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DPRG: GARP motor controller design

Subject: DPRG: GARP motor controller design
From: Eric Yundt eric at sssi.com
Date: Tue Jun 17 11:14:21 CDT 1997

Larry, GARPians:

Larry Kerns wrote:
> 
> James,
> Do you know what the stall current is on the wheelchair motors?  As GARP
> gets heavier he will require more current to run the motors and I was
> curious if the 10A MOSFET H-bridge was enough or overkill.  Cost is not
> a problem since the cost per MOSFET from 5 amps to 46 amps varies only
> about 50 cents per device.  I would rather overkill the devices used and
> possibly avoid some heavy, space consuming heat sinking than to come up
> short.  We should probably consider the inductive "kick-back" from the
> motors also.
> 

That's what I was thinking also...

Here's a nice circuit I came across that I'm about to try on YaTu.
With an IRFZ44 Power Mosfet, this should switch 35 Amps.  Supposedly
a TO-3 heat sink works well for continuous 25 Amp loads with a fan or
bigger heat sink needed for continuous 35 Amps.



                                               V+ (50V max)        V+
                                               .                   .
                                        1K     |                   |
                                    +--/\/\/---+                   |
                                    |          |                ------
                                    |          |                 Load
                                    |      +---+                ------
                                    |     /                        |
                             1K     |   |V_                        |
                        +---/\/\/---+---|   2N3906               |_|D
                        |               |\         100 Ohm    G| 
               10K    |/                  `----+---/\/\/---+---| |-. IRFZ44
   +5V=on  ---/\/\/---|  2N3904                |           |      _|
                      |\                1K     |           |     | |S
                        v         +----/\/\/---+           |       |
                        |         |                 100K   |       |
                        +---------+----------------/\/\/---+       |
                                  |                                |
                                  |                                |
                                 ===                              ===
                                  =                                =


I may not have drawn the power mosfet (IRFZ44) correctly, but the GDS
labels should be correct.
                         
Anyone who knows more about what I'm talking about than I do, have any
comments about this circuit?  I'm thinking about trying 4 of these in
an H-bridge configuration for controlling my main drive motor directly
>from logic pins and planning to do PWM through it.  I expect this to be
much better than relays both for fast switching (nanoseconds?) and
power efficiency.

- --
Eric
                                                          .__
Eric Yundt <eric at ytech.com>                       __ ---   ; \_      //    F
                                                  --- -   ("" a\,   // L  I
"HA!" YaTu laughs as he spits into the wind,       -- -    {\_""   // I  N
tryin' t'catch a fox makes da cow's head spin!    --  ooo__{ _}   // N  I
                                                      (O)---(O)  // E  S
YaTu at YTech.COM  --  http://www.dprg.org/yatu                    //    H

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