# 20 Amp automotive thermatic fan PWM controller

I have an older car which tends to over heat in the summer, so a 20 Amp electric fan was added in front of the radiator. Currently its driven by a on / off relay.

I want to build an Arduino PWM controller but I am not confident switching that much current especially in an automotive environment.

Been testing the VNP35n07 MOSFET but I dont think I am doing it correctly. When tested in the on state there is a noticeable difference in fan speed (noticed via noise) and the draw is about 1 amp less. Understand there will be lose from the MOSFET, but should it be that high?

The wiring for the test is as follows:

VNP35n07
Pin 1: +5.5V
Pin 2: Fan's ground wire
Pin 3: Ground


My intention is to connect the Arduino PWM signal directly to pin 1.

Any feedback / comments welcome.

• Are you saturating the transistor? – Ignacio Vazquez-Abrams Oct 4 '12 at 13:34

You will need to have a higher input voltage to the FET to get it to the lowest RDS-ON. Right from the data sheet that you linked you can see the difference between having an input at 5V versus 10V.

• What would be the best way (part) of driving with 10V? – oden Oct 5 '12 at 1:41
• You can use any MOSFET driver (e.g. a LT1910, a TC4431, or a MIC5011). It gets its power from the 24V rail, so it can supply enough voltage to the MOSFET. The driver accepts a logic signal on its input, so the Arduino can drive it directly. But you also use a LT1981 which is powered by 5V and internally creates the higher voltage needed for driving the MOSFET. – hli Oct 5 '12 at 7:47
• When choosing a MOSFET driver, watch for its maximum voltage - most seems to handle only 24V. (the LT1910 above can do 48V) – hli Oct 5 '12 at 7:48
• Once I increased the input voltage the current flow to the fan increase drastically. Thanks for point that out. Will start looking for a MOSFET driver that can push out 10v as recommended. – oden Dec 2 '12 at 22:51

According to the datasheet, at 5V gate drive the drain-source resistance is 35mΩ, so almost all the resistance will (should) be presented by the fan. There will be some tolerance in the Fans current rating so 19A wouldn't be unexpected (what is the actual current you are reading?)
However if there is a significant difference between using the MOSFET and connecting directly, then it seems there may be something amiss - can you post a diagram of your setup?

I have two suggestions:

1. Measure voltage V_DS and voltage drop across fan. It can help to figure out what happens in circuit.
2. Use simpler power MOSFET. This one that you chosen has some smart circuits inside - like current limiter - and who knows how they behave now? Maybe temperature protection limits the current? If I were you I would try with simple power MOSFET and check if simple control works. If so, then would try to optimize it using smart MOSFET.