# Practical use of voltage divider

Discussion in 'General Electronics Discussion' started by spikey1973, Jan 27, 2018.

1. ### spikey1973

32
1
Jul 16, 2014
hello,

I am stuck with a dilemma which might or might not have a simple solution.

I have a psu supplying 46Vdc

unofrtunately i need to step this down to several different voltages.
1) 14 to 20Vdc (best would be 16Vdc) to power a micro mainboard power supply thingy, using approx 100W
2) 12Vdc form as well a led strip, a fan, temperature sensors (LM35 i believe) and arduino atmega 2560 board
3) 5Vdc

so i was thinking (maybe wrongly) to use a couple of 100W resistors in series (5KΩ - 5KΩ - 5KΩ&2KΩ - 2KΩ) and use it as voltage dividers.

if I calculate correctly this should give me
2/19 * 46 = 4.8Vdc
5/19 * 46 = 12.1Vdc (2 times)
7/19 * 46 = 16.9Vdc

so going from the assumption that all this above is correct, i started to wunder if the is a problem when there is a big difference in power consumption between the different voltages. as the 16.9Vdc "rail" would use approx 90-95 Watt (also pushing to the limits of the resistors used) and approx 1A for the 5Vdc "rail"

if this is not a usable option.. is there an other way to do this correctly?
i have a step down board but this one has a max. input power limit of 40Vdc, en even though i swapped the capacitors to larger once i believe that the actually limit is defined by (an) other component on the PCB, which i would blow up. specially as it would be on 24/7

also voltage regulators can supply this power..

any advice would be highly appreciated.

ps: i understand i faultly use term rail here., and no a different PSU is not an option!

2. ### kellys_eye

4,275
1,147
Jun 25, 2010
Resistors won't do the job and would loose loads of power even if used. The volts they drop vary with the load applied.

The simplest and most efficient way is to use buck (drop-down) regulators where efficiencies of 90% or more can be achieved i.e. not a lot of radiant heat.

These are very common, very small and very cheap.

Cannonball likes this.
3. ### spikey1973

32
1
Jul 16, 2014
thank you!

this would indeed be the best solution.. but as i said (or tried to) i have one of these, but the input voltage is max at 40Vdc, and this is the one with the highest input voltage i could find. well at leat with a high power output too.

kind regards

Matt

4. ### davennModerator

13,798
1,939
Sep 5, 2009
so you need one with a higher I/p voltage
you seemed to miss the point that you will loose lots of power using a resistive divider and will be severely limited in the amount of current it will be able to supply to a load

5. ### kellys_eye

4,275
1,147
Jun 25, 2010
https://www.ebay.co.uk/itm/2x-Elect...092746?hash=item4b227be78a:g:QksAAOSwttRZqLHV

60V input (max), 15A output (max) - use these to get to the 'next lowest' voltage you need and another set of the lower-input devices to get the lower voltages OUT that you need.

At this price for TWO you won't find much better value (of course, being Chinese knock-off's use the appropriate cautions on maximum inputs/loads etc and under run them where possible).

davenn likes this.
6. ### spikey1973

32
1
Jul 16, 2014
@ Davenn: sorry i didn't comment on that but i in my head i discarded tthe resistor option completely...
what i meant with my reaction is that i looked at the buck converter option myself but couldn't find an appropriate one. whch is why i am happy with kellys_eys's reply

@kellys_eye: these indeed seem to fit my requirements perfectly.
also it is a continues converter.. which is important for me to (didn't mention that part yet)

i would need 16 to 20v output with peak power being 10A, but only short if ever.

thank you for this..! much appreciated. i will check the dimensions if it fits neatly (which i promise it will) i will order a set.

Last edited by a moderator: Jan 27, 2018
davenn likes this.
7. ### dave9

1,048
279
Mar 5, 2017
What exactly is this 46V PSU? I wonder if there's a way to modify it to supply less than 40V so you could use your "Step Down Board" but you also didn't mention what it is, if that would solve the 16.9V-ish 10A requirement, or even (just) the 12V so you need to buy one less widget.

8. ### spikey1973

32
1
Jul 16, 2014
thank you, yeah, in theory that would be an idea but to be honest, i'm not really open to that idea.
it is a hypex PSU, high end psu for specifically for hypex amplifiers, but this one i'm not using for that purpose.

i measured and they would fit, fit well actually so i just ordered them. i might upgrade some parts on it (larger cooling block and better caps)

this 16.9V-ish requirement. is for a micro PSU i use to power a pc mainboard i use for a very minimalistic audioplayer. this particular one needs 14-20V, with "best" being 16V

Last edited by a moderator: Jan 27, 2018