# How to choose a MOSFET

Discussion in 'General Electronics Chat' started by zacaj, Mar 3, 2013.

1. ### zacaj Thread Starter New Member

Mar 3, 2013
1
0
I'm trying to drive a solenoid I took off an old pinball machine with an arduino.
The pinball machine ran on 25V AC, and I've found that the solenoid was 2Ω online (so 12.5A). The arduino puts out 5V@40mA. After some googling, it seems I want to use a MOSFET with a logic level gate to do this, however my electronics knowledge isn't nearly enough to understand most of what's going on. I'm looking at different MOSFETs on Digikey, and it gives me choices for a lot of stuff. Some of them I *think* I've figured out, and others I still have no idea about:
FET Type: I'm not sure what the difference is between N and P channel
Drain to Source Voltate (Vdss): I'm assuming this is the max voltage it can handle going through it, so I should be finding a MOSFET that will support 25V+?
Current - Continuous Drain (Id): Assuming this is the max amperage going through it, so looking for one with 12.5A+
Vgs(th) (Max): I think this has something to do with the activation voltage applied to the gate that will make it activate, so I need one with less than 5V?
Power - Max: Assuming this is the max power it can handle. I've calculated the power the solenoid would need as P=V*I=25V*12.5A=312.5W, so I need a MOSFET that can handle more than 312.5W?
I don't know what Rds On (Max), Gate Charge (Qg), or Input Capacitance (Ciss) mean. Are they important for my uses?

Based on searching with these numbers, the cheapest MOSFET returned was http://www.digikey.com/product-detail/en/HUF75345P3/HUF75345P3-ND/965373 . Would this work?

Thanks

2. ### Wendy Moderator

Mar 24, 2008
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Welcome to AAC!

I found this thread moderated. I have approved it. It is a frequent problem for new posters (as in 1st time).

3. ### crutschow Expert

Mar 14, 2008
13,475
3,361
If you want to operate the solenoid from AC then you will need a TRIAC. If you will operate it from DC (at reduced voltage) then you can use a MOSFET.

On AC the solenoid likely takes a lot less than 12.5A due to its inductive impedance.

The power dissipated in a MOSFET when used as a switch is simple its ON resistance times the square of the current through it, so that would be a lot less than 312.5W. The gate charge and capacitance are only important if you want to switch at a high rate (several tens of killoHertz).

4. ### John P AAC Fanatic!

Oct 14, 2008
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A triac sounds right to me. But if you use any kind of semiconductor, be aware of the ground level of the AC system relative to the microprocessor. If they're totally separate, you'll need to run 2 wires, your triac gate plus a return. If they're joined, you'll have to work with what's already there, and it may not be ideal. Even at low voltage, a solid-state relay should work, and it would be the most foolproof way to control AC from DC.

shortbus likes this.
5. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
Keep in mind you will also need a 24-25 vac transformer which can supply the required current.