Advise needed on fixing old LED string

**jimbofish** · 02-08-2009, 01:39 PM

We have a short set of 25 LEDs on a string about 30" long. It has a control box with batteries and an ON/OFF/FLASH switch. There's also a DC jack to run off an adapter. This is one of those mini strings designed to be used on a wreath, mini-tree, etc.

By mistake, I applied the wrong voltage and some of the components on the little circuit board in the battery box have been over-heated and burned. Rather than trying to fix it, I'd like to just remove the flashing circuit and use it direct to a DC adapter.

Of the 25 lamps, it appears that they're wired in series of 12 & 13 LEDs. I measure 4.4 & 4.8 ohms on the two legs with a common ground wire. I hooked up a 3v dc adapter and measure .15 & .17 ma. I have no clue as to what the specs are on the LEDs. They appear to work ok connected this way.

My question is, do I have to put a resistor into this circuit or is it ok to hook these up direct to a 3v supply?

**Ronp** · 02-08-2009, 02:13 PM

Not knowing anything about the leds iIwould guese they are close to 2 v 20 ma each
13 running parellel could use a 4.7 Ohm resistor on 25 it should be fine without one
using 3 v power suply some one should corect me if im wrong

**jimbofish** · 02-08-2009, 04:08 PM

The way it's wired, it appears every other light is on one series. There is only one common lead. If I connect the two series leads together, then to the PS, that will be different than 25 in series, correct? If so, then I should put a 4.7 resistor on each of the series feeds, yes?

**Warlock** · 02-08-2009, 04:49 PM

If you were to connect the leds to the 3 volt dc supply you should be alright...I've had 6 leds connected to 9 volts of dc battery supply for my bike light for the last 4 months..Leds still going strong without a resister at all.The batteries I'm using are simply watch batteries that work very well and last a long long time...4 months and no need to replace em yet.
Hope this helps.
Joe...

**MikeUK** · 02-08-2009, 04:57 PM

I've modified at least ten strings of battery-powered LEDs to work direct from an adapter, never had a problem and no resistor(s) required.

Hope this helps.

Edit: Don't know what colour your LEDs are, if they're red or orange (or green if an old string) then their forward voltage would probably be 2V and there would be a current-limiting resistor in there already (normally soldered to one leg of each LED). If they're modern and green, blue or white, then their forward voltage would be 3.4V and running from a 3V adapter they don't need a resistor.

**jimbofish** · 02-08-2009, 06:46 PM

They're red, green, and yellow.

So it sounds like I can just connect them without a resistor. Any reason to use more than, say, a 200 ma adapter?

**MikeUK** · 02-08-2009, 11:04 PM

Originally Posted by jimbofish

.....

Of the 25 lamps, it appears that they're wired in series of 12 & 13 LEDs. I measure 4.4 & 4.8 ohms on the two legs with a common ground wire. I hooked up a 3v dc adapter and measure .15 & .17 ma. I have no clue as to what the specs are on the LEDs. They appear to work ok connected this way.

My question is, do I have to put a resistor into this circuit or is it ok to hook these up direct to a 3v supply?

This is the bit I didn't understand from your original post: "I hooked up a 3V DC adapter and measure .15 and .17 ma"... I assume you meant 150ma and 170ma? If the current drawn by each LED is about 12.5-13ma, then with 12/13 wired in parallel (as they would have to be to be driven from a 3V battery) that gives the 150/170ma you measured.

You are going to drive both strings at the same time, so the output of the adapter needs to be at least 320ma... I would make it 400ma so the adapter doesn't get too hot.

**jimbofish** · 02-09-2009, 01:11 AM

I'll have to check that current again...

**jimbofish** · 02-10-2009, 01:41 AM

Remeasured and it reads 00.18ma and 00.20ma using a different PS. This an auto-ranging DMM. Perhaps I'm measuring incorrectly. I connected two test leads w/ alligator clips from the PS to the ends of the wires to power on the LEDs, then connected the clip-on DMM leads across the two alligator clips, that is to the + and - clips at the wires. Am I doing this wrong?

**WireWrap** · 02-10-2009, 03:28 AM

Originally Posted by jimbofish

Remeasured and it reads 00.18ma and 00.20ma using a different PS. This an auto-ranging DMM. Perhaps I'm measuring incorrectly. I connected two test leads w/ alligator clips from the PS to the ends of the wires to power on the LEDs, then connected the clip-on DMM leads across the two alligator clips, that is to the + and - clips at the wires. Am I doing this wrong?

That is the correct hookup to measure the voltage supplied to the lights. It is incorrect to measure current through the string.

To measure the current, the circuit needs to be
From the positive power supply terminal to the positive meter lead...
from the negative meter lead to the LED string positive lead...
from the LED string negative lead to the negative terminal on the power supply.

This gives a circuit where the meter is in line with the current flow. Now the meter should read the correct current.

Hopefully, your meter has overcurrent protection, as the way you described your connections, if the meter was set to read current, it would have presented a short-circuit to the power supply.