# Pump does not get enough current via MOSFET

I am trying to build the v4.3 circuit described here: Will this schematic work?

I am building just one branch out of the decoder IC to test on a breadboard. Component info is as follows:

1. The optocoupler is PC817 i.s.o TLP621;
2. The MOSFET is IRF630B;
3. The flyback diode is 6A4;
4. The resistor between MOSFET gate and ground is 5.2K;
5. The peak power rating of the pump is 2.7A @12V.

Now when power is on, RasPi is up and the input to the decoder is set correctly, the pump does not work.

I tried the following to troubleshoot,

1. Connected the pump directly to 12V input, it works. Connected a multimeter in series and found the current draw to be approx 3A to start and then dropping down to 1.8A;
2. Connected an LED via a suitable resistor across the output side of the optocoupler (no mosfet/pump) and it turns on and off depending upon logic input to the decoder;
3. Removed LED and measured the voltage across the output side of the opto-coupler to be ~12V;
4. Added the mosfet and measured the voltage across the drain and source with correct logic input to decoder. This measured ~11-12V;
5. Added pump (w/o flyback diode). Now with correct logic input to decoder, it does not work;
6. Connected a multimeter (in current reading mode) in series between the cathode of the pump and drain. It measures only 0.63A;
7. Replace the pump with LED and proper resistor and enable the decoder, the LED lights up.

It appears to me that connecting the pump via the MOSFET somehow impedes the current flow to a great extent even when it is biased according to specs.

I thought that the MOSFET in question is a voltage controlled device. So when a proper $V_{gs}$ is applied, it should simply conduct the current required by the load.

Why does it conduct only 0.63A when the starting current requirement is around 3A? Am I missing anything?

• What Vgs of the MOSFET can/did you measure? – Fizz Oct 14 '15 at 19:29
• Breadboards are not designed for carrying currents of that magnitude. Try soldering the cables for the pump to the MOSFET and power supply directly. – CharlieHanson Oct 14 '15 at 19:30
• I doubt that's the main reason though. For the breadboard to drop the current to about 1/5, it would have to have a lot of resistance (around 15 ohms), and would have started to smoke with 10W dissipation. – Fizz Oct 14 '15 at 19:51
• I assume you have a heatsink on your FET ? 36watts is enough to damage it. Also try checking the FET with the diode checker on your Multimeter and make sure it still functions and you have your pin orientation right. – BenG Oct 14 '15 at 19:55
• Ok, I connected my multimeter directly across the mosfet before & after the 5.2K resistor. Assuming Iam measuring Vgs correctly and with the decoder enabled, I see that Vgs starts at 10.68v and slowly rises to 11.25. I stopped there because I could sense that the mosfet has become very hot. My apologies to @BenG, I wasnt looking close enough the last time. – Vijay Seshan Oct 15 '15 at 7:14