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I have two regulated 12V 12A power supplies that I want to use to provide switching power to some resistive loads. I currently have them hooked up like this:

schematic

simulate this circuit – Schematic created using CircuitLab

My problem is that when I plug them into mains, the supplies don't consistently turn on. Even if the microcontroller's PWMs are set to 100% low, the power supplies click and their status lights flicker as if they are being overloaded.

The MOSFETs are working, nothing is shorted and each PSU gives 12V when not hooked up to anything. What am I doing wrong?

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    \$\begingroup\$ What do you mean 'nothing is shorted'? I see four MOSFETs shorting your power supplies to ground. Are you sure the circuit diagram is correct? \$\endgroup\$
    – jippie
    Jul 16, 2013 at 15:47
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    \$\begingroup\$ It looks like you are shorting out your power supplies when you activate the MOSFETS. \$\endgroup\$ Jul 16, 2013 at 15:49
  • \$\begingroup\$ Yes, apologies! I mangled the circuit the first time \$\endgroup\$
    – user10256
    Jul 16, 2013 at 15:56
  • \$\begingroup\$ Are those resistors actually 3 ohm? Does the same thing happen if you were to replace the resistors with much higher ones? \$\endgroup\$ Jul 16, 2013 at 16:19
  • \$\begingroup\$ How many goes have you had at getting the circuit correct? I'm beginning to suspect you might not have got the actual wiring correct given you have had what appears to be two iterations on the schematic LOL. Check your mosfet wiring is correct and that you are using N channel devices (not P). \$\endgroup\$
    – Andy aka
    Jul 16, 2013 at 17:56

2 Answers 2

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Since the question has changed, here is a new answer.

I won't state the obvious, that each 3 Ohm resistor would need to be 48 Watts minimum, preferably 100 Watts, which means it needs to be a huge wirewound thing. ... Or a 50 Watt 12 Volt lightbulb. If the resistor is of lower wattage, the PSU won't fail, the resistor will vaporize when powered.

Also, the question does not specify which MOSFET is being used. A datasheet would help figure out whether the MOSFET in question is capable of handling the load at all, or has melted inside and bonded into a short somehow.

Sequence of steps to test:

  1. Preparation: Check if the ground line of the PSU and of the microcontroller are connected together. If not, make that connection. Also add a hefty (1000 uF 18 Volts minimum, more capacitance is better) capacitor between the supply and ground, as close as possible to each set of resistor and MOSFET, + side at resistor's connection to +V, - side at MOSFET source connection to GND.
  2. Power up one PSU with just one 3 Ohm resistor connected between +12V and ground return lines on the PSU. Does this cause the same click/overload? Then the PSU's current capacity is less than 4 Amperes, or current limiting is set for less than 4 Amperes.
    • Is the resistive load getting warm / hot? If not, it is most likely not passing the 4 Amperes calculated from the resistance.
  3. Power down, add a second 3 Ohm resistor in parallel (+12V -- Resistor -- GND), and power up again. Click / overload? Then the PSU cannot handle 8 Amperes current, or current limiting is set for less than 8 Amperes.
    • Are both resistors getting hot? If not, the PSU voltage is sagging due to overload. You need a more powerful PSU.
  4. Power down, hook up one MOSFET with gate and Source both hooked to Ground, Drain to the 3 Ohm resistive load, as shown in schematic at the end of this answer, power up again.
    • The resistor should not get warm this time, the PSU should not have flickering status lights. If the resistor is warm, the MOSFET is shorted, or is hooked up wrong. Check the datasheets, check the pin-outs.
  5. Power down, add the second MOSFET (gate shorted to ground) and resistor, power up again. Should have same results as above.
  6. Power down, disconnect the MOSFET's gate from ground and connect it to the microcontroller's output through a 100 Ohm resistor.
  7. Fire it all up, output a 50% duty cycle PWM from the microcontroller. The resistor should get warm, but not as warm as step 2 above. Hook up a multimeter in DC voltage mode between MCU output and ground. It should read around half the microcontroller's supply voltage.
  8. If any of this fails, then share the MOSFET datasheet, the PSU datasheet, and a photo of the hooked up circuit.

    schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ shhhh don't state the obvious ;) \$\endgroup\$
    – Andy aka
    Jul 16, 2013 at 19:57
  • \$\begingroup\$ Thank you very much! The problem turned out to be that not everything was properly grounded. \$\endgroup\$
    – user10256
    Aug 13, 2013 at 20:10
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The power supplies are being overloaded!

What you have is this:

schematic

simulate this circuit – Schematic created using CircuitLab

In other words:

  • When the PWM signal is high the 12 Volt rail is being shorted to ground through the MOSFET.
  • When the PWM signal is low, the 12 Volt rail is driving 4 Amperes through the 3 Ohm resistor.

If the MOSFET's Rds(on) is, say, 0.1 Ohms, then during the MOSFET on stage, that is 120 Amperes from supply to ground, limited only by the power supply current limiting / overload protection.

What you need, instead, is reshuffle each block thus:

schematic

simulate this circuit

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  • \$\begingroup\$ Yes, this is the way it is currently set up. I didn't make the schematic right \$\endgroup\$
    – user10256
    Jul 16, 2013 at 15:58

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