> Van: Andrius Tamulis <atamulis@INDIANA.EDU>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: AN589 Programmer
> Datum: woensdag 10 juni 1998 18:25
>
> I am a PIC neophyte, trying to put together the AN589 programmer
Microchip
> tells you about. And it seems to not be working. Specifically, transistor
> Q3, the one that is supposed to pull \bar {MCLR} down, keeps burning out.
> Specifically, it's OK while D4/pin 6 is low; \bar {MCLR} is at
> programming voltage. Then I pull D4 high, which is supposed to pull \bar
> MCLR low, but \bar MCLR goes to about 11 volts, and transistor Q3 starts
> to overheat and occasionally smokes. (Well, each _individual_ transistor
> smokes only once, then I replace it, and its replacement smokes too).
>
> I see two possibilities:
>
> 1) It's my as-of-yet unregulated power supply. It's at 15 volts, and can
> go up to an amp (according to specs), so is that burning the transistor?
I
> was going to take care of this before plugging in a PIC, but will that
> solve this problem too? And is the solution in the form of an LM317
> regulator?
>
> 2) possibility 2; there's a bug in AN589 that no one has told me about.
> Anyone else had this problem?
>
> Andrius Tamulis

There is a third possibility though : Line D4 that pulls Q3 low is not
'hard' enough and is pulled up by way of Q1, Q2 and R2.  If that's what
happens both Q3 and Q1 are opened thus creating a path from VCC to GND.
That should create enough current to either blow Q1 or Q3.

By the way : You can throw away Q1, Q2, R1 and R2 because you only need to
have the MCLR pin at Zero and +12.5 volts.  +5 Volts is only required when
_running_.  And you don't want that to happen when programming, do you ?

Pull CLKIN (pin 16) to either GND or VCC. This way the PIC can't generate
clock-pulses, even when the MCLR pin changes too slow.

Greetz,
  Rudy Wieser