Alt-BEAM Archive

Message #07945



To: "'beam@sgiblab.sgi.com'" beam@sgiblab.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Tue, 23 Nov 1999 13:34:02 -0800
Subject: [alt-beam] Re: Parallel 3904's


No. Paralleling of FETs works because, when ON, they are "resistive" and
have a positive temperature coefficient. That means when one gets hotter
than the other it increases in resistance and forces more current through
the other FET, so that both naturally tend to share the current. Bipolar
transistors (2N3904 etc) used in parallel behave exactly the opposite way:
When one of the two overheats, it hogs all the current until it
self-destructs! You can add series resistors in each base and emitter but
that get rather complicated. Just use a higher current (Ic=500ma) device
like a 2N2222 or much higher current like a 2N3055 (Ic=15A) but be sure you
increase the available base current accordingly. Give us an example of an
application for higher current transistors and we can walk through the
simple design procedure for selecting the required components.

regards

Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411

> -----Original Message-----
> From: Craig Limber [SMTP:craig.limber@west.gecems.com]
> Sent: Tuesday, November 23, 1999 9:42 AM
> To: beam@sgiblab.sgi.com
> Subject: Parallel 3904's
>
> Hi there;
>
> Forgive my newbiness, I am just getting into this....
>
> Would running transistors in parallel effectively lower the voltage drop
> across the transistors that run the motors? The reason I am asking is
> that
> for the speed controls in the electric model airplanes I build/fly the
> designers use multiple FETs in parallel to lower the effective resistance.
> I was wondering if this principal might be applicable/useful to these
> little
> robots to give them just a little more oomph.
>
> BONUS QUESTION:
>
> Does anyone know of any BEAM/Robotics events/contests/get-togethers that
> happen in the Calgary, Alberta, Canada area?
>
> Craig



7946 Tue, 23 Nov 1999 14:25:41 -0800 [alt-beam] Re: Surface mount tutorial beam@sgiblab.sgi.com Greg Powell
>> I was wondering where to !get! surface mounted components. I'm also into
>> the "the-smaller-the-better" dept.

I am personaly going to buy surplus surface mount parts from The Electronic
Goldmine. They have packages of diodes, resistors, capacitors, LEDs etc. for
about $2.50 US. It is really cheap and I just want to experiment.

Cheers,
Greg
http://www.engr.uvic.ca/~gpowell/

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