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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: High current semiconductors
Message-ID: <5170034@hplsla.HP.COM>
Date: 30 May 89 16:23:39 GMT
References: <3292@ncar.ucar.edu>
Organization: HP Lake Stevens, WA
Lines: 24

cook@stout.ucar.edu (Forrest Cook) writes:

> Does anybody have any suggestions for a solid state motor control
> that can switch the motor on a fairly heavy wheelchair?
> 
> The chair consumes around 15 Amps at 24VDC during normal use.
> I think the startup current is much higher, probably around 100 Amps.

As someone else suggested, how about a Germanium power transistor?

Or, how about some power mosfets?  Digi-key lists an IRFZ40 for 
$5.88, or $35.28 for 10.
  - 50 volts VDS
  - 0.028 ohms on-state resistance (2.8 volts at 100 amps!)
  - 32 amps continuous at 100 deg. C case temp
  - 51 amps at 29 deg. C
  - 160 amps max pulsed
  - 125 watts max power dissipation

Seems like these come close enough to your requirements that you
could use a single one directly rather than trying to parallel them.
In fact, if you wanted to parallel them, you probably should not even
worry about current equalizing resistors, if you are driving them with
a PWM.