2 years, 7 months ago.

24VDC Supply to 5VDC

I have a 24VDC supply which needs to be lowered in my sub-circuit to 5VDC to power my mbed and Quadrature encoder? What is the "standard" (if any) way of accomplishing this?

You may be long past this stage, but a part I've used with the mbed is this a 3-pin switching power supply. It can handle 36v input and create 5v. The part number is OKI-78SR-5/1.5-W36-C and the link is to that part on DigiKey.

My Project was a general purpose baseboard to which I've also powered displays. You might find helpful ideas on the schematic - including how simple that 5v regulator is to use.

posted by David Smart 03 Mar 2018

2 Answers

2 years, 7 months ago.

Hi E B.

If you are doing your own electronic design, then you might consider a small switching power supply chip, coupled with a few external parts.

  • How "clean" is your 24vdc supply? If it isn't well regulated, it might have 26 or more volts as a peak (and anything you attach has to handle that).
  • What is the current requirement for your mbed?
  • What is the current requirement for the encoder?
  • What else requires power - a display, a serial communication link, ethernet, and so on?
  • What output voltage do you want? 5v, 3.3v, or something else?

For spinning your own design, you might browse the possibilities here:

I hope this helps, and good luck.

Thanks for your reply.

1) 24vdc from a standard adaptor transformer (e.g. https://www.amazon.co.uk/MINGER-Adapter-Transformer-Flexible-Lighting/dp/B01MY96ZQE)

2) How would I determine this?

3) Ditto.

4) Oled display and ethernet. Also need to provide +/-10vdc for op-amp (no idea how this would be done...)

5) Output of circuit is +/-10vdc

posted by b4039 b 03 Feb 2018

Power Supply: It isn't well described on the label, so I would be conservative and assume it might get to 26 or even 30v.

  • Your need for +/- 10v is an important first element. While the mbed runs best on about 5v, and your display is probably 3.3v or 5v, so those are also likely the "high current" part of your need.

From a single-ended supply of 24v, there are a several ways to go to get +/-10v and +5v. Here's two.

  1. Split the supply with a virtual ground, and you'll have your +/- 10v. Then from the +10v add an additional regulator for the 5v.
  2. Regulate from 24v down to 5v, supporting the high current needs, and from there create the +/- 10v from the 5v. if your +/-10v needs are in the low mA range I would go in this direction. In a quick web search, I came across this Maxim part which supports up to 10 mA. There are others I'm sure with a bit higher capability.

There are many other combinations that could also work.

Current consumption:

  • LPC1768 draws about 150mA, and closer to 200 mA when Ethernet is enabled. If it is "well documented", I don't know where, but I've used that mbed module, studied the data sheet and that of some of the other parts, and measured it in my own circuits. [I highly encourage you download the mbed schematic, then the data sheets for each major component and sum them up, but this is probably close enough] And a good input voltage is 5v. This mbed has a its own voltage regulator, but if you feed it 6 to 9v it will get hot and is more likely to fail, so better to feed it from a 5v supply.
  • OLED display - you should try to find out its current requirements, it should have a data sheet. What voltage is the display specified to operate on?
    • OLEDs tend to require roughly the same current as a comparable LED backlight display, but depending on the size, it could be below 50 mA or more than 500 mA.
    • For the signals between the mbed and the other devices (display and sensor), you should make sure they are voltage compatible. The mbed micro runs on 3.3v, so the display should be 3.3v compatible for its signals.


  1. I think your power supply, after knowing the voltage and current requirements, is key to address first.
  2. From that you'll have derived +/-10v and +5v, supporting the current requirements of each.

Web links were from simple internet searched, I do not have hands-on experience with the parts in those schematics.

posted by David Smart 03 Feb 2018

The +/-10VDC will supply power to an op-amp, the result of which will be a +/-10VDC control signal fed into a VFD - not sure what the current requirements would normally be.

The 5vdc will also be used to power a quadrature encoder.

How is it possible to turn 5v into +/-10v? Seems like you'd be creating voltage out of nothing...

posted by b4039 b 03 Feb 2018

Internet search is your friend ;)

By searching for "voltage doubler circuit", I came across this - https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/voltage-multipliers/

As a tutorial, this shows an AC supply. In the Maxim data sheet (referenced above), it has a figure that shows how it works with a DC input and its own internal oscillator to switch the power to first charge a capacitor toward 5v, and then switch it to stack it on top of the 5v rail to create 10v. It does much the same thing to create the negative voltage.

posted by David Smart 03 Feb 2018

Sorry, need to learn to be less lazy :)

The link states you need an AC supply though. Don't most transformers supply DC? Or what if I wanted to power the circuit with a battery? Can the mbed run on AC?

posted by b4039 b 03 Feb 2018

I'll probably be using an AC/DC adapter like https://www.circuitspecialists.com/gfp151u-24volt.html

posted by b4039 b 03 Feb 2018
2 years, 7 months ago.

I assume you want to have ready to use modules? If you want to make your own PCB you can go for the direction where you design it yourself, but otherwise you can just search for DC-DC on ebay or one of the many electronics hobby stores. As example: https://www.adafruit.com/product/1065

Simple PCBs which contain DC DC converters down to 10V can be found on ebay for example. Now the general reliability of random chinese boards from ebay is not that high in my experience, so I would never use something from there for your mains adapter for example, or a real product. But just for your hobby project and after a decent quality power supply I wouldn't worry too much about it.

In general, how would you go about making your own PCB?

posted by b4039 b 03 Feb 2018

By googling for a tutorial, although in general especially since if you have some patience PCBs are very cheap to get, start with something simple. And take into account you also need to be able to solder it, some people can solder everything, others will have trouble with especially smaller SMD components.

posted by Erik - 03 Feb 2018

Sorry, should have been more clear. What components would I need?

posted by b4039 b 03 Feb 2018

For a DC DC converter? Well David showed some ICs you can use, and they will have application diagrams in their datasheet.

posted by Erik - 03 Feb 2018