Stall Current / MOSFET / Transistor selection

I'm looking to use an arduino to control fans.

I'd like to do better than a simple relay, but there's also the issue of stall current. I haven't tackled anything to do with stall current before, but reading this thread: http://arduino.cc/forum/index.php/topic,5325.0.html

Made me think that those car relays are typically rated for around 30A. It's already got a controller in it that appears to operate them at a medium.

What do typical designs do for start up / stall current? The relays that are in there appear to not have failed in the many years it's been running. Perhaps it's the case the fans combined do not draw > 30 A at start up ?

What sort of fans are they? If they use brushed DC motors, you can work out the stall current by measuring their resistance and dividing it into the supply voltage. Measure the resistance several times, rotating the shaft a little between readings, and take the lowest resistance.

PS - one way of reducing DC motor start-up current is to start them using gradually-increasing PWM.

Or a Ballast resistor...

Bob

dc42: What sort of fans are they? If they use brushed DC motors, you can work out the stall current by measuring their resistance and dividing it into the supply voltage. Measure the resistance several times, rotating the shaft a little between readings, and take the lowest resistance.

I think they are brushed DC - typical radiator fan motors.

The resistance differs between them - the measurement I took was 1.8ohms and 2.6ohms

So, taking your calculation, 12 (lowest supply voltage) / 1.8 ohms = 0.15A ? 12 / 2.6 = 0.21A

That seems stupidly low. They have a 50A supply fuse on them.

12V/1.8 ohms = 6.67 amps Charging, 14.7V/1.8 ohms = 8.17A

They still seem very low and this is where I'm struggling - I want to pick a MOSFET to drive them. The fuse is 50A. If the stall current is supposed to be '3 - 8x' the operating current, then 8Amps is very low.

My DMM is fused only for 10A - a 50A fuse suggests I shouldn't measure the current..