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DC DC converter assist

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Pete Laabs

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Hello, I would like to build a converter but am in need of assistance. Input will be -42 to -56 volts dc (-48 Telephone DC Plant) and would like the output to be either +24 volts dc, or +12 volts dc, maximum of 10 Amps. I am wanting to use the reduced voltage(s) to drive LED strips (10 meter). I've seen the modules made for Hybrid and golf cars, but they are way too big, and expensive. Any ideas are welcome. Thank you
 

spec

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Hy Pete Laabs,

Welcome to ETO. I see you are from the US, so component access will not be a problem. Care to tell us which state and put it next to 'Location' on your user page so that it displays in the window at the left of your posts.

You say that you have a -42V to -56V telephone supply and you would like to drive some LED strips; why not put two 24V strips in series or four 12V LED strips in series and connect them directly across the -42V to -56V supply.

Alternatively you could use a standard power converter module to convert the -42V to -56V to 24v or 12V but referenced to -42V to -56V. This approach would use a standard reasonably priced and compact module.

spec
 

Pete Laabs

New Member
Long story as short as possible. The input voltage will be coming from a Fuse Access Panel, the LED strips I am looking at are at +24 & +12. Once I get the +24 or +12 I plan on dropping part to +5 and using an Arduino and PIR (Motion Detector) to brighten the LEDs. No switches, lights either Dim (or some color) and Bright when someone is in the aisle.
 

spec

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Most Helpful Member
OK sounds good.

There is no reason why the LED strings can't operate directly from the -48 to -56 (call it -48V) supply for maximum efficiency and minimum size and cost.

You could then generate a low power +5V supply from the -48V to power the Arduino and motion sensor.

We can sort out the control of the LEDs for you.

spec
 

Pete Laabs

New Member
I can see in theory how this could work, the part I'm trying to wrap my head around is this. The DC Plant has a positive return, the LEDs and most other electronics have a negative return. Of course I could isolate the "controller" and limit the current, but wouldn't the -48 volts harm the LEDs?
 

ronsimpson

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Input will be -42 to -56 volts dc (-48 Telephone DC Plant) and would like the output to be either +24 volts dc, or +12 volts dc, maximum of 10 Amps
From the "Telephone DC Plant" there is a wire called "ground" and one called "negative 48"?????
The LED does not care where ground is.
upload_2016-9-24_9-36-11.png
 

spec

Well-Known Member
Most Helpful Member
I can see in theory how this could work, the part I'm trying to wrap my head around is this. The DC Plant has a positive return, the LEDs and most other electronics have a negative return. Of course I could isolate the "controller" and limit the current, but wouldn't the -48 volts harm the LEDs?
Yes, it can be confusing but, as Ron says, LEDs and electronic components in general, have no idea what you call 0V, ground, or return. All they care about is what they see. So, for example, in Ron's schematic in post #6 above the Arduino would operate quite happily with its OV at -48V and its 5V supply at -43V.

You would not normally drive a LED, or a string of LEDs with a voltage because you then do not define the current that flows through the LED. Instead you would normally drive LEDs with a constant current or semi constant current as shown with the resistor in Ron's circuit.

I assume that the strings of LEDs that you propose to use, either 12V or 24V, incorporate some form of current control. So you could join four 12V LED strings in series and connect them across the -48V or connect two 24V LED strings in series and connect them across the -48V. It would not matter that the -48V supply can vary between -48V and -56V.

spec
 
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