Re: Driving a PNP Darlington transistor from a PIC16F877A

On Thu, 31 Jan 2008 04:39:25 -0800 (PST), scouselad <allanlewis99@xxxxxxxxxxxxxx> wrote:

Hi,

I'm designing an electronically-controlled light dimmer circuit based
on one I found at: http://www.edn.com/article/CA46649.html
I'm replacing the SG3524 PWM chip with a PIC16F877A and I intend to
take a digital input from the user and (scale it, then) compare it
with the 0-5V from the op-amp (digitised using the PIC's ADC). As you
can see from the diagram, the PWM signal drives a PNP Darlington
transistor*. However, the SG3524 can provide 100mA of current, whereas
the PWM pin on the PIC can only provide 25mA. (Note that this current
is not only injected into the base; it also feeds through to R1.) So,
do I need some sort of driver between the PIC and the transistor? If
so, what sort of circuit would I need?

*Incidentally, the model used by EDN is no longer available so I
intend to replace it with a BD678 - can anyone advise me if this is a
suitable replacement?

I would hesitate to say the BD678 is suitable. The data *** I saw gives
almost no info on it's switching speed. I'd look around for a darlington
designed for switching applications, or maybe a power mosfet like an IRF9530.
As has been pointed out already, You'll need a level shifter anyway.

Mike


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.