Variable Voltage Output from 24v 20A PSU

Discussion in 'The Projects Forum' started by ajhalls, May 12, 2016.

  1. ajhalls

    Thread Starter New Member

    May 12, 2016
    I have a few 24V 20A PSU ( and was going to build an enclosure to put one in with bunch of different pinch terminals to make it easy to use for testing things out.

    I am getting started playing with stepper motors using a SmoothieBoard (which is awesome) but I need 24V for the motors, and 5V for the processor on the board. I was thinking it would be nice to have one set of pinch terminals be 24V, one at 12V, and one at 5V, and / or to have one on a variable voltage that you could dial in.

    I had some SCR ( that I had tried to use for a different project (but ended up burning most of them out), I thought I had read here that I could use one of those, but hooking up a voltage meter isn't showing me positive results. Maybe because it was designed for AC, maybe because it is a 2000W, not sure.

    I don't really know what the needed amperage is for each - since it will really just be a hobby PSU to plug random things into, so resistors are out, plus everyone here seems to hate the idea of resistors as they generate heat.

    What is the best way to proceed?
  2. Marcus2012


    Feb 22, 2015
    Hi :)

    Probably best just leaving these supplies as they are, as 24V supplies are useful to have and expensive. If you wanted a bench top supply for hobby/entry level use you may want to consider a modified ATX computer PSU which has 3.3, 5 & 12V rails and around 20A protection on it. There are lots of guides on how to modify ATX supplies and if you search the forum I'm sure you'll find many threads.
  3. ScottWang


    Aug 23, 2012
    1. How much current does the stepping motor need?
    2. You could using LM2576/3A to reducing the power dissipation, you can get +5V, +12V or adjustable voltage.
  4. ajhalls

    Thread Starter New Member

    May 12, 2016
    Thanks guys. Marcus, I wasn't thinking of modifying the actual 24V PSU, just adding some external circuitry in between the supply and the pinch clamps. I do have a bunch of ATX supplies and have used them in the past, I was just hoping for a smallish box because I tend to run it between work and home a lot. I went to a thrift store and was blessed to find a little 1A power supply that puts out both 12 and 5V. It isn't a ton of amps, but it gives me the 3 voltages I need, and I can certainly run some Raspberry Pi projects on that, but I may look for something online like this: to get a little more later.
  5. ajhalls

    Thread Starter New Member

    May 12, 2016
    So yesterday I got a copy machine at the thrift store for $10. It was reporting a paper jam, so I starting taking it apart:
    I never found any paper. Just teasing, my employee took this to post on facebook to joke about what his boss did at work that day :) I was after the stepper motors, gears, polished shafts, springs and other hardware. This morning I was looking at some of the chips on the boards I removed and had saved so I could get the bridge diodes off (love those).

    After googling a few of the chips, I found this one:

    Wouldn't this be easy to setup with a potentiometer to dial in the voltage I want?

    I also found a couple of these: which are like what Scott suggested to give dedicated 3.3V, but just curious what I don't know - and why something like the LM117 wasn't suggested.

    The project I am building will need 24V for the stepper, 12V to power the CCFL Inverter, 5V for the Raspberry Pi, and 3V for any arduino things I throw in. Seems like I need a combination of dedicated voltage ports, and a variable one for little projects like I was doing last night where I needed 0.5-1.5V for doing some nickel plating.

    Maybe I am asking too much of a single box, it just seemed easy to take a high amp 24V and step it down a few times. Maybe I should look at using one of my extra ATX supplies as the starting point and boosting one of the 12V to 24V with one of these: instead of going the other way.
  6. ScottWang


    Aug 23, 2012
    I already told you the reason - using LM2576/3A to reducing the power dissipation.
    When you using the LM117 then you will get more heat and it will need the heat sink more big, why is that, like these 24V - 12V = 12V, 24V - 5V = 19V, 24V - 3.3V = 20.7V, waste voltage as 12V, 19V, 20.7V, when the output voltage more lower then it will get more heat, like when the output voltage just only 3.3V and then 20.7 V will turn to heat for nothing, not for load.

    This is related to the DC to DC conversion efficiency, you will hear the eff% after you read this post, you can read the links several times about LM2576/3A/52Khz, LM2596/3A/150Khz and you will get some more conception,
  7. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    The 24v has a pot on the side to trim it possibly by a couple of volts, these atx psus can be modified to be variable, you need to see what chip they use inside, otherwise use a Lm2596 or other buck down converter.
  8. MaxHeadRoom


    Jul 18, 2013
    I am guessing that is a Chinese wrong interpretation, they appear to be Triac AC only controllers.
    Only one power semi!
  9. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    Linear regulators are conceptually and physically simple, but they do get how. Switching regulator modules on ebay are cheap, and some have built-in digital voltmeters. Add a few external filter caps and you can have a bank of fixed outputs running from one 24 V source in very little time.