0

I'm trying to run a neopixel strip and an arduino from the same power supply using a voltage divider to get 5V to the strip.

Here are the values for my voltage divider:

R1 = 1500 Ω

R2 = 1000 Ω

Vin = 12 V

resulting in:

Vout = Vin*(R2/(R1+R2)) = 12*(1000/(1500+1000)) = 12*(1000/2500) = 12*0.4 = 4.8

When the divider is connected at its inputs but not at its outputs, I measure the expected 4.8V across R2. But as soon as I connect the neopixels, I only get about 1.6V.

Could anybody explain this || Suggest a possible solution?

enter image description here

5
  • You cannot use a voltage divider to supply power. It does not work. It can not work. Learn Ohm's Law and you will know why it can not work. Use a voltage regulator. – Majenko May 9 '16 at 19:48
  • I guess that would be a power limitation of the resistors. Unable to take the current load. That's a great shame, I would like to be able to construct a functional level shifter from compon components. Thanks for pointing me towards a voltage regulator. – xeuari May 9 '16 at 20:01
  • It is not a power limitation. It is Ohm's Law. Edgar's answer tells you why in brief. – Majenko May 9 '16 at 20:02
  • hmm .. rookie mistake. I certainly belive that it's not possible, but I don't understand why... I have some reading to do. – xeuari May 10 '16 at 0:13
  • I should point out here that NeoPixels can draw up to 60 mA each at full brightness. You need quite a hefty 5V power supply to power a lot of them. I suggest a suitable 5 volt DC regulated wall-wart (or similar) rated at a few amps. Even only 20 of them will require 1.2 amps. – Nick Gammon May 10 '16 at 6:40
3

Since you are connecting your pixel strip in parallel with R2, the correct formula is

Vout = Vin × (R2∥Rp) / (R1 + (R2∥Rp))

Where Rp is the resistance of the pixel strip and

R2∥Rp = R2 × Rp / (R2 + Rp)

is the combination of R2 and Rp in parallel. From this you can see that an output voltage of 1.6 V is consistent with a strip resistance of 300 Ω.

This is, however, somewhat oversimplified, as the I(V) characteristic of the LED strip is probably non-linear, thus not really equivalent to a resistor.

3
  • Thank you, that explains the output nicely. Though I'm not familliar with this use of '||' double pipe. – xeuari May 9 '16 at 20:02
  • @xeuari It means "In parallel with" – Majenko May 9 '16 at 20:02
  • @xeuari: It's not a doubled pipe character: it's single character, namely U+2225 PARALLEL TO. Compare: “||” vs. “∥”. – Edgar Bonet May 9 '16 at 20:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.