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Switching negative voltages with a transistor

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dr pepper

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I want to switch a negative voltage with a microcontroller, the controller runs from 0 and 5v, and I need to switch -12v 100ma.
I got the thing to work with a opto isolator, but it doesnt seem very 'elegant', what would be a good way to do this with a transistor or 2.
 

MrDEB

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IMO the opto isolator is a safe way to interconnect a micro with a high voltage or ??
 

AnalogKid

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One PNP transistor driven by the uC, one NPN transistor as the switch, and 3 resistors. 2N4403 and 4401, or 2222 and 2907, or 3906 and 3904.

ak
 

MikeMl

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Transfer function showing switching point. X-axis in the plot is V1. Y-axis shows V(out) and current from PIC port pin.

d3.jpg
 

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crutschow

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Here's the LTspice simulation of a two transistor circuit to switch a negative supply voltage from a positive signal (which I think is what you wanted).
The N-MOSFET can be just about any device with a low enough ON resistance.

upload_2016-11-4_0-50-43.png
 

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AnalogKid

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Oh yeah?!? Well...mine was done first but I had to go to a company meeting before I could post it.

So there.
 

spec

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Oh yeah?!? Well...mine was done first but I had to go to a company meeting before I could post it.

So there.
Well, mine was done before the OP even posted so there.:joyful:

Besides, my schematic is in color and yours is not!:hilarious:

spec
 

dr pepper

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Great thanks all.
I'd had the idea of common a base tranny as a switch but couldnt remember the circuit.
I want to switch the low side rather than the high side, looking at mikes post #6, all I need to do is mirror the circuit around Q2 and use n Npn tranny with its emmitter tied to v-, and have R1 inline with its base.
What does D1 do?
The msofet is a good idea too, I'd not thought about fets, that Aon device is pretty good at 18mohm on resistance.
 
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spec

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Here's the LTspice simulation of a two transistor circuit to switch a negative supply voltage from a positive signal (which I think is what you wanted).
The N-MOSFET can be just about any device with a low enough ON resistance.

View attachment 101980
Cruts, neat circuit, but you need to cut the voltage drive to the AO6408 because the +12V max VGS is exceeded.

spec
 
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crutschow

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....... neat circuit, but you need to cut the voltage drive to the AO6408 because the +12V max VGS is exceeded.
I'm Wally. :)
Most MOSFETs have a 20V rating and I didn't check that one in particular.
But certainly you would need to add a base resistor for that MOSFET to reduce the maximum Vgs.
 

spec

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I'm Wally. :)
Most MOSFETs have a 20V rating and I didn't check that one in particular.
But certainly you would need to add a base resistor for that MOSFET to reduce the maximum Vgs.
I'm a Walley- I got the wrong poster.
A&O seem to make some decent MOSFETs: are they OK? Never used them myself.

spec
 
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