Need advice on suitable transistor for tachometer


New user here, so excuse me if this is the wrong category.

I have a tachometer from a car that I want to control using my Arduino UNO. The tachometer is an old-style air core gauge, with three pins: Ground, 12V and a signal pin that seems to take square wave 12V input.

I've been able to control the tachometer using a mechanical relay that turns on/off 12V to the signal pin. But because of its mechanical nature I hit the limit at about 3000rpm on the gauge, and then the relay won't go any faster. So I need something quicker, and I got my hands on an IRF630 N-channel transistor, but my Arduino doesn't seem to be able to turn it completely on - the transistor will only let through ~2V when I send 5V to the gate - if that's the Arduino or the transistor I don't know, but it "opens" fully when I apply 12v to the gate.

This may be a XY problem, but I could need your advice on what transistor would be a nice fit for this project?


It sounds like you are on the correct track. Use a logic level avalanche rated (UIS SPEC) logic level MOSFET and your problem should go away. The tachometer is inductive but with the avalanche rated MOSFET no diode is needed.

For any inductive load always use a free-wheel diode. Avalanche rating doesn't make a device immune to damage, it just means the level at which damage occurs has been quantified, and you'd need to make
measurements and do calculations to verify if the diode is necessary - its easier to fit the diode always.

I would suggest you read some of the application notes and examples published by manufactures that sell the avalanche rated MOSFETs. Most of the manufacturers test this parameter in each device as part of there final testing process. Many of the H-bridge drivers now have them and no diodes. Mark is correct the max capacity is the UIS rating and is a lot higher and is much faster then the typically 1n400x used by many. Each manufacturer has a different MOSFET process and these numbers will vary by manufacturer. Try this link for an in depth explanation. . It is AN1968 Rev 0.00 Nov 9, 2015. Things have gotten much better since. If you are building one or two parts cost is not that critical. I have seen a lot of free-wheel diodes fail because they are not sized properly.

Hi again, and thanks for your answers - they are highly appreciated.

So after reading your answers it seems like what I should be looking for is a logic level transistor, as it can be turned on/off by using the 5V supplied by the Arduino. :slight_smile:

Another suggestion go on line and find out what a MOSFETs and transistors are and how they are driven.

Letting a MOSFET handle kickback with its avalanche breakdown will lead to lots of completely unnecessary EMI,
don't do that.