# Strange current reading, LED's in series

I have a 19.4v/2.37A power adapter. I have 5 LEDs (3.2v-3.4v 20mA) connected in series. When I measure the current, the reading says 0.08A, which is total of 4 LEDs, not 5, but close enough. Strangely, when I add another LED to the chain, the reading says 0.02A instead of 0.10A. All I'm trying to do is measure how much amps all 5/6 LEDs use together.

When I adjust the units, the decimal places do not change, regardless, if I set 10A or 2mA, it always says 0.08 or 0.02, is my multi-meter broken?

• This is perfectly expected behaviour. And also, you need resistors. – Asmyldof Apr 16 '16 at 18:16
• Doesn't all 6 LEDs together use 0.1A? why does it say 0.02? in the datasheet 0.02A is what one LED uses. – user3578847 Apr 16 '16 at 18:21

Since you dont have a current limiting resistor, the current going through the LEDs is regulated by the sum of their I/V curves:

By luck you did not fry your LEDs and by coincidende they added up to 80mA (which you thought was 20mA * 4).

The current in series does not add up, so if you have 4 LEDs that are rated for 20mA, you should be running them at 20mA. Your reading of 80mA actualy indicates that you are running the 4 of them at 80mA each.

When you added another LED, you added its forward voltage to the bunch and hence move the current further down in the I/V graph of your combined LEDs, hence now you have 20mA.

You should really add a resistor in series with your combination. I'll add that you'll probably have to run 2 in series and 3 in series if you want 5 LEDs with this 19V PSU.

• Ohh I see, thank you very much for the answer! I will accept the answer once the time limit wears off. Quick question, what's the maximum amount of LED's I could connect with my adapter? Thanks! – user3578847 Apr 16 '16 at 18:30
• If you want to be conservative, 4 LEDs in series + resistor to draw 20mA. If you have 100 groups of those in parallel you will draw 2Amps out of 2.37A, so you'll be safe. So easily you could put 400 LEDs in that PSU. – Wesley Lee Apr 16 '16 at 18:32
• You can look into this calculator: ledcalc.com or into this answers for how to calculate it yourself: electronics.stackexchange.com/questions/156406/… – Wesley Lee Apr 16 '16 at 18:35
• wow, amazing tool. As a beginner I have so many questions, this clears things up alot! I'm amazed that I can connect that many haha from a laptop adapter, thank you, Sir! – user3578847 Apr 16 '16 at 18:46