Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unmvax!ncar!stout.ucar.edu!cook
From: cook@stout.ucar.edu (Forrest Cook)
Newsgroups: sci.electronics
Subject: High current semiconductors
Message-ID: <3292@ncar.ucar.edu>
Date: 25 May 89 18:02:46 GMT
Sender: news@ncar.ucar.edu
Reply-To: cook@stout.ucar.edu (Forrest Cook)
Distribution: usa
Organization: Field Observing Facility, NCAR, Boulder, CO
Lines: 45


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.

The current circuit is as follows:
- Two 12V deep cycle batteries
- Two heavy duty DC motors
- A tricky mechanical relay setup that can run the motors in the various
series/paralle configurations, yielding 4 speeds
- a solenoid operated disc brake system

The problem is that 4 speeds are not enough for smooth control in tight areas.

I plan to build a 68HC11 micro based pulse width modulator that can
read the position of a spring loaded speed control pot and control
some kind of solid state switch that would drive the motor.
The pulse frequency will probably be at least a few hundred herz,
depending on the switch used, I suppose.

Now for the questions:

I have heard of devices called auto-commutating SCRs that can be used
to switch high current DC loads on AND OFF.  Would they do the trick?
How about a bunch of power MOSFETs in parallel with small source resistors
to balance the load?

Does anybody make a solid state relay that could hold up to this type of use?

The goal is to keep the switch resistance to a minimum
and the device as simple as possible.

Any suggestions of device types, manufacturers, and/or part numbers
would be much appreciated.

 ^   ^  Forrest Cook - Beware of programmers who carry screwdrivers - LB
/|\ /|\ cook@stout.ucar.edu Work:(303) 497-8799  Home:(303) 499-6333
/|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook
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 ^   ^  Forrest Cook - Beware of programmers who carry screwdrivers - LB
/|\ /|\ cook@stout.ucar.edu (The preceeding was all my OPINION)
/|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook
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