Driving 480 ma from MOSFET without heatsing

I am desinning a water system for my garden which will consist in an Arduino opening and closing water valves. I am trying to accomplish this within the smallest enclosure possible and I was hoping to not require any heatsinks. Somehow although the (overkill) MOSFET I am using is rated 100A at 55V but it is getting really hot when the valve is open. I measured the coil to be 25 ohms so at 12V I figure it consumes in the area of 480 ma. I am clueless as to why the MOSFET is getting hot with so little.

This is the circuit I am working with, obviously the switch will be replaced by an Arduino pin and provide 5V to the gate.

Am I missing something?


Mosfets are efficient when they are either fully closed or fully open.
The datasheet show that a 20v Gate to drain voltage (+-) should fully open it. you are only applying 12v

Use a logic level MOSFET that fully opens already at 3V.

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You can tell a Logic Level MOSFET by looking at the Gate to Source voltage (Vgs) used to measure RDSon (Resistance, Drain to Source when ON). It has to be measured at a Vgs equal or lower than your logic voltage to be a Logic Level MOSFET (often 5 or 4.5V).

The HRF3205 RDSon was measured at 10V but the Arduino pins are 5V (or lower).

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This is something new. Where did you find it?

Thank you all for responding so quick.
I learned something today!
I am looking through the different parts, the specs are not always clear...
For my project would this part be good?

Ok. This will be good.

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Where does it say that? I can't find anything about Gate-to-Drain Voltage (Vgd?)

but it is getting really hot when the valve is open.

What does this mean? I the valve open when to mosfet is conducting? Or the inverse?

Can you measure the actual gate voltage?

The HRF3205 has a max gate voltage of 20 volts, beyond that you are at risk of damaging the device.

With the gate at 12V the Mosfet will be pretty much fully ON. Meaning it will conduct.

With the gate at 0V the Mosfet will be OFF. Meaning it will be an open circuit.

Can you please measure;
Gate to Source Voltage, ON and OFF conditions.
Drain to Source Voltage, ON and OFF conditions.
Power Supply voltage, ON and OFF conditions.

Thanks.. Tom... :smiley: :+1: :coffee: :australia:

I have this water valve which I drew as a coil/solenoid on my circuit.

I am powering the circuit with a 12V 2 Amp wallwart
Gate Source OFF voltage is 0V
Gate Source ON voltage is 3.2V
Drain Source voltage OFF is 12.15V
Drain Source voltage ON is 3.3V
When OFF the voltage source is at 12.15V
When ON the source is at 12.07V
8.3V (on the coil)

The gate to source is too low. It needs to be at least 5V would be better at 10V.

The reason you MosFet is getting hot is it is only partially on.

Your results don't match your drawing so I'll guess the drawing is "simplified"

As @JohnRob has said, you may need to check your circuit

Can you measure the voltage between pin on the switch and gnd in ON and OFF conditions?
Measure the resistance of the 1K resistor to make sure it is correct.

Thanks.. Tom.. :smiley: :+1: :coffee: :australia:

Ok I found my error, to turn ON, instead of connecting the end of R1 to 12V, I was connecting to the Drain pin at point "D".

Since I went through the time to make the measurements, these are with the ON position:

I don't know how I missed that...

But thanks to all.

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Hi, @JABfreak
Great to hear.

Thanks for the two clear diagrams, makes troubleshooting so much easier. :+1: :+1: :+1:

Tom.. :smiley: :+1: :coffee: :australia:

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