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[on:]

I'm trying to control a drill motor with Arduino.  I either need
a) a good DPDT to control 12v/20A circuit w/o frying Arduino or motor, or
b) a recommendation on how else to reverse polarity w/ electronics.  Any ideas?  Thanks.

[Reply #1 on:]

That 20 amp current requirement is going to be a costly H-drive controller and needed if you require both bidirectional and variable speed control. If variable speed not required, then relays will work. Even with relays you need to consider three states, not just two, clockwise rotation, counterclock rotation and motor stop. One DPDT relay will not satisfy all three states, even if you can find relays with 20amp DC contact ratings. So one DPDT relay to control direction and one SPST relay required for on/off control.

 Note that DC current switching is harder on relay contacts then AC switching as AC crosses through 0 voltage twice every cycle, DC does not and causes worst arcing wear on contacts. So if you do go the relay method be sure those contacts are rated for switching 20 amps DC, or higher.

Lefty

[Reply #2 on:]

Big relay like this one
3 pole double throw
http://www.mpja.com/prodinfo.asp?number=18255+RL
http://www.mpja.com/download/18255rl.pdf

[Reply #3 on:]

Big relay like this one
3 pole double throw
http://www.mpja.com/prodinfo.asp?number=18255+RL
http://www.mpja.com/download/18255rl.pdf

I assumed he would want relays using coil voltage the same as the motor voltage for simplicity sake. Even if not one still requires a second relay to stop the motor.

[Reply #4 on:]

Good call Lefty.
Maybe 4 of these then, wired up like the transistors in an H-bridge?
(which I am sure I created a drawing of somewhere)
http://www.mpja.com/prodinfo.asp?number=17155+RL

Yes, how about this with the part above in place of the relays conceptually?

Rats, thats 12VDC isn't - need a different relay still.
Here we go: lots of amps DC,
http://www.mpja.com/prodinfo.asp?number=17289+RL

[Reply #5 on:]

Good call Lefty.
Maybe 4 of these then, wired up like the transistors in an H-bridge?
(which I am sure I created a drawing of somewhere)
http://www.mpja.com/prodinfo.asp?number=17155+RL

No, just two relays are required, one DPDT to handle the polarity reversal and one SPST to remove DC voltage for the stop condition. I don't have a drawing program presently, but the old X wiring across the NO and NC contacts and the motor wired to the two common contacts and power applied to one NO and one NC contact. The SPST relay contact would just be in series with the batteries positive or negative lead.

Lefty

[Reply #6 on:]

Using four SP relays is not a safe thing because you can't depend on all of them to have exactly the same switching times. If one relay is "slow" you could end up shorting the power supply.  Much safer to use a DPDT relay to control direction and a SPST relay (or transistor) to control ON/OFF (or speed if using a transistor and PWM).

Actually, it would be best to use two SPDT relays, with the lever of each connected to the motor, and the NO contacts connected to +V and the NC contacts connected to GND. This way, you could switch either one to control direction, and both off (or on) for brake. A couple of Bosch 20A 12VDC automotive relays would fit the bill perfectly, and can probably be found fairly easily at a junkyard for a few dollars each. They are designed for these kinds of loads (ie, windshield wipers, pumps, windows, A/C fans, etc), and so are engineered to take the abuse. Even brand-new they aren't that expensive.

[Reply #7 on:]

...
Actually, it would be best to use two SPDT relays, with the lever of each connected to the motor, and the NO contacts connected to +V and the NC contacts connected to GND. This way, you could switch either one to control direction, and both off (or on) for brake.

Yes, this is what I was going to suggest. 

At up to 10 amps, these are a good $12.50 solution....  http://arduino-direct.com/sunshop/index.php?l=product_detail&p=155  DISCLAIMER: Ones I found for my shop!