MOSFET - Advise

Other Hardware Motors, Mechanics, Power and CNC
1

Hi,

I'm looking into controlling some LEDs ( The popular ones like the 3528 or 5050 ) - which draw something up to about 3A.

I'm stuck between the 2 MOSFET's - One is much cheaper than the other for bulk.

IRFZ44N - http://www.ebay.co.uk/itm/261497621206
IRLB8721 - http://www.ebay.co.uk/itm/390813572214

I believe both would work perfectly fine. I assume I wouldn't really need heatsink as the AMP would be below what's rated for both of these.

Thanks in advance.

2

Assuming you wish to switch the mosfet with an arduino digital output pin, only the second one is appropriate as it's a logic level mosfet, note the L in it's model number. Standard mosfets require +10vdc gate voltage to reach their rated maximum current capacity (Ron value) where as logic levels mosfets can reach their Ron spec with +5 vdc gate voltage.

3

Hi retrolefty,

Thanks for the quick reply, that answers quite a lot! I wasn't aware.

4

The second one has a guaranteed on-resistance of 16 milliohms when
driven logic level, so is good to about 5A without a heatsink, more with
heatsinking.

Ignore the current rating, that's not a relevant parameter when selecting MOSFETs
unless you plan to liquid-cool them. Compute the dissipation from the Ron
value and I-squared-R.

5

Hi again,

I've purchased the items and I did a little test run last night. I've had to add in some 220 ohm resistors because the LED's was just showing ever so slightly. Would anybody please mind taking a look at my design and tell me if I'm on the right steps? I've noticed people placing resistors between ground and the gate pin. I'm also hoping the 220 ohm I've used is the correct value required.

Thanks!

LEDs_bb.png
LEDs_bb.png2037×1173 285 KB

6

Keep power and ground for the strip separately wired from the Arduino.

Sure Arduino ground has to be common to the LED ground, but don't route
ground currents through the Arduino board like your diagram. Just have one
wire linking Arduino ground to the high-current ground near the FETs.

7

Hi Mark,

I assume you mean the following (attached diagram) when you mean keeping them separately.

Is there any reason why I can't / shouldn't use the 12V supply to power the Arduino, and run it from ground/vin? Or is it just due to the amount of current that would be passing through the Arduino board?

Thanks again.

LEDs_bb2.png
LEDs_bb2.png1944×1182 282 KB

8

I'm just revisiting this.

Realistically, I want to be able to only have one power supply for the Arduino, and would like to use the 12V supply I already have coming in. I've attached another Diagram. Would this one be ok? I've attached the Fritzing file also.

Thanks again.

EDIT:
I originally seen the design from: Overview | RGB LED Strips | Adafruit Learning System

LEDs.fzz (10.9 KB)

LEDs_bb3.png
LEDs_bb3.png2040×1182 223 KB

9

Hi xerof,
You need a 100K resistor between the gate and GND, to pull the gate low, this will ensure that the FETS are OFF until you apply your signal from the Arduino.. Something like 220R is usually from the arduino pin to the gate, just to limit gate current but that is usually very low anyway, perhaps more to protect the Arduino output!! just in case?

Regards

Mel.