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What would I use to variably limit current? PWM and a MOSFET?

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777funk

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I have a solenoid where I'd like to vary the current. I would guess since the solenoid's coil resistance is fixed that'd mean that I vary the voltage since:

V=IR

So what/how would I do this. I would guess it's going to be high current since the coils DC resistance is only 6 ohms.

Would PWM from a uC and a MOSFET be a good way to make this work?
 

Yes! Would be good current if your voltage are good enough. lets say you use 12V.
12V/6 ohms= 2A of current

but if you use 3 Volt (two Cells)
it would become 500mA.

Check for the maximum current that your solenoid can bear.

PWM is a good technique and would work, keeping in mind.
1) MOSFET should be connected in series.
2) Also Rds(on) of MOSFET should be very low (in mili ohms).
 

Chullaa said:
2) Also Rds(on) of MOSFET should be very low (in mili ohms).
Click to expand...

Great. Would I want a Logic level MOSFET since I'm using a uC as the PWM source?

One thing I'm not sure of is how sensitive the gate needs to be. I wouldn't want the MOSFET to be full on with each pulse, would I?
 

Yes, MOSFET work like a switching device, on and off. any MOSFET with good switching speech will work.
Gate should not be sensitive. All you have to provide voltage above Vgs(th) OR below Vgs(th).

If you are driving with uC then you may also need MOSFET driver. (which can be BJT or MOSFET as well)
 

Chullaa said:
If you are driving with uC then you may also need MOSFET driver. (which can be BJT or MOSFET as well)
Click to expand...

This is why I was thinking a low Vgs would be good. Some need 12V to fully conduct and logic level MOSFETs only need maybe 3-4 V to be fully on.

I'm guessing (hopefully) this would eliminate the need for a driver?
 

777funk said:
This is why I was thinking a low Vgs would be good. Some need 12V to fully conduct and logic level MOSFETs only need maybe 3-4 V to be fully on.

I'm guessing (hopefully) this would eliminate the need for a driver?
Click to expand...

Yes generally Vgs(on) is around 3 4 Volts. Try connecting gate directly uC to operate gate. If it doesn't work it is because of uC cant provide sufficient current to gate. Use opto-coupler or mosfet driver between them .
 

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