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[Reply #15 on:]

For a homemade H Bridge of that calibre, you could do it with 2x SPDT relays. If you need PWM control, you need 1 MOSFET between the relays and power source.

This is an interesting option I hadn't considered. Mostly because I have never used relays. Is there anything to really know about them? Working with an relay seems kind of not ideal because the coil in the relay is going to suck a lot of current isn't it?


Also, I found this:

http://www.openservo.com/moin.cgi/Schematic2?action=AttachFile&do=view&target=OpenServo_2.0_032706.pdf

Is this a good approach/ alternative to designing a motor controller? I found it in another thread on these forums and it seems to get around a MOS buffer (or something, I still kind of a wet match in a dark room here). Obviously not using these MOSFETs and I don't really care about the current sensing portion either, just the H bridge (-like) part.

[Reply #16 on:]

because the coil in the relay is going to suck a lot of current isn't it?

Depends on the relay but 20 to 100 mA is not a lot.

[Reply #17 on:]

You do appear to need 2 different PWM signals to the board, and what are the parameters of the current
and voltage signal out? Unfortunately, they rarely seem to have much documentation.

Pretty much standard H-bridge operation. Typical h-bridges either pwm individual foward/reverse lines, or have a single pwm pin and a single foward/reverse direction pin.

[Reply #18 on:]

because the coil in the relay is going to suck a lot of current isn't it?

Depends on the relay but 20 to 100 mA is not a lot.

Are these typical relay currents? I'm looking at automotive relays because they seem to be whats needed to go up to 15 A (Most seem to be 30 or 40 A).  I am making this up as a go with part selection. Also with regard to controlling a relay, I'm guessing I just connect it to my 12V lead acid battery and switch it with like a 2N2222. Do I need some kind of resistor? Or is the coil resistor typically enough to get the current down? They seem to show coil resistances of like 10-50 Ohm, so I'm kind of raising an eyebrow to this 20-100mA range.

[Reply #19 on:]

If you are considering using relays, you might instead get two switches like below (one for each motor).

http://www.radioshack.com/product/index.jsp?productId=2062530

[Reply #20 on:]

If you are considering using relays, you might instead get two switches like below (one for each motor).

http://www.radioshack.com/product/index.jsp?productId=2062530

Lol, I'm trying to build a robot, I want to electronically control my motors, not physically switch them.

[Reply #21 on:]

Here is an interesting relay that is really two independent relays in one package. 25 amp contact rating, dual SPDT contacts and 53 ma for each coil current. so in this one package a arduino using two digital outputs controlling two small NPN transistors to power the coils, one could program control to go forward, reverse, and break. Add a power N-channel mosfet in the negative side of the external DC power supply for the motor and you can add variable speed via an additional arduino pwm output pin. Lot cheaper and smaller then a 25 amp H-drive. And compared to a solid-state H-Drive you only give up a little on the speed with which you can switch from forward to reverse, which for a robot main drive is probably not an issue? I suspect this is used in electric window motor drive, seat control motors, door lock solenoid, etc.

http://www.allelectronics.com/make-a-store/item/RLY-547/DUAL-25-AMP-RELAY-12-VDC-COIL/1.html

Datasheet:

http://www.worldproducts.com/pdfs/ep2.pdf

So high current H-drive on a budget, what's not to love about that. That lets you put more $$$ into sensors and other cool stuff. I may have to pick up a couple of these.

Lefty

[Reply #22 on:]

http://www.pololu.com/catalog/product/708

14A without a heat-sink.  A bit spendy, but no design or testing struggles.