How to reduce 12v to 8v?

I'm trying to add some OLED lights to my brake lights, I've reduced the 12v to 8v using resistors but they won't light up, put my DC power supply to 7.5v and they light up very nice, I'm not exactly an electronic wiz, dont know what I'm missing here, plz help, thx

 

I'd be shopping for 12v product instead of making things more complex.

 

You actually measure 8v with the resistors in?

 

Just looking for answers, but thx for your time

 

The light I'm trying to use is 7.8 volts, if I resist the 12v down to any number close to that they won't light up, but my DC power pack has 7.5 and they light up nice, what am I missing?

 

LEDs require a certain current to operate. I bet your resistance isn't right to give them enough current. Are they just bare LEDs? Do you have a datasheet and/or a multimeter?

Here's a handy calculator
http://www.ohmslawcalculator.com/led-resistor-calculator

 

No data sheet, I do have a multimeter

 

The thing we need to find out is the forward voltage of the LEDs. Typically this is found on the datasheet or wherever they were ordered from, but it can be measured as well. Sometimes you can just put a meter in diode test mode to get it, but if they require too much current it might take a little more work. Here's a video on measuring forward voltage:

https://m.youtube.com/watch?v=jpFpn1a5jRc

 

I pulled it out of a working fixture and I measured 7.8v going to it if that helps

 

Do you still have that fixture available? If you plug the LEDs in and measure the voltage across them that'll be the forward voltage. And then if you put the meter in series and measure the current we'll know what current they're designed to operate at.

 

There's only a positive and negative wire coming from them, here is the hole fixture
The one on the top is it, they are red OLEDs, and the plug fits into a 12v supply,

 

This would be so much easier if it weren't for those black pcbs. Looks like there's a bridge rectifier on the first board so you can plug it in either way and it'll still light up, and then a couple of transistors for managing the current, probably one for each set of LEDs.

I think my next step would be to just plug it in, put my meter across the two pins on the board you want, and see what that voltage is with them running. Then I'd disconnect the red wire, put the meter in current mode, and put the meter in its place to measure the current. Then it'll be just a matter of setting up some resistors to match that.

EDIT: Although if we just do resistors, they'll change brightness when the voltage of the truck changes. An actual regulator would be more better.

 

The voltage is 7.9 to the one I want, not quite sure how to check the current, here's what I got

 

Since I'm not entirely sure how much current this thing pulls, we'll do it the safe way and try not to blow up your meter.

1) cut the red wire to the board you want in the middle. Strip each side of the cut.
2) move the black meter lead to the yellow plug. (20A MAX)
3) Set the meter to the 20A DC setting (just past 6:00, to the left of the vertical line and to the right of that little 200m, in the section with the A and the solid line over the dashed line)
4) Connect one meter lead to each side of the red wire, so that the meter is in series with the LEDs. Use alligator clips if you got them.
5) Turn the power on.
5) If the meter reads less than 0.2 A, you can shut it all down, and move the black meter lead to the white plug, and then change to the 200m DC setting to the left of the 20 setting to get a more accurate reading.

EDIT: When you're done, put the black wire back to the COM port. Leaving it in the amperage measureing position is a great way to blow up a meter when you forget about it.

 

I got zero on both settings,
I really appreciate your time and efforts

 

Hmm. Did the LED light up with the meter connected?

 

No I didn't, but with the wire disconnected, it reads 8.3v verses 7.9 when connected if that means anything

 

Hmm. Sounds like the meter isn't conducting. If you had another meter we could check your meter.

I'll have to give it some thought (company just arrived). Even if we can do this with a resistor, its going to be getting pretty hot.

 

O wanna do it the right way, I dont want any probs with it later, I was just using resistors cause I have a bunch of them

 

So, after eating lots of guacamole, I hit up Mouser. This sort of device is what you need: https://www.mouser.com/ProductDetai...RBMPTWcaRgaVnaXJTVtYzeCn/%2BnvqOhpAqVGeWTIA==

Except, we don't know enough about how much current those LEDs are pulling, so buying the right size is going to be tough. Too much current and it'll toast your LEDs. Too little and they won't be very bright.