24v to 12v Converter

Discussion in 'General Electronics Chat' started by Transatlantic, Jun 24, 2018.

  1. Transatlantic

    Thread Starter Member

    Feb 6, 2014
    44
    0
    I bought a 24 to 12v (20A max) converter to be used with my project, which is a remote controlled vehicle using two 12v windscreen wiper motors running off an Arduino and two BTS7960 43A High Power DC Motor Drivers. I am using 24v as most parts of the project use 24v, but those are not enabled for this issue.

    The problem I am having is that when I do a quick forwards to reverse transistion, the power coming from the converter dies for 3 seconds and then returns. I know that it is the converter as I have a 12v light attached to it, and this turns off too.

    I assume this is because there is a current spike due to the quick transistion, and must be over the 20A max limit?

    Is this normal for a converter to switch off like this when there is a current surge over such a small amount of time?
    Usually the current being drawn when the vehicle is going forwards at full speed is around 5A.

    What kind of converter could I use that won't do this?

    https://www.ebay.co.uk/itm/DC-24V-t...n-Power-Supply-Regulator-For-Car/182676901277
     
  2. -live wire-

    Active Member

    Dec 22, 2017
    873
    73
    My impression is that you need to step 24V down to 12V, for a 5A motor. The motor has inrush currents that you must deal with. Often motors have very high inrush currents. Ideally you would use a supercapacitor or other thing that can supply the inrush current. But one more practical solution is a constant current start-up. You can get yourself a buck converter like this to easily do that. I actually got one for other purposes and can say that it seems to meet up to the claims and is a good product.
     
  3. crutschow

    Expert

    Mar 14, 2008
    23,091
    6,849
    Putting a slight pause (if you can) between the forward to reverse transition might help.

    Otherwise you can add a current-limit circuit to the converter output to limit the current to below 20A.
    That should prevent the converter from shutting down during the high motor inrush current.
    Below is the LTspice simulation of a simple current-limit circuit that should work for that purpose:

    upload_2018-6-24_14-1-1.png
     
  4. Transatlantic

    Thread Starter Member

    Feb 6, 2014
    44
    0
    Thanks for the replies guys I think I might try the buck converter first as they are quite cheap. Managed to find one on ebay UK

    https://www.ebay.co.uk/itm/300W-12A...-7-32V-To-0-8-28V-Battery-LED-HQ/263482557484

    I take that will just replace the converter I originally bought? what is the difference between what I bought and a buck converter?

    Should I use one for each motor? or can one be used for both? It states 300w, which is more than powerful enough, but just checking.

    Also - I would really like to try and measure the inrush current to see how high it actually is. The clamp on power meters in my price range (~£60) state they will only measure 100ms increments. I assume that is no where nearly fast enough?
     
  5. -live wire-

    Active Member

    Dec 22, 2017
    873
    73
    If you get RMS it will get the average current (thought this still may not be the whole picture). If you have a scope you can use a shunt and measure the current that way.
     
  6. Transatlantic

    Thread Starter Member

    Feb 6, 2014
    44
    0
    OK so the buck converter came today. I configured it to run at 12v when powered by 24v and set the current max up as far as it would go, as I want to limit it to the max of the converter.

    I hooked everything up as per the original circuit, and it worked for a bit and then blew the Arduino? :(

    The Arduino was being powered off the same 12v output from the buck converter that the motors are connected to. Should I not have done this? What happened?

    Luckily I had a spare Arduino to replace the broken one, but this time I gave it, it's own power supply (PP3 battery), and it all works.
     
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