Noob Question:

[aerobyrd]

I have planned  an arduino mega which will be driving several motors, two of them are quite large (12v, 25A stall current) I don't need variable speed on these, though it must be reversible, just a simple, cost-efficient reversible on/off. If it matters, I will also be plugging in an adafruit motor shield for the other motors, which obviously won't handle these 2. how best to do this?

[johnwasser]

Some 12V relays and transistors to drive them might be the most cost-effective way of controlling the two big motors.

http://rockingeeks.org/index.php?option=com_k2&view=item&layout=item&id=3&Itemid=61

[winner10920]

Grab some car relays, they are quite cheap and usually rated pretty high

[aerobyrd]

that wouldn't allow for the motors to operate in 2 directions would it? also, would they just be plugged directly into the arduino?

[graynomad]

Any DPDT relay can do two directions, but do you need stop as well?

would they just be plugged directly into the arduino?

No, at this size you'd need a transistor to drive a relay.

______
Rob

[winner10920]

Definetly gonna need those transistors,  and youll need two but they are isually alot cheaper

[lemming]

It may be worth considering configuring some MosFETs in a H-bridge.

[johnwasser]

> that wouldn't allow for the motors to operate in 2 directions would it?

Yes, it would.  That's why I provided a pointer to an article about wiring two relays so that you can drive a motor in both directions.

> also, would they just be plugged directly into the arduino?

Not if the relay coil requires more than 5 volts or 35 milliamps.  I'd be surprised if you could find a 5V <35mA relay whose contacts could handle 12V 25A.  I just checked DigiKey and such relays only had contacts up to 16A.  That's why I said you would need transistors to drive them.  Using a transistor you could power the coil with the same 12V you were switching.  The coil current will be about 50 to 150 milliamps so you don't need a BIG transistor.  The relays should cost about $4 each.