Switching negative voltages with a transistor

[dr pepper]

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]

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

 

[AnalogKid]

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

 

[MrDEB]

go with KISS method
Keep It Simple Stupid
one opto isolator

 

[MrDEB]

here this describes how to do it using either transistors or opticoupler (see page 3 or 4?)
https://www.latticesemi.com/~/media...swithPowerManagerDevices.PDF?document_id=6413

 

[MikeMl]

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

 

[crutschow]

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.

 

[spec]

ISSUE 05 0f 2016_11_04

The transistor and MOSFET sub circuits can be mixed.

DATA SHEETS
https://www.fairchildsemi.com/datasheets/BC/BC546.pdf
https://www.onsemi.com/pub_link/Collateral/TIP35A-D.PDF
https://www.fairchildsemi.com/datasheets/FD/FDN336P.pdf
https://www.infineon.com/dgdl/irfs7534pbf.pdf?fileId=5546d462533600a4015364c3e2c629c7

 

[AnalogKid]

Something *without* a common base amplifier.

ak

 

[spec]

snap

spec

 

[AnalogKid]

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

So there.

 

[spec]

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.

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

spec

 

[MrDEB]

Did you even look at page 2 and 3. It has schematics that are proven to work and are simpler to build (less components)
https://www.latticesemi.com/~/media...swithPowerManagerDevices.PDF?document_id=6413

 

[dr pepper]

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.

 

[spec]

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

 

[Colin]

Here's a simplified circuit

 

[crutschow]

....... 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]

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