3W led's in series HELP!!

Thread Starter


Joined Sep 14, 2011
Before anything, THANK YOU FOR YOUR HELP!

I have 1x Waterproof LED power supply
V: AC 170-250V 50/60Hz
I: AC 0.17A
P: 38W

V: DC 20-45V
I: DC 700mA
P: 31.5W

I Have several 3 to 3.5Vf 700mA LED's and last night I plugged in my power supply and wired up 3 LED's in series and nothing happened.

There's no positive and negative for the AC input, correct? I wired positive and negative correctly on the output with alligator clips and wires. VERY CONFUSED!!! Any help would be appreciated!


Joined Jul 17, 2007
Let's see, the output is 700mA at 20v to 45v.
And, you wired up three 3v to 3.5v 700mA LEDs in series, and connected them to the supply.

3 x 3.5v = 10.5v. 3 x 3v = 9v. So, looks to me like you'll have to have 7 of these LEDs in series before you'll get the minimum of 20v satisfied.

Hopefully, you didn't burn out your supply already.

Perhaps you had one or more of them connected the wrong way around? The anodes need to be connected towards the supply positive.


Joined Jul 17, 2007
I don't know anything about your supply or LEDs except what you've told me.

You may have connected one or more LEDs backwards, which would not be good for either the LED nor the supply.

Since you have not provided a datasheet, part number or manufacturer for either the LEDs nor the supply, nor a photo of aforementioned items, I'm not quite certain what you expect in the way of an answer besides a guess.

However, you HAVE mentioned that you have tried operating the power supply far outside of its' design specifications. I have no idea how long it would take to fail under such conditions. I don't know what its' failure mode might be with such abuse inflicted upon it.

It would seem that you now need a method to test your LEDs using constant current in order to see if they are capable of lighting up still.

Do you have a DC voltage supply that you can use to test them with? Do you have an LM317 regulator and some low-value resistors handy, in the range of 20 to 40 Ohms? If so, you can rig up a temporary constant current circuit. You could even use the 12v supply from your personal computer if you are careful.


Joined Feb 19, 2009
Do you have a DMM with diode check?

Measure the Vf of an LED in diode check mode (most good DMMs will go to 5v and light the LED Dimly).

In voltage mode, measure the DC output of your supply, as well as the AC output on AC volts.

Post results here.

Thread Starter


Joined Sep 14, 2011
I think I will go the DMM route on that one. I have no data sheet for my items and at the moment my computer won't read the card where the pictures I intend to show you are. I live in Shanghai and buy all this stuff online and with my limited knowledge of electronics and low ability to communicate with the sellers except through my friend things are not exactly easy.

I WILL buy a DMM with diode check.
Question: When you mention 5V are you talking about the "DC Voltage?"

I have this spec chart of something I'm looking at online now. Let me know if it would be sufficient for what I'm looking for. My LED's have a Vf of 3-4V so should there be a DC Voltage setting of 3 or 4 V to test them properly?

Auto range 3 3 / ​​4 -bit LCD display of the DMM, the maximum display value of 3999
Auto-off / Data Hold
Display display device: 30 X 60mm LCD
DC voltage: 400mV ± 0.8 , 4/40/400V ± 0.5% , 1000V ± 0.8%
AC voltage: 4/40/400V ± 0.8% , 700V ± 1.2%
DC Current: 0.4/4/40mA ± 0.8% , 400mA ± 1.2% , 10A ± 2.0%
AC Current: 0.4/4/40mA ± 1.2% , 400mA ± 1.6% , 10A ± 3.0%
Electrical  resistance: 400/4K/40K/400K/4M Ω ± 0.8% , 40M Ω ± 1.0%
Continuity test with buzzer
Diode test: to check the positive and negative
Transistor hFE test: 1 - 1000
Power   source: 6F22 (9V) X 1
Dimensions: 91 X 189 X 31.5mm
Weight   capacity: 310g

...and the link (use google translate built into google chrome to check)



Joined Dec 17, 2011
I learned about leds with small solar panels. By small, I mean like little 5 or 6 volt "plastic" ones only a few hundred mA from ebay. Those charge 3 nimh AA's which in turn light up the leds. I use a voltmeter but first use ohms law to resist them, otherwise they usually fry. Then I use ohms law to figure out how many mA it uses, such as,
(let me remember...) volts / amps = ohms, and volts / ohms = amps. The very first led circuit I did was a red, a green and a blue wired in parallel (and 3 AA's without the solar panel). They were the very cheap ones at Radioshack (but was way overpriced) and rated at 20mA. I figured my battery pack was 4.5v...
Red was 2.1v, thus 4.5 -2.1 = 2.4, to be resisted. 2.4 / the max current, .02A = a 120 ohm resister. I used normal 1/4watt ones.
Green was like 3.2v and blue was like 3.3v and so resisted accordingly.
Now, I burnt out my amp meter but still, the volt meter worked. So I found the voltage across the resistor to determine how much current they use. In the case of "red", it would say something like 2.8v...
2.8 / 120 ohms = about 23mA... The battery was a little more than 4.5...

So this might not help because your leds are 35x more powerful, yet explains the math. Now, I use the Cree XPG and XML, which are like the most efficient power leds. I can still use 1/4watt resistors but I wire 'em up in parallel (and sometimes series, too) so they can handle more power.
Kinda like 4 x 3.1 = 12.4v, from 13 (max of lead acid) = .6v, so .6v / 2A (XML's can handle up to 3A with GOOD heat sinking) = .3Ω. How much power rating does this resister need to be? Amps x volts = watts... well you probably know, thus 2 x .6 = 1.2 watts (so I would "twist up" like ten of the 1/4 watt, closest to 3Ω, ones, to handle 2.5 watts).

In any case, I ALWAYS had a pretty good approximation of input voltage (even with batteries!)

In your case, I would get an output with a much more stable voltage! Or use an led driver suited for such a "wild" voltage range. The Cree XML will draw... well let's just do this visually so you know how just a little voltage difference means a lot of current difference!...

I do know that there are led drivers that work for various different voltages. I like to go to deal extreme and buy the AMC for 3.3 to 5v, and go to Led Supply for more expensive drivers for 12v and (much) higher.

I would think a voltage regulator (and possibly, even a zener diode) might work but am not sure. :eek:
In fact that's where I need help (And I hope I did the math right)!


Joined Feb 19, 2009
I WILL buy a DMM with diode check.
Question: When you mention 5V are you talking about the "DC Voltage?"
The 5V is the highest Vf the DMM will measure for a Diode to ensure it is good, some will only go to 1 or 2V, but most go high enough to test higher output LEDs, so any newer model DMM with the Diode test should work.