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 61 Using Arduino / Motors, Mechanics, and Power / Re: MOSFET for DC motors on: January 09, 2013, 07:26:50 am Thanks for the reply but I'm not sure on your values. 150 ohm to limit below 40mA, so 5 / 0.150 = 33mA - OK!What happens with R1 - in the linked circuit it's a pull up resistor right? So a high value is intended to keep current low so that the transistor grounding wins - or have I got how that works exactly wrong?
 62 Using Arduino / Motors, Mechanics, and Power / MOSFET for DC motors on: January 09, 2013, 05:33:18 am I've got two automotive fans I'm wanting to drive from an arduino.The fuse that supplies both of them is 50Amp.So if I take that as the maximum both will draw, I plan to split the fans into two using MOSFETs.I'm looking at the datasheet for the IRF1405 - and I think this will do, but I'm just tossing up whether I need to add a transistor and drive it using battery voltage or direct from the pins.The datasheet is here: http://www.jaycar.com.au/products_uploaded/zt-2468.pdfI think I need a current limiting resistor between the arduino pin and the transistor - but how do I size that resistor?See image for the circuit I'm planning to use - any problems / thoughts?http://www.neufeld.newton.ks.us/files/electronics/mosfet-motor/MOSFET-motor-driver.pngThe MOSFET is different in the schematic linked, so I wonder if the resistor values change compared to the jaycar one I linked to? (it can handle higher current so should be ideal with a transistor).Also, the datasheet shows the gate threshold min max as 2 and 4 volts. Is 2 the off value and 4 the 'barely on' value - and 12 the 'it's full on' value?Using the transistor (2n3904) - I'll want to get 4 - 12V modulating...
 63 Using Arduino / General Electronics / Re: Stall Current on: January 08, 2013, 01:21:16 am 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..
 64 Using Arduino / General Electronics / Re: Stall Current on: January 08, 2013, 01:07:07 am Quote from: dc42 on January 06, 2013, 06:35:13 amWhat 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 2.6ohmsSo, 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.
 65 Using Arduino / General Electronics / Stall Current / MOSFET / Transistor selection on: January 06, 2013, 05:05:44 am 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.htmlMade 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 ?