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I have a Arduino controlled RGB strip, which uses PWM to control the brightness of each colour. I have been using an Arduino UNO to do this but I have have just upgraded to a Photon by particle. Similar to a micro Arduino but with wifi shield built in. The main difference is that the Arduino Uno is 5v and the Particle is 3.3v.

When I do the switch using the same value for analogWrite the particle seems to make the mosfets (STP16NF06L) heat up, not to a burning temperature but warm. I have checked the minimum threshold for the mosfet, which is 1v, and the power being used by the strip is not exceeding the drain current.

I just want to check that I am doing this right and not damaging the mosfet, thanks in advance.

Here is the datasheet for the mosfet http://www.farnell.com/datasheets/1690175.pdf

The circuit diagram circuit
(source: makeuseof.com)

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  • \$\begingroup\$ can you post the circuit you are using? \$\endgroup\$
    – Felice Pollano
    Dec 31, 2015 at 12:51
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    \$\begingroup\$ The datasheet quotes the ON resistance for Vgs=5V and 10V but not 3V. So the MOSFET is turning on, but not fully on. You can estimate the actual resistance from Vgs(th) and Gfs but the easiest thing is to measure Vds at known Id. Or better still, simply increase Vgs to 5V using a level shifter of some sort. Or simpler, find a more suitable MOSFET (with Rds specified at Vgs=3V). \$\endgroup\$
    – user16324
    Dec 31, 2015 at 12:52
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    \$\begingroup\$ BTW.....The thing you just posted is an assembly picture. That is not a schematic. \$\endgroup\$
    – Michael Karas
    Dec 31, 2015 at 12:57
  • \$\begingroup\$ What's your PWM frequency? The higher the frequency, the more current you need to supply to charge and discharge the MOSFET's gate capacitance, and while it's charging and discharging it's not fully conducting so will be dissipating extra power. Even if you introduce a level shifter or substitute the MOSFET, you might consider reducing the PWM frequency if you can do so without it being visible. \$\endgroup\$
    – nekomatic
    Dec 31, 2015 at 13:34
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    \$\begingroup\$ We communicate circuits with schematics here. -1 for posting one of those annoying cartoons instead. \$\endgroup\$
    – Olin Lathrop
    Dec 31, 2015 at 14:43

2 Answers 2

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The answer to this is fairly simple. The MOSFET turns on more (i.e. to a lower drain to source resistance) when it is given more gate voltage. In your case the 5V gate drive was letting the FET go to lower resistance than what you are now getting with the 3.3V gate drive. At the same load current this increased FET resistance is causing more heat to be produced in the FET thus they get hotter.

BTW.....the 1V level that you mentioned is not the instant threshold where the MOSFET goes from Off to fully On. The MOSFET has a range of voltage on the gate over which it turns on.

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The key parameter is the maximum threshold voltage, not the minimum. For this device, they specify the maximum VTH as 2.5V, but this is at a drain current of just 250 µA — not a very useful value for a power MOSFET.

The chart for RDS(ON) doesn't even show any data for VGS below 4.0 V.

Therefore, I would conclude that you need much more than 3.3V to switch high currents with low dissipation with this device.

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