Using MOSFET in place of Relay

Hello,

I have a fairly simple question, but after looking around online, I’m actually more confused than when I started. What I am trying to do is use a MOSFET as a switch for a very low power (3 V, haven’t measured current, but will be low) circuit. The circuit is from a child’s toy, and takes 2 1.5 V AA batteries.

In the past, I have always used relays when making switches for circuits. However, as I would like to limit the size (and power draw) on the circuit I am building, I would prefer to use a MOSFET.

Previously, I did try to replace a relay in a low power circuit with a 2N222a transistor. However, for reasons I never really understood, I was able to use the 2N222a as a switch to turn LEDs on/off in a test circuit, but when I tried to use it as a replacement for a relay in circuit similar to the one I’m attempting again (3V child’s toy), the toy never actually turned on. Thus, I have two questions:

  1. In the attached circuit, is there a MOSFET anyone can recommend that may be suitable?
  2. Is there any reason I can’t use the 2N222A for this circuit? I read this: https://forum.arduino.cc/index.php?topic=212342.0, but then I was a bit confused why it seemed to work fine when simply turning LEDs on/off.

Any answers would be greatly appreciated.

Thank you,

Dustin

Here's OP's pic:

Well those circuits have no common ground so the Arduino isn't actually connected other than as a radio
antenna.

A circuit is just that, a circuit. Current flows round a loop. With no loop, no current.

With a 2N2222 you'd connect the emitter to Arduino ground, base resistor to Arduino pin
With a n-MOSFET you'd connect source to Arduino, gate to Arduino pin.

If you only connect one electrode to the Arduino, its not connected electrically, other than as a radio antenna, and as we aren't talking radio frequencies here that means nothing happens.

I don't see a ground return to the Arduino.

Without it a BJT nor MOSFET will work.

Sorry, yes the circuit drawing was only meant to give a high level overview of what I’m trying to achieve. I just wanted to know if anyone had a suggestion for what mosfet to use as a switch for a 3 V circuit. Any suggestions?

Any MOSFET that can handle the current your circuit requires, and where the minimum VGS, ON is lower than the voltage of the signal you are using to switch it.

TwoChain:
Previously, I did try to replace a relay in a low power circuit with a 2N222a transistor. However, for reasons I never really understood, I was able to use the 2N222a as a switch to turn LEDs on/off in a test circuit, but when I tried to use it as a replacement for a relay in circuit similar to the one I'm attempting again (3V child's toy), the toy never actually turned on.

Post schematics for the specific examples you have questions about. If you don't require galvanic isolation, you should be able to use NPN, PNP, N MOSFET, or P MOSFET to switch power to loads.

There are some cases where relays, solid state or mechanical, are required.

  1. In the attached circuit, is there a MOSFET anyone can recommend that may be suitable?

Which circuit? You showed 4. Number 1 and 2 make no sense. The box you have labeled relay would usually be interpreted as the relay coil. For the contacts, the contact configuration would be drawn.

#3 could could use an NPN transistor or N channel MOSFET (you're switching power low side).

In #4, you've drawn the transistor upside down and backwards; and you have a typo on the transistor number (at least you're consistently wrong). That circuit can switch power for a load up to 70-80% of the rated current for 2N2222.

  1. Is there any reason I can't use the 2N222A for this circuit? I read this: https://forum.arduino.cc/index.php?topic=212342.0, but then I was a bit confused why it seemed to work fine when simply turning LEDs on/off.

From the information you've given, don't see any reason why it wouldn't work; though current draw from AA batteries could be higher than what 2N2222 can tolerate.

When you use MOSFETs, you need to be aware that the VGS(th) specified in the datasheet is the gate-source voltage that just starts to turn the MOSFET on. To minimize power dissipation in the MOSFET, you want to drive it as hard as you can. For example, the inexpensive 2N7000 N channel MOSFET has a VGS(th) of 0.8-3V with 2.1V being typical. At this voltage, Id is specified to be 1mA. Since your load is undoubtedly higher, you need to use the curves for a typical device to estimate on resistance. At a Vgs of 4.5V, on resistance would be in the range of 1.5-3 ohms; depending on load current, which you haven't specified.
2N7000resistanceVsVgs.jpg

2N7000resistanceVsVgs.jpg

A bipolar darlington transistor will work fine. Diagrams here

johnerrington:
A bipolar darlington transistor will work fine.

Seriously? OP wants to switch 3V! I don't think he can afford to lose half that in just the switch.

If you lose more than about 0.3V across a transistor it is not in saturation.

johnerrington:
If you lose more than about 0.3V across a transistor it is not in saturation.

A darlington can never have a saturation voltage that low.

“ "saturation" voltage of a Darlington transistor is one VBE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon.”

“is there a MOSFET anyone can recommend that may be suitable?”

For low current applications:

Si2302DS N-MOSFET rds(on) .085ohms at Vgs = 2.5v

IRLML6402 P-MOSFET rds(on) .145ohms at Vgs = -2.5v

IRLML2402 N-MOSFET rds(on) .35ohms at Vgs = 2.7v

You can always add an extra 1.5v battery to get 4.5v to drive a 5v relay :wink:

Hello larryd,

Thanks for your answer. This is exactly what I wanted to know. In the past, I have always used 5 V relays. However, I believe this will take up a lot more space than the MOSFET. Do you agree? I've never seen a 5 V relay that takes up less space than ~ 2x1x1 cm.

For now, I may try the Si2302DS that you suggested.

Thank you,

Dustin

You can find relays that are ~.8” X .4” X .4”

https://www.jameco.com/z/DS2Y-S-DC5V-Panasonic-DS2Y-Miniature-2C-Relay-2A-5V_842996.html

Yes, but those dimensions are pretty much what I mentioned: 2x1x1 cm. I think the MOSFET should be smaller than that. My space is quite constrained.

MOSFET in SOT23 package, 0604 resistors, that’s about as small as it gets.