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'[EE]: 5V regulator from automotive alternator sour'
2002\03\28@130435 by Stephen Webb

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I'm having trouble with a circuit.  5V regulator, PIC, and a reed relay.

When I run it from a 9v source, it draws ~20 mA-40mA.  It can run
"forever" without problems.

When I run it from my car electric system (12v battery, alternator ~
14v) it gets very hot.  The regulator gets hot, but nothing terrible.  The
PIC gets noticably warm.  Pressing your finger on it for > 5 seconds
causes you to feel pain.

I'm just shooting in the dark, here, but is it possible that the regulator
can't deal with the "noisy" input, and the resulting output is causing the
pic problems?


-Steve

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2002\03\28@131513 by Bob Blick

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Hi Steve,

There's something else wrong. Where's your voltmeter? Start putting it to
work. Your PIC should not get hot, find out how and where the extra
voltage is getting to it.

Do you have decoupling caps anywhere? The regulator surely needs one on
input and one on output, and then one for the PIC. That's sort of a bare
minimum.

Any other connections to 12 volts, perhaps to one of the PIC input pins?
You can't hook it directly to 12 volts.

Cheers,

Bob

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2002\03\28@153015 by Rick C.

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Just posted this on a previous reply today.
rixy

One subject that is often overlooked is the need for bypass capacitors. A
0.1uf cap across the supply
lines of the PIC as close to the PIC as possible. Good wire or circuit track
from the GND pin of the PIC
to power supply ground. If you use any type of regulator, LM78xx or LM340
there MUST MUST MUST be a 0.1uf
cap from the INPUT pin of the regulator to the GROUND pin of the regulator as
close to the regulator as
physically and electrically possible. This will cure 95% of all noise getting
into your PIC and upsetting
the processor/code.
Rick

P.S. Without a bypass cap on the regulator, it may work on the test bench but
when it gets into the real world, with long power supply lines feeding the
regulator, it will "sing" oscillate and get very hot. It will also transfer
its oscillations to the PIC and drive it bonkers.
Rick - (let me know if it cures the problem)

Stephen Webb wrote:

{Quote hidden}

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2002\03\28@174056 by Justin Mierta

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oh!  this happened to me too.  you're providing some inputs to the PIC.
these inputs MUST have voltages <= to the PIC's source voltage,
otherwise what you just described happens

justin


Stephen Webb wrote:

{Quote hidden}

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2002\03\28@193757 by michael brown

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> I'm having trouble with a circuit.  5V regulator, PIC, and a reed relay.
>
> When I run it from a 9v source, it draws ~20 mA-40mA.  It can run
> "forever" without problems.
>
> When I run it from my car electric system (12v battery, alternator ~
> 14v) it gets very hot.  The regulator gets hot, but nothing terrible.  The
> PIC gets noticably warm.  Pressing your finger on it for > 5 seconds
> causes you to feel pain.

Do you have the "recommended" capacitors on the input and output of the
regulator?  Something is probably oscillating.

michael brown

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2002\03\30@102713 by Tom Handley

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  Steve, in addition to comments from Bob and Rick, check the voltage
ratings on your capacitors. Especially if you are using tantalums. If
you get some large spikes, you could easily destroy the cap if it's not
rated to handle the output. Other than that, I would first pull the PIC
and see exactly what voltages you have on those pins...

  - Tom

{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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2002\03\30@125627 by Larry Bradley

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Check the regulators in the automotive section of the National
Semiconductor web site http://www.national.com. They have regulators that
are designed with the automotive environment in mind. They also have some
appnotes that talk about the hazards of this environment.

Larry


At 07:31 PM 3/29/2002 -0800, you wrote:
{Quote hidden}

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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