> Russell McMahon wrote:
>
> > A BETTER WAY .................
> >
> > Using separate TO126 or TO220 power transistors with a modest heatsink
would
> > work much better.
> > As normal transistors will not be able to be driven directly by a
processor
> > use of a darlington would be better eg
>
>
> Just to be argumentative, I always use bipolar power
> transistors directly driven by PIC pin, when I can.

I'll see your argument and raise you a clarification :-)
It's all about Beta (current gain) and how much current you want to switch.
If Beta = eg 100 you need 1 mA of base drive to switch 100 mA of collector
current.

A good everyday small signal transistor such as eg a BC337 has a Beta of
typically 300 and a maximum rated current of 500 mA. This means that you can
drive it with 500/300 = 1.6 mA. Allowing 5 or 10 mA would be wise - a PIC
will handle this easily.

Going to higher currents the sitiuation changes.
You can either buy the utterly superb products from Zetex who shoehorn 3 amp
transistors into an ELine TO92 like package with betas of 100 plus
OR
You can largely buy standard TO220 transistors with Betas of typically 25 to
50 AT HIGH CURRENTS.
Getting good betas at 10's of mA is easy but as soon as you go to currents
of a few amps (as is the case here) beta will usually fall off markedly.

It just happens that I have been Beta testing transistors a few days ago to
see how much bang for the buck I can get. If this was a one off I would use
Zetex parts and be very happy. As it's for a volume task $$$$'s count.

I am using PNP transistors which are generally slightly inferior to NPN but
the same general lessons apply.
With a forced Beta of about 30 (Collector current divided by base current =
30) I get saturations around 0.2 volts using a TIP30 (BD640 equivalent).
With a forced Beta of 100 saturation varies from not much more than this to
as much as 5 volts !!!! :-)
The difference is in the static beta of each transistor. Selecting onntest a
transistor with a "meter beta" (ie measured using a standard meter beta test
facility) of 50 will need a forced beta of 10 or 20 at 1 amp. A transistor
with a meter-beta of 100+ will be OK at 20 or 30 forced beta.
YMMV (nay, YMWV).
With a max PIC current of 20 mA and a forced Beta of even 30 the best
current you can supp[ort is 600 mA! (20*30)
To support 3 amps you ned a beta of 3000/20 = 150.
Reach for the Zetex transistors quickly !!!!!!!!

The BD203 you mention (slightly higher current rating than a TIP41) has a
specified minimum Beta of 30 at 3 amps.

A FET can be the cure for this.
to get a Vsat of 0.2 volts at 3amps you need an Rdson of 0.2/3 = 0.066 ohms.
This is easily achieved at quite low cost. (Under $US1 in 100's for a 14 amp
part with an Rdson of less than this).
To run this from a PIC pin you will need a logic FET.

A Darlington has beta's of typically 200 to 1000 at 3 amps.
With 20 mA drive this gives 20 * 200 = 4 amps at a beta of 200.

> Try BD203,  TO220 pack, (cost 50c)
> = 1A, 0.2v C-E, base current 15mA.
> (some TIP series are just as good and cheap)
>
> There are good cheap low-sat bipolars around, you
> just know where to look! A darlington will get hot
> with 1A continuous @ about 0.9 watt, a good low sat
> transistor only 0.2 watt and stome cold. :o)
> -Roman
>
> PS. Also with the BD203, I measured 19mV C-E saturation
> with a 30mA load! That's almost a relay! :o)

The 3 is a good transistor but is still only as good as its sepcification.
Measure it at 3 amps Ic  tell us what you get.
Also do it at different base drive levels.
Try it with a PIC driving the base directly.
Even with NO resistor twixt base and PIC I reckon you won't get much more
than 1 amp collector curreent at any sort of reasonable saturation voltage.


regards


                Russell McMahon

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu