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Topic: MOSFET for DC motors (Read 2 times) previous topic - next topic

tocpcs

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.pdf

I 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.png

The 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...

MarkT

150 ohm resistor - limit the Arduino pin current to below 40mA.

I doubt the fans pull anything like 50A continuous, but a heatsink for the MOSFET would be a good idea.

If you use that transistor level shifter then R1 is too high at 10k - use 560 ohms.

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tocpcs

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?  :~

LarryD

Quote
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


5V - .7V(Vbe drop) / 30ma = 143.3 ohms
Lets say that HFE DC gain is 30 then .03A X 30 = 900ma maximum
if we use Rb=1K, Ic= 4.3/1000=4.30mA,  Ic=4.30mA X 30 = 129ma max
however 12V - .2Vce sat. /1k = 11.8ma will actually flow. Therefore the transistor is saturated and thus the FET will be turned off.
The way you have it in your schematic isn't the same as how you have it wired up!

MarkT


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?  :~



You want to turn off the MOSFETs fast - the R1 10k pull up will take 250us to charge up the 26nF worst-case input capacitance - at high
current levels that 250us could mean POP!  Blown MOSFET.  You musn't leave MOSFETs switching high powers so long in the linear region,
the dissipation is potentially huge (a sizeable fraction of 12V x 50A here, worst case...).   If PWM'ing the MOSFET you will need a high
current gate driver to keep the average dissipation sensible - switching losses can easily dominate.
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