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Hi I'm looking to turn on a 12V power source. Requirements are:

  1. Switching on the +ve side of the load (I can't find a mofset that can do this with a 3.3V control voltage)
  2. Max peak current is 2 amps maybe 3 to be safe.
  3. I want the switch to be efficient and not waste power.

The control signal is 3.3V. I'd prefer 0V control = on and 3.3V off but it can be reverse if needed.

I've looked at Mosfets but this can't work switching the +ve side of the load.

What hardware should I use? Should I consider an IC switch.

OK. So this is what I ended up with. A p-type mofset and 2x NPN Transistors. 1 NPN acts as a not gate and the other switches the mofset. Thus the mofset is open when the control voltage "PWR-DIABLE" = 0 and the mofset is closed when PWR-DISABLE = 3.3V.

Did i get it right? Seems to work in the sim.

enter image description here

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  • \$\begingroup\$ Welcome! Please follow schematic best practices and use transistor symbols for transistors, not blocks. Draw positive voltages pointing up, ground and negative pointing down. Signal flow is from left to right. \$\endgroup\$
    – winny
    Commented Jan 15 at 7:41

2 Answers 2

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Here is a schematic grab of the most common way to do what you describe, from this site:

https://itecnotes.com/electrical/electrical-pmos-high-side-switch/

enter image description here

The 5 V source can be replaced by a 12 V source. Also, R1 can be increased to 10 K to reduce the base current. R3 represents the load device or circuit, whatever is being powered through the switch.

When the I/O signal is low, both transistors are off and there is no 12 V current other than off-state leakage through Q1 and M1.

Q1 Can be almost any small signal NPN transistor, such as a 2N4401, 2N2222, etc. M1 should be rated for at least 25 V and 2x the peak load current.

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At 3.3V, there's a lot of bipolar transistors and a few MOS that work well. I'd do this

schematic

simulate this circuit – Schematic created using CircuitLab

At zero V out, the output should handle 100 mA or so. Some LDO regulators, with enable inputs, might work too, like MCP1804 regulator

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  • \$\begingroup\$ :-) I was going to add a comment on Tim's answer that noted you could drive the emitter and connect base to 3V3 to achieve the inverted logic that he wants - but you beat me to it. This topology makes some people's head explode :-). \$\endgroup\$
    – Russell McMahon
    Commented Jan 14 at 11:08

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