Author Topic: Switching 12V with 28V, 12V with 3.3V (mosfet?)  (Read 1271 times)

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Offline namlu

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Switching 12V with 28V, 12V with 3.3V (mosfet?)
« on: February 18, 2015, 08:50:27 pm »

Both 12V output voltages should be able to drive at least 1A load. Load is unknown. Should be like "voltage source". As if I put voltage regulator / relay.

1) I want to switch 12v with 28v pin. 0v output if 0v input, 12v output if 28v input. I made it with voltage divider of 28v to 12v, and then a simple CMOS buffer (two cmos inverters - nmos and pmos x 2). Seems to work. Is it ok that I used voltage divider, or voltage divider should not be used in "real circuits"? Any other way to do it?

2) switching 12v with 3.3V FPGA output. How it is done? I could not figure out. In pmos - if vg=0 or vg=3.3, and vs=12, both times vgs<vthreshold. Any good way to do it?

Thank you.

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Offline suicidaleggroll

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Re: Switching 12V with 28V, 12V with 3.3V (mosfet?)
« Reply #1 on: February 18, 2015, 09:03:37 pm »
2) Use an NMOS to isolate the line.  PMOS gate is pulled high and connected to NMOS drain, NMOS source connected to ground, NMOS gate is pulled low and connected to FPGA pin.  FPGA pin is low or high impedance, NMOS is off, PMOS gate is pulled high, PMOS is off.  FPGA pin is high, NMOS is on, PMOS gate is pulled low, PMOS is on.  Just make sure you use an NMOS with a Vgs threshold below 2v or so to ensure it's good and on at 3.3v.

You could do the same thing for #1 as well, provided the NMOS you use can tolerate a Vgs > 28v.

Offline Whales

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Re: Switching 12V with 28V, 12V with 3.3V (mosfet?)
« Reply #2 on: February 19, 2015, 12:25:22 am »
For (1) I presume the last buffer is supplying the output power, so the voltage divider is fine as it won't be affected by the load you put on the output. 

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