# How to generate a -24V signal

I have to interface a specific motor, it has a weird driving board, I was in contact with the technical team from the manufacturer, and they told me that I have to send a -24V on a pin to allow it to turn. I have a +24V power supply, several step down modules, on 5V and 3.3V for my microcontrollers and other things.So here is my question: What is the simple way to produce a -24V voltage source from a 24V or lower one? I have searched a lot, but found nothing... My first idea was an inverting opamp, but if I were able to power it down to -24v then I would'nt need it at all :)

Any Ideas are welcome.

My ideas are now oriented to step down ICs able to output negative voltage from a positive one... I guess something around -18V should be enough to be understood by the driver.

• how much current do you need on the -24V? Commented Jul 26, 2016 at 9:36
• google polarity inverting buck boost e.g. ti.com.cn/cn/lit/an/snva022e/snva022e.pdf Commented Jul 26, 2016 at 9:57
• Are you sure that this is merely a signal and not a power supply rail? Commented Jul 26, 2016 at 10:03

Linear tech produce a number of inverting switchers like this: -

Check the data sheet to see if it can produce -24V. If not then use the search engine on this page here

There's also this one: -

• No, it can't. Its specification is 45V maximum between Vin and ground. Commented Jul 26, 2016 at 10:05
• @OleksandrR. I did say in my answer to check the data sheet to see if it can produce -24V. Anyway I've added another option. Commented Jul 26, 2016 at 10:06
• Yes, and I checked it and found it's not suitable. The second option is better and I upvoted your answer. I wonder, if not the datasheets, where are you getting these images from? Commented Jul 26, 2016 at 10:07
• @OleksandrR....... enter your search phrase into google then select images from the top line! Is anyone of these pictures you: google.co.uk/… Commented Jul 26, 2016 at 10:09

Assuming the -24v is a control input, so doesn't need much current, and only needs approximately -24v, then a cheap, quick and dirty way is to use a charge pump. Like this.

simulate this circuit – Schematic created using CircuitLab

What's drawn as an opamp is any circuit capable of making a rail to rail square wave.

The output is unregulated, and drops with increasing current.

If you do need exactly -24v, then you can use a 2 stage charge pump and regulator.

If you need more current than a few mA, then it's worth going to a magnetic solution, an inverting boost, rather than a charge pump.

• It takes an oscillator to drive "OA2", and diodes D1 and D2 are reversed. As shown, this would generate positive 24V. Commented Jul 26, 2016 at 19:42
• OA2 could be the oscillator, that's not a full circuit of course. Diodes the wrong way round ... I'd like to say 'congratulations, you're the first to spot my deliberate mistake', but I must actually say 'doh! facepalm'. Commented Jul 26, 2016 at 20:12

An easy way is to use a DC-DC converter with an isolated output. For example, an PDS1-S24-S24-S. This particular one, while cheap, has a minimum output load requirement of 4mA so add an LED indicator or whatever if your load is insufficient (the consequence of insufficient loading is that the magnitude of the output voltage rises above the expected value).

What is the simple way to produce a -24V voltage source from a 24V or lower one?

You don't have to produce a new source. Because +24 and GND/0V is just a name for a potential we can switch the postitive and negative potential to get a negative voltage. Connect the "0V/GND" for your -24 Voltage to +24 and your -24V connector to 0V/GND.

• It's probably not that simple. If the motor needs positive voltages, too, this trick won't work. But generally spoken you are right. Commented Jul 26, 2016 at 12:15