# Selecting a MOSFET for driving load from logic

I'm looking to drive a magnetic door lock from an Arduino. I've found a question about driving a solenoid from an Arduino, which includes a circuit that looks perfect for this kind of situtation:

What I don't understand is how to select a MOSFET for the job. What properties should I look for, if I know my logic level, device voltage and device current?

In this case it's 5V logic, and the load runs at 12V / 500mA, but it'd be nice to know the general rule.

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You've got a luxury problem: there are thousands of FETs suitable for your job.

1) the logic level. You have 5 V, and probably less than 200 mV or so when off. What you need is $V_{GS(th)}$, that's the gate's threshold voltage, at which the FET starts conducting. It's given for a specific current, which you want to keep an eye on too, because it may be different for different FETs. Useful for you could be maximum 3 V @ 250 µA, like for the FDC855N. At 200 mV (or lower) you'll have a leakage current much lower than that.

2) Maximum $I_D$ continuous. 6.1 A. OK.

3) the $I_D / V_{DS}$ graph:

This one's again for the FDC855N. It shows the current the FET will sink at a given gate voltage. You can see that it's 8 A for a 3.5 V gate voltage, so that's OK for your application.

4) $R_{DS(ON)}$. The on-resistance determines the power dissipation. For the FDC855N it's maximum 36 mΩ at 4.5 V gate voltage, at 5 V it will be a little less. At 500 mA that will cause a 9 mW dissipation. That's more than good enough. You can find FETs with better figures, but there's really no need to pay the extra price for them.

5) $V_{DS}$. The maximum drain-source voltage. 30 V for the FDC855N, so for your 12 V application OK.

6) package. You may want a PTH package or SMT. The FDC885N comes in a very small SuperSOT-6 package, which is OK, given the low power dissipation.

So the FDC855N will do nicely. If you want you can have a look at Digikey's offering. They have excellent selection tools, and now you know the parameters to look out for.

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Awesome, I'm pretty sure I get it now. I was looking at the IRF520N from International Rectifier, which has a Vgs of 2.0V, but mentions a Vgs threshold max of 4.0V in the same table. What does that mean? It then shows a Vgs/Id graph with Vgs figures going as high as 10V. From the graph, it looks like the Id at Vgs=5V is more than high enough for my needs. I looked at the IRF520N because I can buy them locally for ~£0.21 each in TO220 cases. –  Polynomial Jul 19 '12 at 10:23
2 V is minimum 4 is maximum. That doesn't leave you much headroom, remember that's for only 250 uA, but then the graph shows typically 4.5 A at 5 V, so probably OK. I'd lean more towards the FDC855N, though, since that's maximum 3 VGS(th). –  stevenvh Jul 19 '12 at 10:35
I'm having trouble sourcing the FDC855N, and I'd prefer through-hole cases. Is the STP14NF12 a reasonable substitution? Vgs threshold is 2V min, 3V typ, 4V max @ Id=250uV. –  Polynomial Jul 19 '12 at 10:47
@stevenvh how do we consider the power dissipation when we select FET transistor (given the the question scenario)? –  JeeShen Lee Jul 20 '12 at 1:59
@JeeShenLee - power = current x voltage = current squared x resistance. Also, voltage = current x resistance. Say you have a 2 A current, and you want the voltage drop to be 120 mV maximum, that's 1 % of a 12 V supply. Then resistance should be less than 120 mV/ 2 A = 60 m$\Omega$. You won't have trouble finding FETs which do even better. The power dissipation will then be 120 mV x 2 A = 240 mW, which even a FET in a small SMT package can handle. It's about choice. You can't do much about the current, but you can choose how much voltage drop or power dissipation you will allow. –  stevenvh Jul 20 '12 at 5:01