Simplest way to switch a negative voltage using logic level signals

Thread Starter

dannybeckett

Joined Dec 9, 2009
181
Hi guys,

I have the need to switch a -36V load on and off, driven by a 3.3V microcontroller. A relay is overkill - what's the easiest way to achieve this using solid state devices?
 

dl324

Joined Mar 30, 2015
11,530
You could use a transistor or two.

You'll get better responses if you give more information. What is the drive capability of the microcontroller output and what is the load current? Do you want to drive high side or low side?
 

crutschow

Joined Mar 14, 2008
25,423
Below is the LTspice simulation of a negative voltage, high-side switch that uses one PNP BJT and one N-MOSFET, plus three resistors..

1598379846964.png
 
Last edited:

Thread Starter

dannybeckett

Joined Dec 9, 2009
181
Thank you guys for the very rapid help!

The application is specifically for the LM3886 mute circuit.

https://www.ti.com/lit/ds/symlink/lm3886.pdf

Page 6 of the above datasheet shows Rm, Cm and S1 forming the external mute circuitry, and the table on page 8 (items 12 and 13) detail what those components do. I'm choosing 33k and 100u to a -36V supply rail for a filter that will settle at about -1mA coming out of pin 8 on the LM3886.

The control will be coming from the pin of an STM32H747 microcontroller, which can:

"The GPIOs (general purpose input/outputs) can sink or source up to ±8 mA, and sink or source up to ±20 mA (with a relaxed VOL/VOH)."

@crutschow unfortunately your attachment 404's when i click it, mind reposting it or showing us a screenshot?

Thank you!
 

Thread Starter

dannybeckett

Joined Dec 9, 2009
181
Attachment should now show.
That's really cool. Thank you for your voluntary effort. I have always been a little confused when I see a control signal on the emitter of a transistor. I will simulate the circuit myself and have a play to learn. I'd also like to offer thanks for the absolute massive contribution you have personally provided this forum - are you a staff member, or someone who simply wants to make the world a better place?

Dan
 

crutschow

Joined Mar 14, 2008
25,423
I have always been a little confused when I see a control signal on the emitter of a transistor.
The grounded-base transistor is non-inverting with a low input impedance, and no current-gain, only voltage gain.
So current into the emitter (for the PNP), will cause transistor action, with a small base current to ground, and the rest of the current going to the collector.
This current then makes the collector voltage more positive (less negative).
In this case the collector voltage goes from -36V with no input, to near 0V with a 3.3V input.
are you a staff member, , or someone who simply wants to make the world a better place?
Not a staff member.
I don't know if it's making the world a better place. :rolleyes:
I'm just a retired electronics engineer who enjoys solving a circuit design problem to generate one that performs a function the TS needs, with as simple a circuit as possible.
Sorta like some people like to solve crossword or jigsaw puzzles.
 

ronsimpson

Joined Oct 7, 2019
932
plus three resistors..
You used R1,2 as a voltage divider to get the Gate voltage right.
I used R4, Q1 as a current source that puts 5X voltage on R1. The actual voltage of V2 is not important or even in the formula as long as V2 is larger than M1's turn on voltage.
1598401144611.png
R2 could be removed.
 

Papabravo

Joined Feb 24, 2006
14,412
Anytime you use a MOSFET as a switch, be careful that you don't try to turn it on and off at too great a rate. For this circuit if you turn it on and off much faster than about 430 kHz it will stay on constantly because the gate doe not have enough time to discharge. In the time domain you need to wait about 3 milliseconds before trying to tun it off.
 
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