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'[PICLIST] PIC 16F84A Self-Destruction'
2001\10\08@040502 by Jonathan Luthje

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G'day all,
   I have a question relating to the 16F84A processor - basically all I'm
doing is switching on and off a couple of relays via a 1.6mA base current to
a pair of BF246 NPN transistors. There are 3 pins (RB0,1,2) set to output -
the rest are set to input, some tied high, some tied low, all via 2k0
resistors, all unused pins are tied together, set to input and tied to
ground via a 2k0 resistor. I have regulated and filtered 6vdc supply rails
powering the thing.

The circuit and program were working beautifully up until the time where the
circuit "browned out" and caused the PIC to get very hot (hot enough to put
a blister on my thumb - ouch). I've since installed a reset-on-brownout
circuit as given in the 16F84A datasheet. This is PIC #3 that this has
happened to ... it's starting to get a little expensive! :)

Is there some fundamental design flaw I'm making here?

Thanks in advance,


Regards,


Jonathan Luthje

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2001\10\08@050942 by dr. Imre Bartfai

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Hi,

I think you have right saying "fundamental design flaw". It is your power
supply. The absolute maximum of Supply Voltage (D001) is 5.5V as described
in the recent datasheet. As I researched your problem, I was surprised
there are some newer PICs allowing up to 6.0V, but for the said type this
is not the case.

I hope this helps.

Regards,
Imre


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On Mon, 8 Oct 2001, Jonathan Luthje wrote:

{Quote hidden}

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2001\10\08@052021 by wzab

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On Mon, Oct 08, 2001 at 02:45:18PM +1000, Jonathan Luthje wrote:
> G'day all,
>     I have a question relating to the 16F84A processor - basically all I'm
> doing is switching on and off a couple of relays via a 1.6mA base current to
> a pair of BF246 NPN transistors. There are 3 pins (RB0,1,2) set to output -
> the rest are set to input, some tied high, some tied low, all via 2k0
> resistors, all unused pins are tied together, set to input and tied to
> ground via a 2k0 resistor. I have regulated and filtered 6vdc supply rails
> powering the thing.

Probably, when the relay gets switched off the spike of high voltage is
generated on the NPNs collector, which then breaks down the CB junction and
further propagates to the PIC's output (BTW what is the resistor between PIC
output and the NPNs base?).
Add the diode parallelly to the relay's coil (anode to the NPNs collector,
cathode to the Vcc), it should fix the problem.

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2001\10\08@052849 by wzab

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On Mon, Oct 08, 2001 at 11:19:43AM +0200, Wojciech Zabolotny wrote:
>
> Probably, when the relay gets switched off the spike of high voltage is
> generated on the NPNs collector, which then breaks down the CB junction and
> further propagates to the PIC's output (BTW what is the resistor between PIC
> output and the NPNs base?).
> Add the diode parallelly to the relay's coil (anode to the NPNs collector,
> cathode to the Vcc), it should fix the problem.

Ooops... sorry I've sent the message before I thought carefully. The effect
described above should not lead to PIC destruction, because the high voltage
spike will be shorted to the GND by BE junction :-(. But anyway you need the
protection diodes to avoid NPN destruction.
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2001\10\08@211529 by Dwayne Reid

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At 02:45 PM 10/8/01 +1000, Jonathan Luthje wrote:
>G'day all,
>     I have a question relating to the 16F84A processor - basically all I'm
>doing is switching on and off a couple of relays via a 1.6mA base current to
>a pair of BF246 NPN transistors. There are 3 pins (RB0,1,2) set to output -
>the rest are set to input, some tied high, some tied low, all via 2k0
>resistors, all unused pins are tied together, set to input and tied to
>ground via a 2k0 resistor. I have regulated and filtered 6vdc supply rails
>powering the thing.
>
>The circuit and program were working beautifully up until the time where the
>circuit "browned out" and caused the PIC to get very hot (hot enough to put
>a blister on my thumb - ouch). I've since installed a reset-on-brownout
>circuit as given in the 16F84A datasheet. This is PIC #3 that this has
>happened to ... it's starting to get a little expensive! :)
>
>Is there some fundamental design flaw I'm making here?

One possibility: separate all the unused pins and turn them into
outputs.  I can imagine a situation where one port pin tries to go LO and
another tries to go HI.

Its a long shot but is easy to try.

dwayne



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2001\10\09@222944 by myke predko

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Jonathan,

Could you please show us the circuit that you are using?

I'm interested in seeing what the circuit is (including relay power),
because I can't imagine what the problem is.

myke


At 02:45 PM 10/8/01 +1000, Jonathan Luthje wrote:
>G'day all,
>     I have a question relating to the 16F84A processor - basically all I'm
>doing is switching on and off a couple of relays via a 1.6mA base current
to
>a pair of BF246 NPN transistors. There are 3 pins (RB0,1,2) set to output -
>the rest are set to input, some tied high, some tied low, all via 2k0
>resistors, all unused pins are tied together, set to input and tied to
>ground via a 2k0 resistor. I have regulated and filtered 6vdc supply rails
>powering the thing.
>
>The circuit and program were working beautifully up until the time where
the
>circuit "browned out" and caused the PIC to get very hot (hot enough to put
>a blister on my thumb - ouch). I've since installed a reset-on-brownout
>circuit as given in the 16F84A datasheet. This is PIC #3 that this has
>happened to ... it's starting to get a little expensive! :)
>
>Is there some fundamental design flaw I'm making here?

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2001\10\10@012949 by Ned Konz

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>  At 02:45 PM 10/8/01 +1000, Jonathan Luthje wrote:
> >G'day all,
> >     I have a question relating to the 16F84A processor - basically all
> > I'm doing is switching on and off a couple of relays via a 1.6mA base
> > current to
> >a pair of BF246 NPN transistors. There are 3 pins (RB0,1,2) set to output
> > - the rest are set to input, some tied high, some tied low, all via 2k0
> > resistors, all unused pins are tied together, set to input and tied to
> > ground via a 2k0 resistor. I have regulated and filtered 6vdc supply
> > rails powering the thing.

I tried to look up BF246 but all I could find was a N-channel JFET of the
same name. I couldn't imagine that you were using a JFET to run relays with...

Anyway, have you tested the transistors for shorts when the problem happens?

Do you have diodes or snubbers across your relay coils?

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