Go Down

Topic: Stall Current / MOSFET / Transistor selection (Read 897 times) previous topic - next topic

tocpcs

Jan 06, 2013, 11:05 am Last Edit: Jan 08, 2013, 12:07 pm by tocpcs Reason: 1
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 ?


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.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dc42

PS - one way of reducing DC motor start-up current is to start them using gradually-increasing PWM.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Docedison

Or a Ballast resistor...

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

tocpcs


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.

LarryD

12V/1.8 ohms = 6.67 amps
Charging, 14.7V/1.8 ohms = 8.17A
The way you have it in your schematic isn't the same as how you have it wired up!

tocpcs

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..

Go Up