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Power 30 3mm LEDs (from battery powered string lights) with 5V 140mA wall power block?

I have a string of 30 3mm LEDs attached to a battery pack which holds 3 C-size cells and has a switch, and a 10ohm 10% resistor (brown black black silver). I'm not sure of the exact specs for the individual LEDs, but they look pretty standard (warm-white, 3mm.) I think they are connected in parallel (2 wires in to each, 2 wires out)

I also have a power block (wall wart) from an old vacuum which outputs 5V DC 140Ma.
Is it possible to power the LED string with the power block, and if so, what kind or resistor should I substitute for the original? (and, for future reference, how did you arrive at your answer?)

How does supply wattage matter?

Might as well throw this in as well: what resistor would I need to power them with USB?

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Get a meter and measure the actual current through the LEDs - with that you can work out everything dead on.
Vick Jr (author)  steveastrouk7 years ago

My multimeter is a bit...twitchy. But I'll try...

The leds are getting about 2.3 volts. (should be a little higher, but my batteries are weak) 
I tried connecting it across the terminals of the switch (when it was open) and it sais 0 current. I tried breaking it into another led test circuit and it alwais sais 0. I think my multimeter got overloaded or something. Sorry.

Hi Vick,
Put your meter ACROSS the 10Ohm resistor and measure the voltage. Current = voltage/resistance.
Vick Jr (author)  steveastrouk7 years ago
The voltage across (in parallel with) the resistor is 1.39ohms
(Don't you measure current by putting the meter in series with the circuit?.
Ohm's law says that the current would then be .139 amps or 139 milliamps.
(1.39/10 * 1000mA/amp)

I guess my question is: does the wall wart output current make any difference in the resistor calculation? I know how to calculate the required resistor from the supply voltage, LED current, and LED voltage drop.
(Resistance = (supply volts-LED volts)/LED amps
There are also many online calculators to do this.
But supply current never comes into the equation. Does this mean that it doesn't matter, and I can hook the leds up to any supply current, as long as the resistor is appropriate for the supply voltage and led voltage and current?

Thanks!
By measuring the voltage across the resistor, and doing the math, you HAVE measured the current - without breaking into the circuit. Its a trick.
Vick Jr (author)  steveastrouk7 years ago
Ahh. Very clever. Thanks you  steveastrouk for your comments.

Ok. So, for parrallell leds, resistance = (supply voltage-voltage drop for each led)/(number of leds*led current).
Assuming these leds use 3.4 volts, I calculate that each uses about 3.67mA.  (I thought most LEDs use about 20Ma. Maybe they overdid the resistor to be on the safe side.)
Using these values, i calculate that replacing the 4.5V supply with 5V, I would need (ideally) about a 15 ohm resistor.
Does this sound right?
If they're parallel, then you get very very roughly 139/30 mA each, or 4.7mA.

Parallel running is a really bad idea, unless you have well controlled LEDs by the way - you can get runaway, where one LED pulls more current, so it gets warmer, so it pulls more current, the others can only get less current so they cool down, and then they take less current.....


Yes, should be OK.

Steve
But supply current never comes into the equation. Does this mean that it doesn't matter, and I can hook the leds up to any supply current, as long as the resistor is appropriate for the supply voltage and led voltage and current?

So long as the supply can provide at least as much as your LEDs you're good to go
BobS7 years ago
1) Are the C cells alkaline or rechargable? That means 4.5 V vs 3.6 V.

2) Assuming the LEDs are meant to last more than an hour (just let it stay on overnight to test this), put one diode in series if it meant to run on alkalines, or 2 in series if it was meant for NiMHs. The diodes are cheap (1N4001), but can be free: Look in a recycling bin for broken compact fluorescent lights. Each bulb has at least 4 of them!
(Put the remains back, to prevent mercury and other bad stuff to enter the environment).

I am running 3X 21 white LEDs on an old phone charger this way as countertop lights in the kitchen.
Vick Jr (author)  BobS7 years ago
1) They run on regular alkaline batteries, not rechargables. My batteries are a bit dead so there's a little less then 4.5volts.

2) They definitly last for more than an hour. I had another set of lights like this in a halloween costume.

What do the diodes do? The wall wart already outputs DC. Don't I need resistors?