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Testing a 3 watt LED? Answered

I recently purchased 13- 3 watt LED's. I'm trying to establish a high out-put LED light.
I also purchased a LED Driver Transformer 48 Watt 120 Volt to 12v DC 4 Amp.

First I would like to test each LED using the same divider.

I'm thinking that if I create a voltage divider two 330 ohms to limit the current and reduce the voltage to about 6 volts that would allow me to test with the driver that I purchased.

Secondly, will this driver be enough to drive all my LED's and would I have to reduce the voltage down too since it appears to push out enough wattage and amperage for the total number LED's I'm putting together?

I'm building my circuit based on this video.


Link to website data of 3 Watt LED.

Assistance would be greatly appreciated.



What you are trying to do isn't compatible with what you've got. The LEDs drop about 3 V each, but will run on about 1.2 A

I think the video isn't clear. The power supply is giving 670mA to between 9 and 12 LEDS, not 9-12V

If I were you, I'd have to consider a different supply I'm afraid.


Yes, after I posted this, I realized that it was not enough to drive my 3 watt LED's. Now I'm thinking maybe going to my old computer and snagging the 200 watt Power supply.

I'm still interested in the tester though. I wonder if a 9 volt battery will do the trick?

Anyway, I don't want to waste my initial investment though.

This has been quite a learning experience for me.


The problem you have is a distinct lack of voltage !

If I were you, I'd try and get 13 x 3 volts to drive them, and use 40 volts to drive them, via a circuit like this:-

Thank you for your help. I've uploaded photos of my success. I put in a variable power supply with a 32 volt max.


I'm using a LM317. I believe that should work?


They haven't burned up yet. I'm letting it run for 24 hours. It hasn't generated any real heat.

It's a very similar circuit in your basic design, but I needed something that could have enough voltage. First, I had to find a properly rated step down transformer like around 24 to 32 volts AC. Then I found these kits with a the LM317 that had a bridge rectifier that would convert AC to DC. It also provided the current control features that you spoke of. I actually also pieced everything together and bought the items separately too, but I liked the printed circuit board that the kit came with, so I went with it. The pot was really not necessary since I want the max current. Comparing the price for piecing it vs the kit, the kit is substantially cheaper. In my next note, I'll send the schematic. There was know schematic with the kit. I have to draw it out based on the circuit board design. The company hasn't published the circuit diagram,


Although it does have a variable resistor. I've read that to effectively dim the circuit I would need to use PWM. I guess that will be my next project. The resistor does dim the LED's, but through time may damage the LED's? I've found some circuits that provide PWM even Arduino can provide PWM. A cheaper route is to use a 555.

I'm still working on drawing up the circuit from the kit I purchased on my original project. I've almost completed it.


No, it won't damage the LEDs to dim them - they'll last longer in fact.

I guess I have to do more testing and research with the dimming. Anyway, I had fun putting the LED circuit together and it works. Now it has given me more ideas to try.

My project as far as I'm concerned was a success thanks to your help.


No, a 9V battery won't supply the current you need.

You HAVE to have current control of these LEDs or you will soon have 13 very small black spots.....


LED Spec

I did measure the voltage on the LED Driver. It is a DC power supply with a regulated 12 volts. The reason for the voltage divider is to reduce the voltage on a single LED to for testing purposes.

I do realize now that for all these LED's to work in series I need a bigger power source. I haven't figured out the right size though. I have an old PC that I can gut out for the 200 watt power supply and it's regulated, so that might work?

More detail on LED

What you have to have is a regulated CURRENT, not voltage. The simplest method is to use a circuit like this one, but try and get the supply voltage right up.

LED driver.JPG

I'm defiantly checking out the LM317. It could help me with the individual tester. Thank you so much for providing a clearer picture of what I have to do.

I wish this could be easier like details spelled out. I learn by doing and making mistakes as I go a long. I just don't want it to be a costly mistake.

I know my LED's are still good, and I believe I've found the right driver after our discussion

http://www.ledsupply.com/docs/36watt-lightech-data.pdf. Although this one seems to only work with 12 LED's in series. I believe I can make it work with this power-supply driver.

I'm still going to have a long discussion with LED Supply tomorrow to make sure they are on the same page as me.


Hey well done, getting to grips with something really new like this. Some people would be too scared.

LM317 - caveat: Fasten it to a GOOD heatsink !!! And if you don't, be prepared for it to get very hot very quickly.

If your PSU has a 5V output, with say 2 A of current rating it will be perfect for a single LED test.


Actually I have an old burned out power unit I replaced some years ago in my computer. It has two good size heat sinks that I'm thinking of using to try out my 3 Watt LED tester.

I went to school many years ago for electronics and I'm trying to remember all the things that I learned. Cleaning out the Cobb webs so to speak. The main thing that the training taught me was to respect it and not be afraid to try and experiment.

This may turn out to be a good lesson for me.

Thank you.


Great reply. Best of luck.
Any questions, just ask.

No problem. You've convinced me to get a different power supply. I've since gone to LED supply, and I stumbled onto their calculator for sizing my little experiment.


I would still need a Power supply that will produce at least 32 volts though.