559 by Gavin Jackson
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Hi there once again

Correct me if I'm wrong, but I though that the 78XX voltage
regulators can only take 3X the output voltage on their input,
so the maximum supply that a 7805 can have as its input
is 15V.

Let me know

Gavin
--------------------------
.....vulcanKILLspamspam@spam@ihug.co.nz
www.geocities.com/TheTropics/Cabana/2625
--------------------------

{Quote hidden}

1998\03\14@041743 by bam-mon

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Gavin Jackson wrote:
>
> Hi there once again
>
> Correct me if I'm wrong, but I though that the 78XX voltage
> regulators can only take 3X the output voltage on their input,
> so the maximum supply that a 7805 can have as its input
> is 15V.
>
> Let me know
>
> Gavin

Sorry to correct you, but the LM78xx-series are spec'd as follows :
all 78xx devices execpt 7824 : max input voltage 35 V DC
7824 : max input voltage : 40 V DC

with regards,  Reelf

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1998\03\14@093753 by John Payson

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> Hi there once again
>
> Correct me if I'm wrong, but I though that the 78XX voltage
> regulators can only take 3X the output voltage on their input,
> so the maximum supply that a 7805 can have as its input
> is 15V.

There are two input-voltage limitations on 78xx regulators:

[1] Maximum input voltage; this may vary with manufacturer and product line
   (7805 vs 78L05, etc.) but is independent of the output voltage.

[2] Maximum power dissipation: since any voltage between the input and the
   output is simply converted into heat, a 7805 will have to dissipate four
   times as much heat converting 25 volts to 5 at a certain output current
   than it would converting 10 volts to 5 at that same current.  If you need
   only a few tens of mA, a heat-sinked 7805 should be able to go from 35 to
   5 without problems; if you're drawing a full amp, the chip will overheat
   if it tries to drop that much voltage.

1998\03\14@114211 by Herbert Graf

picon face
> -----Original Message-----
> From: PIC microcontroller discussion list
> [PICLISTspamKILLspamMITVMA.MIT.EDU]On Behalf Of Gavin Jackson
> Sent: March 14, 1998 00:42
> To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
> Subject: Re: Power Supply
>
>
> Hi there once again
>
> Correct me if I'm wrong, but I though that the 78XX voltage
> regulators can only take 3X the output voltage on their input,
> so the maximum supply that a 7805 can have as its input
> is 15V.

       Not as far as I know, I believe the whole 78XX series has a common max
voltage, around 35V if memory serves, You can check the data sheets if you
want, I believe NS has them online for that product...
Excerpt from LM7805C datasheet, page 2:

Absolute Maximum Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Input Voltage (V O e 5V, 12V and 15V) 35V
Internal Power Dissipation (Note 1) Internally Limited
Operating Temperature Range (T A ) 0 'C to a70 'C
Maximum Junction Temperature
(K Package) 150 'C
(T Package) 150 'C
Storage Temperature Range b65 'C to a150 'C
Lead Temperature (Soldering, 10 sec.)
TO-3 Package K 300 'C
TO-220 Package T 230 'C

TTYL

1998\03\14@150743 by Mike Keitz

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On Sat, 14 Mar 1998 18:42:09 +1300 Gavin Jackson <EraseMEvulcanspam_OUTspamTakeThisOuTIHUG.CO.NZ>
writes:
>Hi there once again
>
>Correct me if I'm wrong, but I though that the 78XX voltage
>regulators can only take 3X the output voltage on their input,
>so the maximum supply that a 7805 can have as its input
>is 15V.

They are rated for up to 35V, except the 7824 is rated up to 40V.

But, there is a problem with higher input voltages: heat.  Since they are
linear regulators, the amount of power dissipated depends almost entirely
on the load current and the difference between input and output voltage.
For example, if your circuit uses 5V at 50 mA, with an input of 15V, the
7805 will dissipate about .05(15-5) W, or 500 mW of heat.  This is about
the limit for a bare 78XX device.  Above that, a heat sink would need to
be added to keep it cool.  If the input voltage were 25V, the dissipation
would increase to a full Watt, even though the load circuit is using only
1/4 Watt of power.  So the 3x input voltage rule is a good rule of thumb
for design, though not an absolute limitation.

In the case of the boat project being discussed, heat isn't a big problem
since the load current is only 5 mA.  In this case, the 7805 would
dissipate about 380 mW.  This is too much for a 78L05 as someone
suggested (max. dissipation on these is 100 mW), but a standard TO-220
regulator with little or no heatsink would do fine.  One of the newer
ones intended for automotive use should be used along with additional
protection in order to withstand the likely high voltage spikes on the
24V input from other large electrical machines connected to the 24V
battery.

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1998\03\17@215805 by William Chops Westfield

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This weeks EDN Magazine has a "design ideas" note on using switched-capacitor
voltage doublers (backwards) to cut power dissipation in cases where the
input voltage is more than twice the desired output voltage.  It's by a
Linear technology guy, and the example uses the LT1054CS8, so you can
probably find it on the LT website as well.

Their example does 5V or 3V @ 200mA from 12V input, using an SO8 chip and
no heat-sinks.  Pretty neat idea...

BillW


'Power Supply'
1998\05\12@215047 by TONY NIXON 54964
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picon face
Has anyone got any pointers to a low noise 5V power supply with good
temp stability. Mains operated, 1A max O/P.

I need it to power 24 strain guages for data aquisition.

TIA

Tony


PicNPoke Multimedia 16F84 Beginners PIC Tools.

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1998\05\13@005308 by Chris Eddy

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part 0 2768 bytes content-type:text/x-vcard; charset=us-ascii; name="vcard.vcf" (decoded 7bit)

I can toss in some thoughts on the subject.

At all costs, remain linear for powering the system.  Switchers are the
harbinger of death in this sort of application.

Strain gauges like to be driven with a current source, such that you
regulate the current through the bridge.  I read in an AD data book that
this compensates for wiring loss and noise (?!).  A bridge can be driven
by an op amp, NPN, and sense resistor (IE 10 ohms) in the bottom.  I
would do ascii art, but it pisses me off.

Investigate the ratiometric method.  A/D's these days are more and more
likely to be capable of rigging the reference to an external voltage.
If you skip the precision reference and link the reference to the bridge
exite voltage, you get better tracking (really).

You must use an instrumentation amp on the return pair.  The old standby
tripple op amp 'electrometer amplifier' configuration requires hand
matched resistors for CMRR and you DONT want to spend the rest of your
life with an ohm meter in your hand and a fistful of resistors.  And
even at that the performance still cannot beat today's cheap IA's in
plastic, like the AD620/621/622.  You might need another IA on the
supply to the bridge to drive the reference on the A/D to be ratiometric
IF you opt for the bridge current regulation in the low side.  Unless
your A/D has ref+ and ref-.  Get all of that?

What kind of performance / resolution are you looking for?  I saw the
new AD7730 sigma delta family in the Analog Devices book, and it blew me
away.  Without pulling off a genuine miracle in layout teqcniques, you
could probably do 16 bit resolution and skip all of the above.  See the
data sheet and you will see what I mean.  BIG bucks, though.

In answer to your original question on supplies, you can simply use
linear regulators, and filter very well with a resistor, electrolytic,
and ceramic on each power feed.  That will clean up to, oh, 5mV or
less.  MORE IMPORTANT THAN THE REGULATORS, though, is to use either a
superb starred ground layout, or even better a genuine ground plane.
When you do, you will begin to see databook performance from your
circuit, and it will make you beam with pride.

G'luck.
Chris Eddy, PE
Pioneer Microsystems, Inc.

TONY NIXON 54964 wrote:

> Has anyone got any pointers to a low noise 5V power supply with good
> temp stability. Mains operated, 1A max O/P.
>
> I need it to power 24 strain guages for data aquisition.
>



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Content-Transfer-Encoding: 7bit
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Attachment converted: wonderland:vcard.vcf (TEXT/CSOm) (00007F96)

1998\05\13@012000 by Mike Keitz

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On Wed, 13 May 1998 09:54:48 +1000 TONY NIXON 54964
<@spam@Tony.nixonKILLspamspamENG.MONASH.EDU.AU> writes:
>Has anyone got any pointers to a low noise 5V power supply with good
>temp stability. Mains operated, 1A max O/P.
>
>I need it to power 24 strain guages for data aquisition.

The real problem isn't so much in regulation, it's distribution.  You
need to know the excitation voltage at each strain gauge to take an
accurate measurement.  Regulating them all from one central point will
make loss in the cables a critical variable.  Maybe a 4-wire arrangement
so the excitation voltage and the bridge voltage can both be measured.
Or use the sense leads from each gauge to control the drive to a constant
amount.  Use a ratiometric conversion so the accuracy of the reference
isn't too important.


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1998\05\15@131804 by Josef Hanzal

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>Investigate the ratiometric method.  A/D's these days are more and more
>likely to be capable of rigging the reference to an external voltage.
>If you skip the precision reference and link the reference to the bridge
>exite voltage, you get better tracking (really).

SNIP

>What kind of performance / resolution are you looking for?  I saw the
>new AD7730 sigma delta family in the Analog Devices book, and it blew me
>away.  Without pulling off a genuine miracle in layout teqcniques, you
>could probably do 16 bit resolution and skip all of the above.  See the
>data sheet and you will see what I mean.  BIG bucks, though.
>

I would fully agree. The AD7730 achieves a very high resolution (24 bits),
but there is also AD7715, 16 bit lower cost (~$5), with differential input
_and_ differential reference. You can do four wire connection without any
instrumentation amplifiers at all.

What is the required resolution of your readings ? It will influence lot of
other things.

Josef


'power supply'
1999\10\30@004256 by camerlin
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Hello everyone,

I am using a 16F84 to activate a motor that runs off of 5Vdc or 12Vac
and pulls about 1A.  I am using a 12Vac / 200mA wall transformer for the
power supply, rectifiying it, and then regulating it at 5V for the PIC.
(I choose the 200mA supply because of the PIC maximum sinking currents)
If I use the 12Vac to activate the motor will it still affect the PIC
even though the power supply would have gone through rectifying and
regulating before it got to the PIC?

Could I use the 5Vdc instead and just put a diode across the relay I'm
using to trigger the motor to prevent kickback or would that affect the
PIC?

My last question is that if the motor requires 1.0A and the wall
transformer supplies 200mA, will it turn on the motor?

I would appreciate any help.  Thanks in advance.
Chris Camerlin

1999\10\30@020055 by William J. Kitchen

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face
Choose a supply that can provide enough current for the motor + PIC +
associated circuitry.  A bigger supply will not harm your PIC as long as
the voltage is regulated.

Whether a motor powered by the 5V supply will affect the PIC depends
on how big your power supply filter caps are and on whether the supply
is big enough to keep the voltage up under load.  Bigger is better, within
reason.  Also, the PIC will be less likely to be affected if it is not in the
current path between the motor and power supply.  That is, it's better if
the PIC (and associated circuitry) and the motor have seperate power
and ground paths (especially ground) back to a common point on the
power supply. If the motor isn't terribly big (and it sounds like it isn't),
then this may not matter, but still it's a good practice when switching
large loads or with circuits that are very sensitive to noise.

{Quote hidden}

---
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                                       William Kitchen
The future is ours to create.

1999\10\30@021353 by Jinx

face picon face
If the figure of 200mA is on the label then that will be the current
at the labelled voltage. In all probability the unloaded voltage will
be higher, perhaps 15VAC. I would think it unlikely you would
get 1.0A out of this supply no matter how low you are prepared
to let the voltage drop to under this load. I'd say a 500mA supply
would be getting closer. Only a rated 1.0A 12VAC would really
deliver the goods. Much of this really depends on whether the
motor can work at lower voltages and if the reduced torque it
can develop is acceptable. Is it possible to use this motor on
DC ? Some bridge configurations will give you more usable
current but at reduced voltage and vice versa.

Joe

> My last question is that if the motor requires 1.0A and the wall
> transformer supplies 200mA, will it turn on the motor?

1999\10\30@025128 by Mark Willis

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face
Larger 'wall warts' are fairly cheap, too;  ComputerGate (for example)
sells a 220 or 110 VAC in, 1A at 3, 4.5, 6, 7.5, 9, and 12V DC adapter
(p. 52 of their catalog, "ACDC1000L" is the part number) for $4.95.
About 6 different connectors on the end of the 6' cord there.  Of
course, that's a DC output, but you get the idea, and could strip the
transformer out of there <G>

Also:  Halted, or BGMicro, among others, also usually have small power
lumps that are cheaper than that & have about the same current
capability or more.  Why try to solve a "power supply's not big enough"
problem with pricey engineering, when you can just plug in a better,
larger power supply, for cheap?  <G>

BTW, Jinx, not true that you'll see 15VAC out unloaded - We're not
talking after rectification here, but before - when you don't load a DC
supply, it peaks up (and also you see less ripple than at heavy loads
<G>

 Mark

Jinx wrote:
{Quote hidden}

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1999\10\30@035515 by Keith Causey

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Every Thrift Shop I ever go in has a full bin of orphaned wall warts of
every conceivable description for prices ranging from $0.25 to $3.50. As a
consequence I have a bucket full of wall warts rather than a bucket full of
pennies.

{Quote hidden}

1999\10\30@035930 by Jinx

face picon face
Correct, I know exactly what you mean about DC plugpacks. I would
still not trust a label and consider it prudent to measure the AC voltage.
You never know what, if any, standards the manufacturer has. If you'd
agree that a loaded transformer's voltage will drop, then it's not
unreasonable to suggest that a manufacturer could install one with a
higher voltage that will drop to the stated voltage at the stated load.
As an example I have a small labelled 18V 0.3A AC adaptor for a
UV EPROM eraser (no retification or caps at all). It measures 20.3VAC
unloaded but 17.4VAC in use.

Whether we agree or agree to disagree I think a point about not
assuming labelled voltages on plugpacks is worth mentioning. I've
stopped at least one of my customers blowing a circuit I'd made for
him by checking his chosen 4.5V plugpack before he used it. Admittedly
it was a DC one and if he'd been in the know he could have expected
it to be as high as it was (7.5V). With so many low voltage CMOS
devices now on PCBs and the consequent general reduction in current
required, if you aren't using a regulator you'd be ill-advised not to check
a bought plugpack (please don't think I just said you give ill advice).

All of this is academic in Chris' case. It looks as though the supply he's
got will be inadequate anyway.

WBR Jinx.
{Quote hidden}

1999\10\30@165428 by Mark Willis

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I usually use DC 'wall warts' - Interesting!  I hadn't measured an AC
one, I just let someone else use those, usually.  I didn't anticipate
any real drop in voltage - guess I forgot about internal resistive
losses inside the transformer, which is already part of why a DC
plugpack "peaks" sans load.  Need a vacation here, obviously <G>

I used to use a DC Lump, put it through a reverse protection diode for
safety <G>, then a fuse & Zener for over-voltage protection (cheaper:
Use the reverse protection diode AS a fuse, not ideal but cheap!  Even
lower cost: Skip the series diode, put DC In - Fuse - Reverse diode to
ground, or skip the fuse & let the plugpack's diodes blow if you must
<G>)  Then a nice Cap, regulator, etc.  I'm migrating to using a bridge
rectifier for input protection, (yeah, you lose 2 diode drops, but it's
always SAFE, and it's cheap, and accepts DC or AC input. <G>)

True, on the rest <G>  I like the recent trend to try to use different
plugs for different voltage ranges;  Even though it is a pain in terms
of finding all those sockets, makes it less likely that someone will
plug a 15VDC plug into their 4.5VDC device, then complain to the
manufacturer about poor design quality <G>  Hoping Chris gets a new
plugpack!

 Mark

Jinx wrote:
{Quote hidden}

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'power supply'
2000\02\21@210529 by Joe Hamilton
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face
It sounds like you know what your doing, but I would just like to make sure
that you have checked the input V going into the regulator, it must be about
2 V higher than the designed output V without ripple ( the -peak of your
ripple must be 2V higher - this must be checked under load-).  Other
problems that I have had when designing high current power supplies is to
much noise or ripple on the output, resulting in improper feedback, and
number 1 is a series pass transistor with a low beta, and finally *ground
loops* can cause havoc with the feedback.


'Power supply'
2005\08\17@123221 by Vasile Surducan
face picon face
On 8/17/05, Mauricio Jancic <KILLspaminfoKILLspamspamjanso.com.ar> wrote:
{Quote hidden}

A full bridge rectifier. SMD or not.

Vasile

{Quote hidden}

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