Voltage divider producing unexpected voltage

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? • 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. 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. 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. May 10 '16 at 6:40

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.

• Thank you, that explains the output nicely. Though I'm not familliar with this use of '||' double pipe. 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. “∥”. May 9 '16 at 20:28