Alt-BEAM Archive
Message #00691
To: Justin jaf60@student.canterbury.ac.nz
From: Bob Shannon bshannon@tiac.net
Date: Sat, 20 Feb 1999 10:58:56 -0800
Subject: [alt-beam] Re: dumb questions
Justin wrote:
>
> I'm contemplating a design that involves two motor h-bridges getting
> their +ve feed from a single BC327 transistor, which is probably going
> to exceed that transistor's limits on a regular basis. What can I do to
> avoid this?
> The obvious answer is to use a bigger transistor, but I don't how to
> determine which higher-current transistors are functionally identical.
>
> Can anyone tell me how to find this out?
> Failing that, can anyone name a suitable replacement transitor?
>
> BTW - I'm curious - can you wire two transistors in parrallel to allow
> higher-current use?
Yes, but you need to add a resistor per parallel transistor. This will
be
a very low value resistor, less than 10 ohms usually.
The reason for using this transistor is because no two transistors are
perfectly
matched, there will be some small variation. when the are connected in
parallel this small variation will cause one transistor to turn on
harder than the other and draw the lion's share of the current. this
eliminates most of the current capability advantate of using parallel
transistors.
We solve this by placing small value resistors in series with the
emitters of each parallel transistor. IF one transistor turns on a bit
harder, its emitter resistor drops more voltage and equalizes the total
current.
If you parallel connect two 2n2222 style transistors with 1 ohm series
resistors on the emitters, you can draw over half an amp without
problems.
But its usually easier to select a single larger transistor and save
space, weight and those pesky interconnections. You need a selection
guide for the parts available. Find a transistor with the same gain,
but a higher maximum collector current rating, and try it out.
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