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'[PIC]: 16F877 draws huge current with minimal desi'
2000\12\31@013759 by Jacob M Hartman

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I am having a serious problem getting started with the 16F877.  All the
pins are pulled low (~10k resistors to ground) except for VSS and VDD,
which are respectively connected to ground and +5V (coming out of a 7805,
with a few caps to level the voltage).  Yet the chip draws around an amp
and starts heating....  It's as if the VSS and VDD pins are internally
shorted to each other.  I'm positive that everything is hooked up as
described; e.g., I didn't swap the supplies to VSS and VDD, causing a short
through the protective diodes.  While I suspect that I've toasted this
chip, I'm wondering what I did wrong, in the interest of not toasting
another one.  I'm also very curious what could cause such a critical
internal short.

Thank you, and have a happy new year!

Jake

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2000\12\31@080518 by Peter Anderson

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Check your 7805 and be certain it is grounded.

I recently had much the same thing happen, heating my ICD, PIC and an
LCD only to find the 7805 output was at +8.0.


Peter H Anderson, http://www.phanderson.com, pha(at)phanderson.com
PIC C, Serial ICDs


---- Begin Original Message ----

From: Jacob M Hartman <.....jmhartmaKILLspamspam@spam@US.IBM.COM>
Sent: Sun, 31 Dec 2000 01:27:46 -0500
To: PICLISTspamKILLspamMITVMA.MIT.EDU
Subject: [PIC]: 16F877 draws huge current with minimal design


I am having a serious problem getting started with the 16F877.  All the
pins are pulled low (~10k resistors to ground) except for VSS and VDD,
which are respectively connected to ground and +5V (coming out of a 7805,
with a few caps to level the voltage).  Yet the chip draws around an amp
and starts heating....  It's as if the VSS and VDD pins are internally
shorted to each other.  I'm positive that everything is hooked up as
described; e.g., I didn't swap the supplies to VSS and VDD, causing a short
through the protective diodes.  While I suspect that I've toasted this
chip, I'm wondering what I did wrong, in the interest of not toasting
another one.  I'm also very curious what could cause such a critical
internal short.

Thank you, and have a happy new year!

Jake

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---- End Original Message ----




Peter H. Anderson, EraseMEphaspam_OUTspamTakeThisOuTphanderson.com, http://www.phanderson.com
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2000\12\31@081726 by uter van ooijen & floortje hanneman

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> Check your 7805 and be certain it is grounded.
An check that it is actually a 7805, I once used a 7808 by mistake, it is
sometimes hard to read....
Wouter

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2000\12\31@093314 by mike

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On Sun, 31 Dec 2000 01:27:46 -0500, you wrote:

>I am having a serious problem getting started with the 16F877.  All the
>pins are pulled low (~10k resistors to ground) except for VSS and VDD,
>which are respectively connected to ground and +5V (coming out of a 7805,
>with a few caps to level the voltage).  Yet the chip draws around an amp
>and starts heating....  It's as if the VSS and VDD pins are internally
>shorted to each other.  I'm positive that everything is hooked up as
>described; e.g., I didn't swap the supplies to VSS and VDD, causing a short
>through the protective diodes.  While I suspect that I've toasted this
>chip, I'm wondering what I did wrong, in the interest of not toasting
>another one.  I'm also very curious what could cause such a critical
>internal short.
Usually reverse polarity, or connecting one pair of vss/vdd lines the
wrong way round so the internal interconnects pass current.

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2000\12\31@102133 by Roman Black

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wouter van ooijen & floortje hanneman wrote:
>
> > Check your 7805 and be certain it is grounded.
> An check that it is actually a 7805, I once used a 7808 by mistake, it is
> sometimes hard to read....
> Wouter

And make sure it's NOT a 7905!! Ouch!
-Roman

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2000\12\31@131637 by Jacob M Hartman

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Thanks for the feedback.  There doesn't seem to be any problem with the
7805, which is indeed a 7805 and is properly connected.  My voltmeter
confirms that it is outputting 5V.  (Until I connect the 16F877, at which
point the voltage drops significantly, as if due to a short.)  To be sure,
I replaced it with another regulator (a LM2940), and nothing changed.  And
the VDD and VSS pins are properly connected.  I've tried connecting pin 11
to +5, 12 to GND; pin 32 to +5, 31 to GND; and both 11 and 32 to +5, 12 and
31 to GND.  All configurations have the same effect:  The chip draws about
an amp and heats up.  It really is like there is an internal short between
pins 11-12 and 32-31.  But I'm quite baffled what would cause such a thing.
Maybe I unknowingly sparked it, and the surge caused a short of some kind?

Jake

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2000\12\31@140034 by Bob Blick

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Hi Jake,

You must use both sets of VSS and VDD. If you did not do that initially,
that could be your problem and your chip is now toast.

-Bob

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2000\12\31@154556 by Dan Michaels

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Jake  wrote:
 I've tried connecting pin 11
>to +5, 12 to GND; pin 32 to +5, 31 to GND; and both 11 and 32 to +5, 12 and
>31 to GND.  All configurations have the same effect:  The chip draws about
>an amp and heats up.  It really is like there is an internal short between
>pins 11-12 and 32-31.  But I'm quite baffled what would cause such a thing.
>Maybe I unknowingly sparked it, and the surge caused a short of some kind?
>

Have you tried sticking your ohmmeter across the chip Vdd and
Vss pins - out of circuit?

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'[PIC]: 16F877 draws huge current with minimal desi'
2001\01\01@062251 by Tsvetan Usunov
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> I am having a serious problem getting started with the 16F877.  All the
> pins are pulled low (~10k resistors to ground) except for VSS and VDD,
> which are respectively connected to ground and +5V (coming out of a 7805,
> with a few caps to level the voltage).  Yet the chip draws around an amp
> and starts heating....  It's as if the VSS and VDD pins are internally
> shorted to each other.  I'm positive that everything is hooked up as
> described; e.g., I didn't swap the supplies to VSS and VDD, causing a
short
> through the protective diodes.  While I suspect that I've toasted this
> chip, I'm wondering what I did wrong, in the interest of not toasting
> another one.  I'm also very curious what could cause such a critical
> internal short.

if everything is connected as need you have to check the 7805 output
voltage
PICs are very prone to reverse voltage connection .I very often put the 18
pin PIC reverse in the socket, so I know this from my own experience ;)

Best regards and
Happy New Year!

Tsvetan
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PCB prototypes for $26, http://run.to/pcb

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2001\01\01@085634 by mike

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On Sun, 31 Dec 2000 13:17:22 -0500, you wrote:

>Thanks for the feedback.  There doesn't seem to be any problem with the
>7805, which is indeed a 7805 and is properly connected.  My voltmeter
>confirms that it is outputting 5V.  (Until I connect the 16F877, at which
>point the voltage drops significantly, as if due to a short.)  To be sure,
>I replaced it with another regulator (a LM2940), and nothing changed.  And
>the VDD and VSS pins are properly connected.  I've tried connecting pin 11
>to +5, 12 to GND; pin 32 to +5, 31 to GND; and both 11 and 32 to +5, 12 and
>31 to GND.  All configurations have the same effect:  The chip draws about
>an amp and heats up.  It really is like there is an internal short between
>pins 11-12 and 32-31.  But I'm quite baffled what would cause such a thing.
>Maybe I unknowingly sparked it, and the surge caused a short of some kind?
>
>Jake
If in doubt, put a current-limiting resistor on the input side of the
regulator, say to limit current to 100mA - this is unlikely to smoke
anything.

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