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'[OT]: Hot LM7805A cause strange behaviour?'
2001\05\07@035457 by Tim Thompson

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Has anyone out there ever had hot LM7805A regulators be the cause of
strange pic software behaviour? I have a
PIC communicating with a program on a PC, and it was working flawlessly the
first run, but after I reset the PIC and
restarted the software it stoped parsing the serial data..I noticed the
regulator was fairly hot (its trying to regulate a BAD dc output from a
wall wart), and after letting the reulator cool the PIC ran fine..and im
trying to see if the problem was directly the cause of the regulator being
hot (I thought these had thermal shutdowns?)
or the cause of some software glitch...I'm lacking any debugging of the PIC
so i'm resorting to re-reading the code (about 1K assembled) and commenting
everything out trying to limit it down to what could be causing this, 2
days of this so far...). It would be nice to know if this is the regulators
fault.

Thanks in advance,
Tim Thompson


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2001\05\07@040952 by David VanHorn

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At 12:55 AM 5/7/01 -0700, Tim Thompson wrote:
>Has anyone out there ever had hot LM7805A regulators be the cause of
>strange pic software behaviour? I have a
>PIC communicating with a program on a PC, and it was working flawlessly the
>first run, but after I reset the PIC and
>restarted the software it stoped parsing the serial data..I noticed the
>regulator was fairly hot (its trying to regulate a BAD dc output from a
>wall wart), and after letting the reulator cool the PIC ran fine..and im
>trying to see if the problem was directly the cause of the regulator being
>hot (I thought these had thermal shutdowns?)

They do, they dump your VCC, which would likely cause non-deterministic
execution.

Can you get rid of some of the load on the reg?

Then again, is there enough dissipation (calculated) for it to be hot?
Oscillating 7805's get hot, I've seen them doing a couple watts of 1-2 MHz.
That's not likely to do your CPU much good either.


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2001\05\07@042033 by Tim Thompson

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The whole circuit is drawing like 70ma from the wall wart...
What do you mean about the dissipation?
I'm starting to think this might be a software problem..I took out my handy
little logic probe, and when the PIC (16F877 btw) locks up, the TX pin on
the PIC is neather high or low (tri-stated i beleave its called)..So i have
no clue wtf is going on. Everything was working fine, then i did some code
addition and now its broken totaly, and I cant tell what I did to break it
or how to fix it. If anyone would be willing to look it all over (its
fairly well commented) please let me know.

Tim Thompson


At 03:08 AM 5/7/2001 -0500, you wrote:
{Quote hidden}

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2001\05\07@063512 by Dave Dilatush

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Tim Thompson <spam_OUTiruleTakeThisOuTspamIRULE.NET> wrote...

>Has anyone out there ever had hot LM7805A regulators be the cause of
>strange pic software behaviour? I have a
>PIC communicating with a program on a PC, and it was working flawlessly the
>first run, but after I reset the PIC and
>restarted the software it stoped parsing the serial data..I noticed the
>regulator was fairly hot (its trying to regulate a BAD dc output from a
>wall wart), and after letting the reulator cool the PIC ran fine..and im
>trying to see if the problem was directly the cause of the regulator being
>hot (I thought these had thermal shutdowns?)
>or the cause of some software glitch...I'm lacking any debugging of the PIC
>so i'm resorting to re-reading the code (about 1K assembled) and commenting
>everything out trying to limit it down to what could be causing this, 2
>days of this so far...). It would be nice to know if this is the regulators
>fault.

Tim,

If you suspect regulator problems, first thing to do is measure the
output voltage of the regulator.  Is it 5 volts, give or take a few
percent?  If not, it might be malfunctioning.  Is the regulator output
the same when hot as it is when cold, within a few hundredths of a
volt?  If it is, it would seem that it's working OK.

How much voltage is your wall-wart delivering to the input of the
regulator?  Is it safely below the regulator's maximum rating?

If you have an oscilloscope or can get to one, look at the regulator
output.  Is it steady, or is it oscillating?  If it's not rock-solid
steady, that's a problem.  Also, look at the regulator input voltage;
is there a lot of ripple on it?  Wall-warts tend not to have very
hefty output filters, an their outputs can have a lot of ripple at
high loads.  If the minimum voltage on the input drops below about 7
or 8 volts at any point in the power line cycle, your 7805 output will
droop, causing problems.

As for the regulator being too hot, a crude but fairly reliable rule
of thumb is that if you can stand to hold your fingertip on the device
for at least five seconds, it's probably not too hot.  If it's so hot
you can't bear to touch it that long, it might be.

That's about all I can think of; hope this helps.

Dave

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2001\05\07@092531 by David VanHorn

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At 01:21 AM 5/7/01 -0700, Tim Thompson wrote:
>The whole circuit is drawing like 70ma from the wall wart...
>What do you mean about the dissipation?

(Input voltage - output voltage)  * current = watts


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2001\05\07@093511 by Spehro Pefhany

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At 08:24 AM 5/7/01 -0500, you wrote:
>At 01:21 AM 5/7/01 -0700, Tim Thompson wrote:
>>The whole circuit is drawing like 70ma from the wall wart...
>>What do you mean about the dissipation?
>
>(Input voltage - output voltage)  * current = watts

Current (in this case) = input current = output current + regulator current
~= 75mA (assuming a 78xyy regulator).

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2001\05\07@094952 by David VanHorn

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At 09:35 AM 5/7/01 -0400, you wrote:
>At 08:24 AM 5/7/01 -0500, you wrote:
> >At 01:21 AM 5/7/01 -0700, Tim Thompson wrote:
> >>The whole circuit is drawing like 70ma from the wall wart...
> >>What do you mean about the dissipation?
> >
> >(Input voltage - output voltage)  * current = watts
>
>Current (in this case) = input current = output current + regulator current
>~= 75mA (assuming a 78xyy regulator).

I knew that...
And the other variable is?
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2001\05\07@100237 by Spehro Pefhany

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At 08:48 AM 5/7/01 -0500, you wrote:

>I knew that...

Of course, but probably not everyone did, and it adds a bit to Pd.
More of a factor with 78Lxx regulators with relatively high Vin.

>And the other variable is?

Lots of other variables.. the 'common' current is fairly constant with
output current and Ta on the 78x05 regulators, not so on some others,
So Pd = (Iq + Io)  * (Vin - Vout) is pretty close.

What are you getting at?

Best regards,

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2001\05\07@101103 by David VanHorn

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>
>What are you getting at?

Just that we still don't know the input voltage, which is a major factor.
He mentioned a dirty 18V, but I'd like to see at least a meter measurement,
if not scope.

With the power levels mentioned though, I'm beginning to suspect oscillation.

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2001\05\07@103400 by Spehro Pefhany

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At 09:10 AM 5/7/01 -0500, you wrote:

>Just that we still don't know the input voltage, which is a major factor.
>He mentioned a dirty 18V, but I'd like to see at least a meter measurement,
>if not scope.

Ah, yes. If it was 18VAC, poorly regulated and full wave rectified, it might
be more like 30V, leading to almost 2W dissipation. At 65'C/W and 25'C Ta,
you'd need 1.9W to get to the 150'C Tj typical shutdown temperature.

>With the power levels mentioned though, I'm beginning to suspect oscillation.

I've never been able to get a 78M05 to oscillate, even without any load
capacitor. I suppose with really, really bad layout it might be possible, but
they are pretty sluggish devices.

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2001\05\07@104403 by David VanHorn

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>
>I've never been able to get a 78M05 to oscillate, even without any load
>capacitor. I suppose with really, really bad layout it might be possible, but
>they are pretty sluggish devices.

I've only seen it a couple times.
They got much hotter than you'd expect in the circuit.


I've had really bad luck with the M devices just dying for no apparent reason.
Never had that problem with the L or standard sevices.
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2001\05\07@122803 by rottosen

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Spehro Pefhany wrote:
>
> At 09:10 AM 5/7/01 -0500, you wrote:
>
> >Just that we still don't know the input voltage, which is a major factor.
> >He mentioned a dirty 18V, but I'd like to see at least a meter measurement,
> >if not scope.
>
> Ah, yes. If it was 18VAC, poorly regulated and full wave rectified, it might
> be more like 30V, leading to almost 2W dissipation. At 65'C/W and 25'C Ta,
> you'd need 1.9W to get to the 150'C Tj typical shutdown temperature.


30 volts is also beyond the maximum input voltage of some 5 volt
regulators.


>
> >With the power levels mentioned though, I'm beginning to suspect oscillation.


What if it is a TO-92 packaged 7805? Then the 75ma could cause a thermal
shutdown.


>
> I've never been able to get a 78M05 to oscillate, even without any load
> capacitor. I suppose with really, really bad layout it might be possible, but
> they are pretty sluggish devices.


He is using a wall wart with the filter capacitor at the end of a cable
away from the 7805. The manufacturers always say to put a bypass
capacitor at the input of the 7805 in this situation.


-- Rich


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2001\05\07@145016 by Spehro Pefhany

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At 10:10 AM 5/7/01 -0600, you wrote:
(sorry for the double copy..)
>What if it is a TO-92 packaged 7805? Then the 75ma could cause a thermal
>shutdown.

Then it wouldn't be a 78M05, it would be a 78L05. Too bad they don't package
the 78M05 in the TO-92 case, BTW.

>He is using a wall wart with the filter capacitor at the end of a cable
>away from the 7805. The manufacturers always say to put a bypass
>capacitor at the input of the 7805 in this situation.

Yes, that could do it. ;-)  The input capacitor is *mandatory* in that
situation. A small electrolytic (say 22uF ~ 100uF, 35V) should do it.

Best regards,


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2001\05\07@145813 by David VanHorn

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>
>He is using a wall wart with the filter capacitor at the end of a cable
>away from the 7805. The manufacturers always say to put a bypass
>capacitor at the input of the 7805 in this situation.

Ouch.

I'm the kind of guy that plants an SMD 0.1 right between the pads.

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2001\05\07@150837 by Dal Wheeler

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> I've had really bad luck with the M devices just dying for no apparent
reason.
> Never had that problem with the L or standard sevices.
Hmmm...  I've had bad luck with M devices as well.  I thought it was just a
batch problem.  Oh well, comfort in shared misery.

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2001\05\07@151655 by Spehro Pefhany

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At 01:06 PM 5/7/01 -0600, you wrote:

>Hmmm...  I've had bad luck with M devices as well.  I thought it was just a
>batch problem.  Oh well, comfort in shared misery.

Can't concur; have used over 100K of them with almost no problems. Mind you,
I seldom ask them to dissipate more than 1W, usually 400-600mW.

Mechanical damage of the TO-220 package (if heat sinked) is a common problem.
And JRC makes their leads too stiff (steel rather than copper) IMHO.

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2001\05\07@152524 by David VanHorn

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>
>Mechanical damage of the TO-220 package (if heat sinked) is a common problem.
>And JRC makes their leads too stiff (steel rather than copper) IMHO.

We saw that problem, and made a die to bend the leads. Still had loads of
failures, operating well within spec.
Gave up, and used 7805's instead, and went back to near-zero failures.
I don't remember the vendors though. Probably NS and TI.

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2001\05\07@154259 by Spehro Pefhany

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At 02:24 PM 5/7/01 -0500, you wrote:

>We saw that problem, and made a die to bend the leads. Still had loads of
>failures, operating well within spec.
>Gave up, and used 7805's instead, and went back to near-zero failures.
>I don't remember the vendors though. Probably NS and TI.

Probably ours were ST or JRC, judging by a few boards within reach, the 7805s
tend to be NS (or the LMxxx equivalent)

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2001\05\07@183028 by Tim Thompson

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The wall wart outputs 12V DC at 800ma max.

Tim Thompson
At 09:10 AM 5/7/2001 -0500, you wrote:
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2001\05\07@184313 by David VanHorn

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At 03:30 PM 5/7/01 -0700, Tim Thompson wrote:
>The wall wart outputs 12V DC at 800ma max.

What's the input voltage though?
As lightly loaded as that wart is, I'd expect it to be closer to 16-18V.

In any case, we've covered the heating issues, dissipation, oscillation,
and just plain broke.

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2001\05\08@025124 by Tim Thompson

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Its outputting at 16.23, still well within the LM7805's specs.

At 05:43 PM 5/7/2001 -0500, you wrote:
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2001\05\08@034127 by Russell McMahon

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> The wall wart outputs 12V DC at 800ma max.


Summary of various inputs -

Here are some rules for you to apply :-)

A lightly loaded 12v a wall wart without an internal regulator (most types)
will have  a higher output voltage when lightly loaded.

Vout could be as much as say 20 volts (unusual) or so at light loads.
7805 is good for 1 amp IF PROPERLY HEATSUNK.
At 70 mA load, 5v out and 12v in dissipation is (12-5)*70 = 490 mW
This would (probably) be OK in an unheatsunk TO220 package.
1 watt dissipation will occur at 5 + 1000/70 = 19 volts odd.
At this voltage a flag heatsink would be a good idea.
Check data sheet for dissipation in free air appropriate to your package.

If it burns your finger it is hotter than it needs to be.
If it boils water (or spit :-) ) then it is too hot.
Data sheet allows it to be this hot under controlled conditions but you are
asking for trouble.

Many regulators will oscillate under certain load conditions unless
CORRECTLY bypassed by placing a  capacitor near the regulator at output
and/or input. Actual values needed and characteristic vary with regulator -
read your data sheet. An oscillating regulator will still usually produce a
5v out[put as measured on a standard meter but will cause you endless woe.
Use proper bypassing. Check with a scope. Instability may vary with load and
regulator condition (hot or cold etc).

An overheated 7805 regulator (and many others) will shutdown automatically
and reduce output voltage. Your circuit will probably malfunction.




   Russell McMahon

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