Calculating resistor for LED's when little is know

Hi All,
I purchased a Click CLK053 1m led strip which has 30 led's. All I know is that it is powered by 3x1.5v batteries and has a 7.5 ohm resistor, Click cannot give me any further information i.e., Vf & amp rating. I want to cut the strip into various lengths to suit model I'm building for my model railway. My BUS line runs on 18v dc so is there a way to calculate what size resistor to use for a given number of led's when little is know.

Best Rgds,
Dave R.

As the LEDs are white, they will be about 3 V. The design uses 3 batteries to so that the current is reasonably stable. There will be around 4.5 V total, of which 3 V will be across the LEDs, leaving 1.5 V across the resistor, so the current will be about 200 mA in total. That will probably give over 6 hours on alkaline batteries until the voltage of each cell is down to 1.2 V. A t that point the current will be under half the starting current.

Anyhow, 200 mA is about 7 mA per LED.

To run one LED from 18 V at 7 mA, there will be 15 V across the resistor, so you need about 2200 Ohms or 2.2 kOhms, and there will be about 0.1 W of heat dissipated in the resistor.

For more LEDs, divide the resistance by the number of LEDs, and multiply the power. If you run all 30 of the LEDs, you need 2200/30 = 73 Ohms and 0.1 * 30 = 3 Watts.

Small changes in resistor value will give small changes in current, so if you used a 68 Ohm resistor for all 30, the current would be 7.35 mA per LED, which is probably fine.

I suggest you run resistors well under their power rating, so if you do want to use one resistor that dissipates 3 W, I suggest that you use a resistor rated at 6 W or more. I'm suggesting that because a 3 W resistor running at 3 W will be really hot, while a 10 W resistor running at 3 W will be much cooler, but probably still hot enough to burn your fingers.

If you have separate strips, use separate resistors for each strip. Otherwise, if one strip becomes disconnected, the other strips will take more current and may burn out.

White LEDs are typically rated at near 3V forward drop, so possibly about 1.5V across the resistor at nominal battery voltage.
1.5V / 7.5R = 200mA, which is divided over 30 LEDs from the info in the ads I can see.

That would be around 6 - 7mA per LED, which is quite a low value and should be safe on pretty much any LED there is.

To get roughly the same current per LED on 18V, so 15V across the resistor, around 2K or 2K2 for each LED.
(Divide that by the number of LEDs being fed & use the nearest standard value - eg. 1K for two, 680R for three, 470R for four etc.)

FWIW... I made a low-load automotive test light with a red LED and a 10k ohm resistor, and it still lights up at 5V.

My point is that LEDs often work ok with much less current than advertised.

Just a thought.

MacFromOK said:

FWIW... I made a low-load automotive test light with a red LED and a 10k ohm resistor, and it still lights up at 5V.

My point is that LEDs often work ok with much less current than advertised.

Click to expand...

Fairly obviously, the current simply sets how bright they are - so they don't 'work' with a specific current at all - they work anywhere between zero and more than maximum current, with varying degrees of brightness (and destruction). It's pretty amazing how little current can produce useful light from a modern LED.

You are far better off with a different stripled. Yours is plastic and easily melts with soldered wires. And 3V draws too much current with all in parallel and wastes too much power dropping 18 to 3.1V or so. I suggest smaller warm white and use magnet wire tinned to solder and paste with hot glue for mechanical strength over the wire bond.

then buy a 2A~3A 12V supply or so. Cheap or better a cheap “buck converter” variable from 18 to 12 as a dimmer. Banggood is a great source, . Here is Ebay https://www.ebay.ca/itm/252371081346?var=551219054051&hash=item3ac27d1882:g:~PkAAOSwZjhXIjLv

the stronger flex circuits use polyamide rather than flimsy plastic film. the stronger ones can be cut to length.

Some include supplies but 5m reel is standard is multiples of 3 LEDs with resistors for 12V included. RGB color ones have a remote and supply includes but again not easily cut and extended.

if you just want single LEDs I suggest a 5mm LED with a resistor (18-3V)/20mA= use 1k from 18V and they will get hot if only 1/4W

I have thousands of surplus. I can send 200 if you want. I used to sell 50k /yr to a client in NZ From Canada, with my custom 16,000 mcd specs and +/-0.1 V tolerance in matched 4500’K white and other colours.

Hi Tony,

I hear what you are saying about the plastic backing but trying to get white at 150 per 5m ain't easy. Most are 300 plus, mostly coloured, and were far too close for the model that I was building for my model railway. What I also liked was the ability to cut them up singularly, ideal for house and shop models. The voltage is actually 15V, not much of a drop, the 18V was a/c, which I don't use.

I did pick up on the temperature problem when I mentioned to a friend that I was going to use the 300 led and paint over the odd ones in black.

Thanks all the same with your thoughts and ideas, all go into the memory banks.

Best Rgds,
Dave R.

Painting black will not raise the temperature since IR dissipation is in the Rs of the substrate and not I n the emitted spectrum and if there is a lens, that acts as a thermal insulator. Otherwise it might raise temp a bit.

60/m is pretty standard or 300/5m but I found some 24/m or 120/5m in fancy remote RGB strips here at Banggood.