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!

Edited to add a schematic:

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-1 for the wiring diagram in place of a real schematic. No, I'm not going to trace the wires only to find what seems to be connected to what. SHOW A SCHEMATIC – Olin Lathrop Jul 17 '14 at 15:31
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. – JimmyB Jul 17 '14 at 16:10

1 Answer

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 on each LED so the things I mention shouldn't be a problem unless the constant current driver is doing its job poorly or failing.

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 those two things don't help you out, without more diagnostic information on the circuit, there's not much anyone can tell you.

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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. – DoxyLover Jul 17 '14 at 15:32
@DoxyLover Interesting point. Added an edit to address it. – horta Jul 17 '14 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. – davidwiththequestions Jul 17 '14 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. – horta Jul 17 '14 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? – davidwiththequestions Jul 18 '14 at 1:35