Alt-BEAM Archive
Message #07944
To: "beam@sgiblab.sgi.com" beam@sgiblab.sgi.com
From: "Dan Larson" dlarson@citilink.com
Date: Tue, 23 Nov 1999 15:31:24 -0600
Subject: [alt-beam] Re: Parallel 3904's
On Tue, 23 Nov 1999 15:21:45 -0500, Jim Taylor wrote:
>
>James Taylor
>URL: http://fly.to/springmeadows
>
>----- Original Message -----
>From: Craig Limber
>To:
>Sent: Tuesday, November 23, 1999 12:41 PM
>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 think we do this more to handle more ampers in the ESC.
>
FWIW,
I wouldn't do this *without* adding a resistor to the emitter of *each*
bipolar transistor because bipolars conduct *more* as they heat up. The
strongest transistor will draw more current, heat up, draw even more
current, heat up even more.... thermal meltdown of sorts. The resistor
is to limit the current of each transistor so this doesn't happen.
Learn about transistors and ohms law then calculate R = (Vb - Vbe) / I,
where I = even fraction of of total I desired through motor. Experiment
with large values and work your way down until it works, if you must do it
by trial and error. Try about 43 ohms on the emitter of each transistor
for a max of 100ma per transistor, assuming a Vb of ~5V.
Better yet, use a transistor with a lower Vcesat and higher max current, like
the 2N4401 or PN2222A...
It works with FETS because they conduct less as they heat up, not more
like bipolars do, and won't melt down.
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