Help with LED's

[Jonney_boy]

I'm in the process of building a DYI hood for my 10 gal. A long work in progress.

The main lights are done and working, but now i'm trying to wire up the moon lights and got stuck.

the plan was to use a dozen regular white led's connected to a 12v supply with on a pot so it could be dimmed. The problem is that I have forgotten how regular pots work and the math behind it. Searching on the net and it seems all the sites now are talking about the new high powered led's and not much left on the older stuff.

The plan is to have 4 strings of 3 led's. The led's are 3.2v and 20ma.

From what I can see each string would need one 100ohm resistor on it. I wanted to put a single pot on the "main" line coming from the dc transformer. What size and rating pot would I need/want? Would a 1/4w 10k pot do?


My goal is to have 1/4 to 1/2 the brightness of what the led's are producing.

if anybody can point me to a website that can give me a quick crash course I would be more than grateful

[Theratboy_101]

I've never dimmed leds but have wired lots.... If I remember the math... watts = amps X volts. So a 1/4 watt pot should just do it... your at .24watt. A 10k pot would work but a lower ohm pot would make for easier and more precise control.

also you need a 120ohm resistor not a 100ohm. the formula to work out what resistor you need is starting voltage minus voltage of the leds divided by watts of 1 led equals the ohm needed (20ma=.020watt) ... to make is simple (12v-3.2-3.2-3.2)/.020=120ohm

now that I feel that your good and confused here is a site that tells you how to wire leds. Quick guide on how to wire LEDs in a circuit

If you want to save a little money wire your leds in to 3 strings of 4... they'll be running on 3volts and give you a small head start on dimming them

[ngo911]

Don't think 3 strings of 4 LEDs would work since each branch would require 3.2V x 4 = 12.8V which is more than the supply voltage.

If you go with 4 branches with 3 LEDs each wired in parallel, that would work (3.2V x 3 = 9.6V). The remaining 2.4V drop would go across your current limiting pot on the "main" line. Assuming 20mA for each branch, you would need 60mA coming from the "main" line.

Therefore, the pot range should be 40 Ohm and up (2.4V / 60mA = 40 Ohm). I would recommend using a 1/4 W 39 Ohm resistor in series with a small 1/4 W pot so that the max current would be limited by the 39 Ohm resistor.

Hope that helps.

[Jonney_boy]

Wow, thanks for the reply's...

Theratboy_101 Thanks for the suggestion of running 3 strings. In this case tho I will stick with running 4 strings as it fits into the hood better.

So, correct me if i'm wrong. there are 2 ways of wiring up this led array.

a) Theratboy's method.
Each string of 3 led's will have one 120ohm resistor on it. On the "main" line I will have a pot. The smaller the better.

Locally, the "smallest" one I can find is.

1/2 Watt with 1A Switch - 1K Linear

b) ngo's method.

Wire all the led's into branches of 3 leds. On the main branch only put a 40 Ohm resistor on it and the smallest pot I can find (see above).

[Theratboy_101]

Don't think 3 strings of 4 LEDs would work since each branch would require 3.2V x 4 = 12.8V which is more than the supply voltage.

It will work, I've tested it with some 3.4v 30ma blue leds and they were very bright at 3v. I did a quick bench test and found that my blue leds will run on as little as 2v. (but just barely glow) After all the point is to under power and there fore reduce the light output of the leds.

after doing some tests and a little more math if you run it in 3 strings of 4 with no resistors and a 150ohm pot in the main line should let you go from just a glow to almost full light... don't quote me on it but I'd think that running 4 strings of 3 with 120ohm resistors or a 40ohm on the main line and a smaller 50ohm pot would have a similar effect

small pots of less then 1k can be hard to find but you can get them. you may need to go to an industrial electronics supply store.

[kaptin99]

Guide to DIY (LED) Moonlighting

Here is a something I found hopefully that helps.

[ngo911]

Ah fair enough. I was just going by specs and theory Never actually tried it so I'll take your word over mine

It will work, I've tested it with some 3.4v 30ma blue leds and they were very bright at 3v. I did a quick bench test and found that my blue leds will run on as little as 2v. (but just barely glow) After all the point is to under power and there fore reduce the light output of the leds.

after doing some tests and a little more math if you run it in 3 strings of 4 with no resistors and a 150ohm pot in the main line should let you go from just a glow to almost full light... don't quote me on it but I'd think that running 4 strings of 3 with 120ohm resistors or a 40ohm on the main line and a smaller 50ohm pot would have a similar effect

small pots of less then 1k can be hard to find but you can get them. you may need to go to an industrial electronics supply store.