# Choosing the right MOSFET for 5V power relays

I am planning following circuit PR_SIG: a 5V logic level coming from a uC +5V: is powered trough a external power supply (max 10A) Power_relay: 5V, needs 70mA to switch

My problem now is choosing the 'right' MOSFET. I used the search on Mouser using the parameters that come to mind: - Vgs = 5V - Id > 250mA (3x70mA when all relays are switched on) - Another parameter is the power loss, but can also be neglected

With my research I found two candidates PMF250XNEX, DMG1012TQ-7

Comparing the two both should work as switch but I am not sure that I consider all necessary parameters, since there are ~450 different MOSFETS which apply to my search.

Do I need to look at more characteristics or can I choose the MOSFET using only Vgs and Id?

Since relays are a lot slower than any MOSFET, your only concern are the DC parameters.

The MOSFET must be able to handle a $$\V_{GS}\$$ > 5 V

The MOSFET must be able to handle a $$\I_D\$$ > 250 mA

You also need to be sure that the MOSFET switches on properly when you apply $$\V_{GS}\$$ = 5 V.

When the MOSFET is switched on it needs to have a low enough resistance $$\R_{DS,on}\$$ so that it will not drop much voltage. At only 250 mA a value of less than 0.5 ohm should be OK. Many MOSFETs will have a much lower $$\R_{DS,on}\$$.

It is also OK to use a hugely overspecified MOSFET. For example, if you have some MOSFETs lying around that are large and can handle 20 A but their size isn't an issue, you could use them, I mean, that would work fine.

The two MOSFETs you found look quite expensive to me. To keep things cheap (and for hobby projects, not professional use) I look on Ebay and find the cheapest MOSFETs which can do the job. A very common and cheap type is the AO3400 which costs around 10 Euro cents (including shipping) if you buy 20 from China.

• We have some MOSFETS here which are indeed highly overpowered for the task (up to 50A, 150V). I read in some literature that the input capacitance should not exceed 100pF in order to switch it using a uC. Reducing Rds(on) makes also sense. – v3xX Jan 13 at 7:53
• I have the luxury that I do not need to look at the prices (only 2 PCBs are needed). – v3xX Jan 13 at 7:54
• I read in some literature that the input capacitance should not exceed 100pF in order to switch it using a uC Next time, when you read that ask yourself: Why?. I can guarantee you that even when the input capacitance of a MOSFET is 10 nF (that's 100x 100 pF) it will work just fine when switching something on/off slowly (as will be the case with a relay). For a PWM signal, sure then speed matters more and the load capacitance should be limited. But for switching some relays: the capacitance really doesn't matter much. – Bimpelrekkie Jan 13 at 7:59
• Makes sense for slow switching speeds. Thanks – v3xX Jan 13 at 8:02