Apptech wrote:
>>>> R1 1K 1/4W
>>>> R2 10K
>>>> R3 6K8
>>>> NPN: 2N/MMBT4401
>>>> PNP: 2N/MMBT4403
>
> I took it to mean (possibly wrongly) that the PNP needed to
> be in saturation or close to it.

Yes, that's exactly what I meant.

> Without looking back I don't know if the 250 mA was
> specified when those resistor values were given.

It was specified by the OP.

> As will be
> seen below, the circuit is in trouble as shown at 250 mA
> with that transistor and resistors.

No, you flipped R2 and R1 apparently.  The base current is 12V accross R1
(1Kohm) minus a B-E drop and saturated C-E drop.  Figuring 750mV for B-E
drop of Q2 and 200mV for the saturated on voltage of Q1, that gives a base
current of (12V - 750mV - 200mV) / 1Kohm = 11V / 1Kohm = 11mA.  At 250mA max
load current, Q2 must have a gain of at least 250mZ / 11mA = 23, which is
quite safe for a MMBT4403 at 250mA.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist