# Very hot mosfet when powering an led strip

I'm basically following this instructable which is using an arduino to control an rgb led strip. I have wired it up exactly as shown, although I didn't use a breadboard i just soldered everything.

I'm using an old laptop power supply that's 12v 5.83a, and for the mosfet I'm using irf530n. I've noticed that when red is full power the mosfet that's connected to the red led gets really hot after a few minutes of being on. It seems to only be affecting the red one, even if i have it on full white only the red one is hot. The other two barely get warm.

Are the mosfets supposed to get that hot? Why does it only get hot on the red side? Are there mosfets out there that are better suited for this application? Are there any ways to reduce the heat?

Thanks for any help!

• The FET's datasheet seems to indicate that at Vgs=5V Rds should be about 200mOhm. If you can figure out the current the LED(s) draw you can calculate the power lost in the FET. If you end up with more than say 5A (->1W) the FET will get hot. At 1A or less I think it should get warm at most but not hot; discard FET and take another. Commented Jul 17, 2014 at 16:10
• Lose the stupid and annoying cartoon. Commented Feb 12, 2017 at 19:30
• Lol Olin, you are so smart. Commented Feb 15, 2017 at 4:57

Red LEDs have a much lower voltage drop than other colors so in order to compensate, the mosfet is having to drop a significantly higher amount of voltage/power.

Red LEDs usually have a drop of 1.8 V while Blue and green are up into the 2.5-3.3 range generally. This means that the Red mosfet will have to drop about an extra volt at whatever current you're running it at. Power wasted is V*I.

The way to get around this would be to have a switching regulator that brings the voltage down closer to the range you're after for the Red LED and then power it that way. Otherwise, you'll always end up with wasted power and more wasted power from the red LED channel.

If you don't want your red mosfet getting hot, you could determine how much current and voltage it is dropping compared to the others and then add a series power resistor to absorb some of the power for the Red LED. In this way, you distribute the heat between the resistor and the mosfet driving the red LED.

EDIT:
Doxy has pointed out that there's already a constant current driver/resistor on each LED so I suspect that the problem is actually that they're using the same resistors for all colors of LEDs. This results in a higher voltage dropping across the same value resistor which means an increase in current for the red LED. Then your mosfet switcher is actually always seeing a larger current than the others. The easiest fix here is still as I mentioned originally. Add another resistor to the red LED path to balance out the current between all of the colors.

Alternately, you may try reducing the duty cycle on the Red LED's and see if that alleviates the problem.

It could be a part issue. In that case, swap the MOSFET's around between say Red and Blue and see if the Red MOSFET still heats up the worst.

Otherwise, if these three things don't help you out, without more diagnostic information on the circuit, there's not much anyone can tell you.

• If you read the linked Instructable (yuck, I know), the LED strips in use have built-in dropping resistors and are intended for direct use with 12V. The MOSFETS shouldn't be dropping the extra power. Commented Jul 17, 2014 at 15:32
• @DoxyLover Interesting point. Added an edit to address it. Commented Jul 17, 2014 at 15:54
• Thanks for you help. I have tried replacing the transistor and still the same thing where that one is getting hot. Can you recommend some ways of testing this circuit out, I do have a multimeter. Commented Jul 17, 2014 at 18:39
• @davidwiththequestions check the current flowing through the mosfet compared to the current flowing through the other two mosfets. It should be the same because you have the constant current drivers on the LEDs. If not, you need to make sure you have the same number of led's on each branch. Make sure the voltage dropping across each mosfet is the same as well. Commented Jul 17, 2014 at 20:15
• @horta, thanks for getting back with me. I tested both the voltage and the current, the voltage is about 4v for each of them, the current is ~.6a for the blue and green, and ~.8a for the red one. It doesn't make sense that it would cause that much of a difference in temperature. Any other things you can think of for me to check? Commented Jul 18, 2014 at 1:35

just see your maximal power output for your Arduino. It is possible, that you are sourcing too many mili-amps from your Arduino, so you have lower voltage on some pins. Since on Gate of MOSFET is lower voltage, its not fully open, have bigger resistance and warm more. Therefore it is very usual to put resistor between PWM Pin of Arduino and Gate of MOSFET (min 120 Ohm, typically 200-250 Ohms, depending on which arduiono do you have also 500 Ohms suits).

Also try to exchange MOSFETs, because one can have thicker capacitor layer so it will need more volts (after exchange you will see, it heats by another color).

I also find that potenciometers are not so good while it has on "0" also 0 Ohms (or 10 nano-Ohms) that means all the current from source is sinking to Arduino. Imagine, there is also limit for that, you can sink only 150 mili-amps to Arduino on one port. That means at least 33 Ohms between source of power and arduino. So maybe your analog inputs are burned.

https://startingelectronics.org/articles/arduino/measuring-voltage-with-arduino/