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### Topic: Could use some help understanding resistors (Read 395 times)previous topic - next topic

#### donldmn

##### Jan 18, 2019, 06:24 pm
I am wanting to control LED strip lights.

I am brand new to Arduino and don't actually have anything yet. I am in the process of ordering parts.
My knowledge is limited to some Java I learned in college so I'll cross that bridge when I come to it.

I have done some research and have found that transistors should be used when they are able to handle a load and relays should be used when they can't.

I was wondering if I could use this transistor (IRLB8721) to power 3014 chipset LED strips and if so, how many LED strips could one transistor handle?

It would be helpful for me to understand how to tell what a certain transistor is capable of so could anyone explain it to me or have a link to somewhere that does this?

#### Grumpy_Mike

#1
##### Jan 18, 2019, 07:16 pm
I think all your questions are answered here
https://learn.adafruit.com/rgb-led-strips/overview

#### donldmn

#2
##### Jan 18, 2019, 07:48 pm
Thanks for the link. I actually came across this site which is where I found the IRLB8721.

If I understand correctly, the IRLB8721 can support 16 Amps at 12 Volts.
So if 5M of LED strips uses 72 Watts that is equal to 72 divided by 12 which is 6 Amps.

If my math is correct (I really don't know), I can comfortably use one IRLB8721 transistor for  two 5M strips of lights for a total of 144 Watts at 12V 12A.

Does this work? I'm just trying not to burn my house down.

#### larryd

#3
##### Jan 18, 2019, 08:01 pm
Actual specifications are difficult to see.

You may have to experiment some to confirm things don't get too hot.

Suggest you get a power supply rating 1.5 to 2 times what you think you need.

Order  extra MOSFETs.

They can be run in parallel.

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#### donldmn

#4
##### Jan 18, 2019, 08:12 pmLast Edit: Jan 18, 2019, 08:13 pm by donldmn
I didn't even think about running them in parallel.

Thanks for the help I think I'll experiment when I get them.
I was thinking of using a spare computer PSU.

#### MarkT

#5
##### Jan 18, 2019, 11:29 pmLast Edit: Jan 18, 2019, 11:30 pm by MarkT
Thanks for the link. I actually came across this site which is where I found the IRLB8721.

If I understand correctly, the IRLB8721 can support 16 Amps at 12 Volts.
Its worst case on-resistance when driven from 5V is 16 milliohms, which means 4W dissipated
for a 16A load (power = I-squared-R  =  16 x 16 x 0.016 = 4)

4W would need a middle sized heatsink.

With a big heatsink it could handle more current, with no heatsink about 8A is the limit
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### MorganS

#6
##### Jan 19, 2019, 01:00 am
I didn't even think about running them in parallel.

Thanks for the help I think I'll experiment when I get them.
I was thinking of using a spare computer PSU.
"Regular" transistors can't be paralleled but MOSFETs can. It's still better to buy a transistor of the appropriate size as then there's less wires to get tangled.

One technique for large LED arrays is to use a number of power supplies and a number of MOSFETs but don't try to parallel them. Each supply+transistor controls its own strip. The grounds of all power supplies should be connected or you would need extra electronics to control each one.
"The problem is in the code you didn't post."

#### DrAzzy

#7
##### Jan 20, 2019, 12:46 am
I'd probably use an IRF3708PBF; the Rds(on) is lower, and they're a good go-to power MOSFET because they can be controlled from 3.3v logic levels too. Will still need a heatsink.

One concern - if you're PWMing it, you may find that the MOSFET gets hotter than you expect - this is because during the time that it's being turned on and off, for a brief moment, it is partially on with a much higher resistance (the gate acts like a capacitor, so switching takes a non-zero time). Depending on the amount of current and how strong the drive is, this may or may not be a problem. If it turns out that it is, you need a MOSFET driver (this is a little IC with a pair of caps on it and it's own internal switches that is capable of dumping very high current into (or drawing it out of) the gate to switch it on and off very quickly.

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