Re: Driving a PNP Darlington transistor from a PIC16F877A

scouselad 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?
If you are designing from scratch, consider low side switching (i.e. +ve supply -- Load -- Switching device -- ground (-ve supply).

Does away with the need for level shifters and NPN power devices are
somewhat more common. You could also easily drop in a power mosfet.

Alternatively, look at automotive lamp control ICs, High side switching and protection against lamp faults.

Whatever you use, it should be seriously overrated to drive a lamp load.
If you dont allow for lamps taking typically 10 times the current during startup and failing often with a momentary short as the filament blows your expensive semiconducter *will* fail protecting your cheap fuse ;-(


--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:
.

Relevant Pages