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I need a bit of help for a lighting project I am undertaking. I have a couple of 12 volt DMX LED controllers driving various zones in a bathroom with LED strip.

I want to replace the controlbox for the LEDs in the bath and drive it via my DMX system. The output of the RGB bath controller is 5 volt per channel.

How can I reduce the output voltage from a 12 volt DMX RGB controller to be 5 volts?

Note the output voltage will need to vary based on the change of input voltage as the controller works through the various the various RGB colours.

Thanks for any input in advance,

Olly.

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  • \$\begingroup\$ Can you have a separate 5 V supply? If yes, then all you need is three transistors and three resistors. \$\endgroup\$ – winny Mar 18 '18 at 19:25
  • \$\begingroup\$ Hi Winny - I could easily add a 5V supply if needed. What would your proposed idea look like? Apologies, I never got to grips 100% with electronics at school and years later wish I had!!! \$\endgroup\$ – Olly Thomas Mar 18 '18 at 20:02
  • \$\begingroup\$ I’m mobile so no schematic editor but take a look at electronics.stackexchange.com/questions/124526/… and replace your 12 V in the top right hand corner with 5 V and you are pretty much done. \$\endgroup\$ – winny Mar 18 '18 at 21:07
  • \$\begingroup\$ thanks Winny. So I take it, the PWM in would be my variable voltage (up to 12V) feed, and the 5v would just go to ground through the transistor. Do I need R1? if so, how would I calculate what resistance it should be? I know the 5V LEDs are rated at 0.6 amps based on the manufacturer details. \$\endgroup\$ – Olly Thomas Mar 19 '18 at 13:29
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If you can "inject" 5 V from a separate source, you can build your own low-side PWM like this:

EDIT: New schematic due to lowside PWM from your driver. This is kind of dirty. If you want to improve the efficiency and probably dimming at very low duty-cycles, you should consider an inverting gate driver instead of R1 and M1.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Wow - thanks that's amazing! I'll head out to Maplin tomorrow to buy my first ever transistors! and try to make this up :o) \$\endgroup\$ – Olly Thomas Mar 19 '18 at 23:16
  • \$\begingroup\$ @OllyThomas You're welcome but hold your horses. If your 12 V PWM output isn't a true 12 V driver but rather another low side PWM, you will need pull-up instead of pull-down. If you have a wireing diagram for it it would be easy to determine. \$\endgroup\$ – winny Mar 20 '18 at 8:26
  • \$\begingroup\$ Hi Winny, Not been shopping yet! This is the controller I am using: ultraleds.co.uk/… and this is the spec sheet for it: ultraleds.co.uk/fileuploader/download/download/… Do appreciate you taking the time to look at this for me! Olly. \$\endgroup\$ – Olly Thomas Mar 20 '18 at 13:59
  • \$\begingroup\$ Ah! As I expected! Not a true 12 V output but rather low side PWM. I'll update the schematic for you to reflect this. \$\endgroup\$ – winny Mar 20 '18 at 14:48
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    \$\begingroup\$ Hi Winny just wanted to thank you again and to let you know that it's working fine with the +5V from the transformer using the low side switching on the 12V LED controller. Thanks again for your help - I really do appreciate it. What a great thing the internet is to put a question out on a forum like this and get it answered and resolved so quickly. Plus - I've learned something new! Great community. Thanks again, Olly. \$\endgroup\$ – Olly Thomas Mar 22 '18 at 18:03

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