# Mosfet Abnormal Temperature?

Hi guys,

Recently I've been using IRLZ44N; a logic level mosfet. I have been passing 9v, 1.6A to the mosfet. I did not use any pwm.

The mosfet is connected to pin arduino D10 and a 10k resistor between gate and source.

The mosfet got up to 65 degC which I think it is high comparing to the theoretical calculations.

Theory
RdsOn @ 5Vdc is 25 mOhm
Power dissipate is 0.064 W (P = I2R)
Temperature rise should be around 4 degC (P * Rja) + ambient

Hope you all could give me some advices. Thanks alot.

Please post a complete schematic. "Passing 9V, 1.6A" does not make sense.

I think the 25 mohm is understated. You will have to look a the device curves to see what the operating point is when switched fully on with 5V on the gate.

I believe you have two possible failures, first a bad MOSFET which I doubt, second you are not fully enhancing the MOSFET. I would suggest you place the 10K resistor from D10 to the source and something in the few hundred ohm range between D10 and the gate. I believe you will then find your calculations right on! Be sure all the grounds are connected together including the source.

Since you do not PWM the MosFet the 10K is OK. I would but a 1k there to keep the heat down during on and off transitions.

BTW are you using a 3.3V or 5 V signal? I assumed 5v which will but about 4v on the Mosfet gate.

Looking at the part specification:

You likely have about 0.1 or 0.2 volts across the fet. and at 1.6 amps so about 0.3 watts. That might get you to 40°C. Not so far off your 65°C.

You should measure the voltage from drain to source to calculate the watts not estimating the resistance.

A word about thermal calculations. With crude off the cuff calculations you will be very lucky to get within 20°C of the actual part temperature.

The published Θja if for a part hanging in free air. It is an unrealistic condition and likely the same for any TO-220 device.

Yes, something is wrong. But you failed to post your circuit and your code so we have no idea what yet....

BTW did you source your MOSFET from a reputable source? If not its likely to be counterfeit, pretty much all random sources of semiconductors on eBay etc these days are counterfeit as the genuine sources are undercut by the criminals.

Hi all,

Thank you so much for the advices! You wouldn't know how much it means to me. Thanks alot.

I think it was due to me buying cheap mosfet from online platform instead of buying from a reputable supplier. I've spent only a dollar for 5 pcs of these IRLZ44N Mosfet. =(

Hi,
Can you please post a circuit diagram of your project?
You have got the gnd of the Arduino connected to the gnd of the 9V supply?

What model Arduino are you using?
When you apply power to the gate.
What is the load in the Drain circuit?
What is the Drain to Source voltage?
What is the load?

Thanks..Tom..

That might be the case but not 100% positive.

Can you measure Gate to source, Drain to source? and post the values.

Are you confident the 1.6 amps is correct?

Hi all, thanks for the replies.

Attached is the simple schematic on how the Mosfet is connected. Kindly enlighten if there's any mistake.

The load is consuming 1.6 ampere @ 9 Vdc; measured using a multimeter. And I have a Ruideng AT35 tester too.

I have measured the voltage values;

1. Gate-drain voltage is 4.8 Vdc
2. Drain-source voltage is 50 mVdc when cold and 60 mVdc when above 50 degC
3. Gate-source voltage is 4.8 Vdc

That gives 0.0375 Ω which is equivalent to 37.5 mΩ and it is relatively higher than the built specification. However, at these values, the power dissipation should be still well below 65 degC; theoretically is 6 degC + ambient temperature.

I measured the voltage values directly on the Mosfet legs when it is on load.

Something doesn't jive.

If your drain current is indeed 1.6 amps the power dissipated is 0.06 * 1.6 = 0.096 Watts.

So either your current is more than you think or your temperature measurement is in error.

Physically you can't have both conditions unless you have your MosFet insulated in foam.

Hi,
Can you please post a picture of your project, so we can see your component layout?

What are you using for a 9V power supply?

Thanks.. Tom...

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