Circuit Issue with MOSFET

Question

Hey,

I am having issues with this circuit:

Background: This circuit is to control an RGB LED strip (so three of the above would achieve this). The reason for this particular design is: IRF540 to drive load (large current) and the two 2N7000 drive the MOSFET, reducing current drain from Controller (possibly a 3v3 Arduino). The double 2N7000 allows the IRF540 to be active when logic high.

Can someone please tell me if there is a problem with the above schematic, otherwise I think it would be my breadboard layout is not correct. (too hard to post that right now).

Any comments would be much appreciated.

Many thanks,

wallarug

dougw · Accepted Answer

You are correct on all counts - the gate current will by much less than 1 mA. It looks to me like the circuit will work fine with no pull-down or pull-up on the first FET, Even R1 is not needed with this FET. To ensure full switching operation R2 should be relatively high - between 30 K and 100 K should be safe, although it will likely work even with a 10 K resistor. My previous comments only apply if the FET doesn't work quite as well as the datasheet indicates.

johnbeetem · Answer

wallarug wrote: 
 
 What is the reason for such a high R2 (R3 in new diagram) resistance? 
 
 Do you recommend any values for the other resistors (new diagram: R2, R4)? 
 
 FET gates have very high impedance, so you can get by with high resistance on the pull-ups and pull-downs. If you use a low resistance on R3 (in the new diagram) you're wasting current when the left-most transistor is on. I suggest 10K - 47K for R2-R4. I suggest zero Ohms (i.e., short circuit) for R1. Since a FET gate is high impedance, you can drive it directly from a microcontroller output as dougw suggests. I like R2 since it forces the LEDs off when you power up.

dougw · Answer

If you want to save as much power and cost as possible then R1 = 0 (replaced with a wire), R2 is removed (infinite resistance), R3=100K, R4=100K.

If you want to modulate the control with PWM to achieve variable brightness then R1=1K, R2 is removed, R3=10K, R4=2.2K.

If you really need R2 to keep the LEDs off for the microsecond it takes to power up, it should be 100K, and R1 should be replaced with a wire.

Doug

Circuit Issue with MOSFET

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