Need help with wiring

Empty V · Nov 2, 2008

Hey everyone this is my first post, thanks in advance for your help.

So I've got 20 5mm LED's that I would like to wire in my car and have the ability to dim them. They require 2-4v DC and consume no more then 30mA each. Would I wire them to a linear pot in order to create this dimming effect? If so how big of a pot do I need? I've some lying around here are the sizes.

10K
20K
50K
500K
10MEG

I've never wired something like this and I'm a little confused by the amount of connectors on some of these pots. They vary from 3-6.

Thanks again for your help!

Billy

Mr Happy · Nov 2, 2008

Empty V said:
Hey everyone this is my first post, thanks in advance for your help.

Well, hi! :welcome:

So I've got 20 5mm LED's that I would like to wire in my car and have the ability to dim them.

Ah, one of the hard questions.

They require 2-4v DC and consume no more then 30mA each.

Unless they are special LEDs, this is not quite true. They can very easily consume much more than 30 mA and then evaporate in a puff of smoke :poof:

Your primary job when wiring them is to restrict the current to less than 30 mA so this doesn't happen.

Would I wire them to a linear pot in order to create this dimming effect?

Unfortunately, no. LEDs do not dim very well with pots because the apparent brightness does not change very linearly with pot position that way.

Before you try to dim them, your first task is to know how to connect them up to a 12 V car supply so they light up without burning out. How familiar are you with how to do that?

Starting from there, we can work onwards.

Empty V · Nov 2, 2008

Mr Happy said:
Well, hi! :welcome:

Ah, one of the hard questions.

Unless they are special LEDs, this is not quite true. They can very easily consume much more than 30 mA and then evaporate in a puff of smoke

Your primary job when wiring them is to restrict the current to less than 30 mA so this doesn't happen.

Unfortunately, no. LEDs do not dim very well with pots because the apparent brightness does not change very linearly with pot position that way.

Before you try to dim them, your first task is to know how to connect them up to a 12 V car supply so they light up without burning out. How familiar are you with how to do that?

Starting from there, we can work onwards.

To be completely honest, my knowledge of power does not go very deep. I've built 12v rechargeable battery packs for a certain voltage, wired car audio/video gear and cut-off switches but that's about it. I understand voltage and current to a limited degree.

So starting from the ground up, albeit asking a lot, would be great. The more I understand the better off I'll be.

Thanks for the help!

Billy

Mr Happy · Nov 2, 2008

Well basically, LEDs are different from bulbs. With bulbs you have to give them the right voltage and the current works itself out. So if you connect a 12 V bulb to a 12 V battery everything is good. With LEDs you have instead to give them the right current and let the voltage work itself out. Also, you must connect an LED the right way round, unlike a bulb that you can connect either way. If you connect an LED backwards you can destroy it.

If you look at an LED description, you will find a forward voltage, often written as Vf, and a current. You mentioned a voltage from 2-4 V, but this can vary with the color. A red one might be 2 V and a blue one 3.5 V.

Suppose we want to connect a single LED to a 12 V battery. To avoid exploding the LED it must be connected in series with a resistor calculated to limit the current to 20 or 30 mA. There are lots of guides and tutorials on the Internet, and quite a few "resistor calculator" gadgets on web pages.

Perhaps you can do a bit of searching and reading up on the subject, and then return with further specific questions.

Light Sabre · Nov 2, 2008

Here's a couple of online resistor calculators that I came across that might help you out:

LED calculator for single LEDs

LED series parallel array wizard http://led.linear1.org/1led.wiz

Empty V · Nov 2, 2008

Mr Happy said:
Well basically, LEDs are different from bulbs. With bulbs you have to give them the right voltage and the current works itself out.

So I did a little bit of research and learned something that will help me along in life now, I understand what a resistor does and how to apply use it

After checking out the website of the manufacturer and further examination of the LED's that I bought, they were designed to be wired into a 12v system, resisters attached and all. Here's a few pics of one where I sacrificed some of the heat-shrink on it to expose the resistor.

Mr Happy said:
You mentioned a voltage from 2-4 V, but this can vary with the color. A red one might be 2 V and a blue one 3.5 V.

FYI they are blue LED's and when I went to color match the bands on it it came out 18ohm short. But this resistor looks a lot different from the other ones I've seen.

Mr Happy said:
Suppose we want to connect a single LED to a 12 V battery. To avoid exploding the LED it must be connected in series with a resistor calculated to limit the current to 20 or 30 mA.

So I plan to wire them all parallel to a distribution block and inline with the block and my remote power wire is where I would put the "dimmer."

How do the LED's react if you wire them into a pot? Do they not dim proportionally to the knob position? Do they just go from on to off? If I get the right pot for the job could this issue be resolved or would a rheostat work better by regulating the voltage going to the LED's? Is there anything else that might work?

I hope this isn't too basic for the forum, like I said this is all new to me. Thanks again for your help!

Billy

Mr Happy · Nov 2, 2008

OK, let's do a few quick calculations.

Suppose the LED drops 2 V, we want 30 mA through it, and it is a 12 V supply. The resistor value would be (12-2)/0.030 = 333 ohms.

Suppose 20 mA instead: (12-2)/0.020 = 500 ohms.

Suppose 3.5 V dropped by the LED: (12-3.5)/0.030 = 283 ohms.

Suppose 14 V supply (car running): (14-2)/0.030 = 400 ohms.

You can see with all these numbers that 18 ohms difference is neither here nor there. Any resistor in the 400 to 500 ohms range will work OK.

Actually, you will not easily see much difference in brightness between 20 mA or 30 mA. So usually the resistor is chosen to be a "safe" value, but the exact resistor value is not extremely critical.

The resistor in your picture looks like 470 ohms, so that is right in the middle of the range I have illustrated. It seems fine.

Concerning the pot, it is as you say. They will not dim very proportionately with the knob position.

Suppose you want 1 mA at the dimmest position. The resistor value would be (12-2)/0.001 = 10k. So you could put a 10k pot in series for dimming purposes. What you would likely find is that the LED dims very slowly until you get to the last bit of knob travel, and then it goes out almost immediately.

A second thing you have to worry about is that you have 20 of these LEDs in parallel. So that is not 20 or 30 mA, but rather 400 to 600 mA. If you start putting that through a pot with a 10 to 12 V drop, it will be dissipating 5 W or more of heat. It will need a big, chunky wire wound pot to take that amount of power dissipation. (Of course with 20 LEDs in parallel your pot will not be 10k any more, but 500 ohms, which is more achievable in a wirewound pot.)

You can try the pot for dimming, but dimming LEDs is hard, and normally needs an electronic circuit using pulse width modulation.

Empty V · Nov 2, 2008

Mr Happy said:
OK, let's do a few quick calculations.

Suppose the LED drops 2 V, we want 30 mA through it, and it is a 12 V supply. The resistor value would be (12-2)/0.030 = 333 ohms.

Suppose 20 mA instead: (12-2)/0.020 = 500 ohms.

Suppose 3.5 V dropped by the LED: (12-3.5)/0.030 = 283 ohms.

Suppose 14 V supply (car running): (14-2)/0.030 = 400 ohms.

You can see with all these numbers that 18 ohms difference is neither here nor there. Any resistor in the 400 to 500 ohms range will work OK.

Actually, you will not easily see much difference in brightness between 20 mA or 30 mA. So usually the resistor is chosen to be a "safe" value, but the exact resistor value is not extremely critical.

The resistor in your picture looks like 470 ohms, so that is right in the middle of the range I have illustrated. It seems fine.

Concerning the pot, it is as you say. They will not dim very proportionately with the knob position.

Suppose you want 1 mA at the dimmest position. The resistor value would be (12-2)/0.001 = 10k. So you could put a 10k pot in series for dimming purposes. What you would likely find is that the LED dims very slowly until you get to the last bit of knob travel, and then it goes out almost immediately.

A second thing you have to worry about is that you have 20 of these LEDs in parallel. So that is not 20 or 30 mA, but rather 400 to 600 mA. If you start putting that through a pot with a 10 to 12 V drop, it will be dissipating 5 W or more of heat. It will need a big, chunky wire wound pot to take that amount of power dissipation. (Of course with 20 LEDs in parallel your pot will not be 10k any more, but 500 ohms, which is more achievable in a wirewound pot.)

You can try the pot for dimming, but dimming LEDs is hard, and normally needs an electronic circuit using pulse width modulation.

Wow, thanks for all the info, it's definitely starting to make sense. Well I tried a few diff pots on a 12v 2700mAh battery I made a while back and I was able to come to a few conclusions.

100ohm pot constant light,
10K pot constant light
500k pot dims but doesn't go out completely
10meg pot has a small amount from off to on and a lot of travel for off

From this experiment it seems that a
I thought that if you wire the LED's parallel 12v will be going to all of the LED's, at least that's how it looks here:

http://www.theledlight.com/ledcircuits.html

Am I missing something?

If this page is accurate and I can distribute 12v to each LED do you think a 2meg pot would work? It seems to me that it would be right in the pocket of the right resistance. When it was dimming on the 10 MEG and 500K pots it seems to dim pretty smoothly, I didn't notice any stepping. I'm also curious as to how all of this will act when I take it off the testing bench and to my car because they're obviously different power supplies..

If I'm totally off track let me know.

Billy

Mr Happy · Nov 2, 2008

Yes, since your LEDs have a built in resistor, you can wire all of them in parallel to the 12 V supply. It would be like the third picture on that page with different color LEDs. Sure, you could wire all the LEDs to a terminal block try the 2meg pot between that and the power supply, but be careful about overloading it with too much current.

My calculations for pot values were only estimates. Your actual experiments will be more accurate than my guesses.

Need help with wiring

Empty V

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Light Sabre

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