# using 1.8V, max 40mA to switch a 12V, 70mA load (MOSFET data sheets?)

I recently asked a question about controlling a magnetic relay with a solid state relay.

The reason is that I'm controlling the circuit with a MCU that has a max ouput of 40mA per output pin and the coil on the relay draws 70mA. Also, the MCU is operating at 1.8V and the relay coil is 12V (as is the external load).

It was suggested that I use a MOSFET rather than a solid state relay. While researching that I have become very confused trying to iron out the information on the MOSFET data sheets.

Can anyone give me some tips to make it more understandable for a newbie hobbyist?

EDIT - what I think will work to see if I'm totally confused:

gate source threshold voltage - 1.2V (MOSFET will "turn off" under 1.2V?)

gate source voltage - 1.8V (MOSFET is designed to be fully "turned on" at 1.8V?)

continuous drain current - 2A (MOSFET is designed to handle switching up to a 2A load?)

drain source breakdown voltage - 40V (MOSFET is designed to switch up to a 40V load?)

drain source resistance - 260 mOhms (using ohm's law to calculate the load that will be placed on the MCU output pin, this is way below 40mA?)

Am I reading all of this correctly?

• Have a read of this question and the answers Commented Aug 16, 2017 at 16:37
• I just keep getting more confused. Maybe I should stick to the solid state relay that I understand or just stick my head back in the sand and go back to mechanical design where I belong :(
– LsD
Commented Aug 16, 2017 at 16:45

For the type of application you mention in your question you could think of an N channel MOSFET as a relay (but without isolation between contact and coil). In other words the "relay contact" is between drain and source and the "coil" is between gate and source.

In your application, the source connects to 0 volts and the gate connects to your digital control voltage. The voltage applied between gate and source turns the "relay" on and connects drain to source. As this imaginery device is equivalent to an N channel MOSFET current should only flow between drain and source.

As you apply a gate voltage the "contact" doesn't suddenly change from open to closed - it happens gradually as gate voltage increases gradually. If gate voltage increased rapidly then the "contact" resistance changes from "open" to "closed" rapidly.

Lowest contact resistance is achieved (for an N channel MOSFET) when gate voltage is highest with respect to source AND, like a normal relay, if you put too much voltage across the coil (gate to source) you will damage your MOSFET.

If you don't apply enough voltage between gate and source you get a half-on "contact" of anything between mega ohms and a few tens of ohms. You must choose a MOSFET that can near-fully turn on with your logic control voltage of 1.8 volts. This will usually mean that the gate threshold voltage in the data sheet will need to be sub 1 volt.

So, it's like a three terminal relay with coil between gate and source and contact between drain and source.

• Thank you. I think that helps. I THOUGHT I understood how it worked, but just didn't understand the data sheet and relevant specification requirements. Now I'm not sure if I understand any of it? So- the gate is the equivalent to the input to the coli? And the source is just like the battery voltage terminal of the relay, and the drain is like the relay (output) contact going to the load? Is that correct? Basically I can just imagine it's a relay with a coil that doesn't require a ground?
– LsD
Commented Aug 16, 2017 at 16:51
• Gate is one side of the coil and source is the other side of the coil. Source is also one terminal of the normally open contact; drain is the other terminal. Source connects to ground for an N channel device in your application. Commented Aug 16, 2017 at 16:53
• Ahh... getting clearer. I need to use the MOSFET to switch the relay ground, not the relay power. I just saw a similar comment in my other question. MCU output to the gate, relay coil "ground" to the drain, and source to the ground plane on the PCB? correct? Still foggy on the specs...
– LsD
Commented Aug 16, 2017 at 16:57
• Basically yes, proper relay coil connects up to 12 volts and other side of coil to drain. Source to 0 volts and drive the gate (don't forget about a flyback protection diode across the relay coil). Commented Aug 16, 2017 at 16:59
• OK. Great. That all makes perfect sense now. I'm still having a hard time understanding the specs. Are the interpretations I edited into my question right?
– LsD
Commented Aug 16, 2017 at 17:06