Burning some more PIC16F84A - mystery solved

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Hi All,

For those who read my previous post about burning PIC16F84A chips. I was
working on a very simple circuit, driving two leds in series with 470 ohm
resistors and I managed to blow another chip. Obviously, I suspected the lab
power supply. I put the oscilloscope on its output and the voltage was
perfect, no ripple, no noise, no anything. But in the moment I decided to
turn it on and off, I noticed some very short spikes of voltage that get
toward 30V, if not more (I actually suspect they get up to the maximum
voltage of the transformer which is close to 40V). I was trying to be a
"good boy" and to protect the chips by turning the power supply on and off
while inserting/removing the IC in circuit and this was actually what was
destroying them... It cost me a couple of microcontrollers to discover my
lab power supply has a transient fault, but at least there are no more
mysteries... I used this lab power supply very often for a few years, with
other boards with Zilog microcontrollers, but I ignored the fact those
boards had their own regulator on board and this is why none of them blew
up...
Thanks to all who helped me with ideas/suggestions.

Regards,
Nicolae



Re: Burning some more PIC16F84A - mystery solved


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Now a solution is very simple!

Just add a voltage regulator in the circuit, which is always a good
practice.
Also if you use a bread board, make this and some other protective circuits
a standard on the board.
My board makes some voltages and has some protection because of an external
circuit I designed for it.
I can choose the folowing regulated voltage 3.3V, 5V, 12V, 15V all Positve
and Negative execpt 3.3V and 1 Amp, +5V also has 3Amp.
I can also add my Lab PSU it's a HP6634A which is (almost) perfect, in
ripple protection and calibration.

Good luck with no more blown chips!

Alexander



Re: Burning some more PIC16F84A - mystery solved


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Thanks Alexander,
I was trying to minimise my work, because it was just an educational board,
unfortunately the most I learnt is what not to do in the future :-)
I found the experience useful, although costly in both time and money.
However, that was only the beginning, now I run into other problems, for
which I will post another questions.

Regards,
Nicolae



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