cook@stout.ucar.edu (Forrest Cook) (05/26/89)
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
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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
/|\ /|\ {uunet|ucbvax|allegra|cbosgd}!nbires!ncar!stout!cookmmm@cup.portal.com (Mark Robert Thorson) (05/27/89)
You might want to look at the products from a company called GERMANIUM POWER DEVICES. They are the only remaining vendor of germanium transistors. A germanium transistor has a lower Vce(sat) than silicon, so the heat generated is lower. I don't have the address handy, but will post it after the next issue of Electronic Design arrives.
tomb@hplsla.HP.COM (Tom Bruhns) (05/30/89)
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.