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Question for Electronics Experts: LED current

lr172

Well Known Member
Hoping the electronics experts can help me out.

I am making an LED map light and will be using some LEDs that are 12 volt. The specs recommend a current of 20 ma. How do I limit the current to these LEDs? I am guessing that a resistor would do it.

I will be using 4 of these in the lamp and would like to get the 20ma at 14 volts. Can you point me to a calculator or formula to determine the resistor size? Also, please advise if the formula is for series or parallel configuration.

Thanks so much for the assistance.


Larry
 
Hi Larry,

This is what you would do to figure out the size of resistor to limit the current:

Each forward biased (ie, light "on") has about a 0.7V drop across its terminals. For your application, I would wire all 4 LEDs in series, which would lead to a 4 * 0.7V = 2.8V total drop.

Given a 14V power source, we can use ohm's law to calculate the size of the resistor:

V=I*R
(14v - 4*0.7v) = 0.02 A * R

R = ~560 ohms

Be sure to wire them the correct way .. a reasonable DVM with a diode setting will tell you which side should be connect to positive, and which to ground.
 
Hi Larry,

This is what you would do to figure out the size of resistor to limit the current:

Each forward biased (ie, light "on") has about a 0.7V drop across its terminals. For your application, I would wire all 4 LEDs in series, which would lead to a 4 * 0.7V = 2.8V total drop.

Given a 14V power source, we can use ohm's law to calculate the size of the resistor:

V=I*R
(14v - 4*0.7v) = 0.02 A * R

R = ~560 ohms

Be sure to wire them the correct way .. a reasonable DVM with a diode setting will tell you which side should be connect to positive, and which to ground.

Thanks so much for the help!! Is an 1/8 watt resistor big enough here?

Larry
 
Are you sure...

Are you sure that is not the current draw of those LEDs?

As long as the voltage is correct, the LED will only draw 20 ma. You could hook it up to a car battery and it will still only draw 20ma as long as the voltage is correct.
 
Are you sure that is not the current draw of those LEDs?

As long as the voltage is correct, the LED will only draw 20 ma. You could hook it up to a car battery and it will still only draw 20ma as long as the voltage is correct.

If it is a raw LED, it will draw ?infinite? current, for a very, very short period of time.

How about a data sheet of what you are using? It would s possible it is more than just an LED, but an LED with current limiting already built in to the circuitry.
 
Most red LEDs drop about 1.7 volts across them, while blue and white LEDs can drop 3.4 volts across them, so Bill's maths applies with revised numbers... Four red LEDs in series would need a 330 ohm resistor, 1/4 watt.

The calculated values are fairly loose. You're not going to pick the difference in brightness between 16 and 20 mA, likewise you're not going to cause any damage unless you get the values horribly wrong.

You called out a 12 volt LED. Do you have more info? LEDs with built in resistors do exist but are not that common.

Calculator here.

 
.7 volts is for a NP junction of a pure silicon diode (or any NP silicon junction)
LED’s are not pure silicon as that won’t produce light. You need to have the exact data from the product you want to use if your using a raw LED, which is unlikely.
 
Series

Due to the individual voltage drop across each LED, wouldn?t wiring them in series cause each successive LED to be dimmer?
 
Due to the individual voltage drop across each LED, wouldn?t wiring them in series cause each successive LED to be dimmer?

For LEDs without built-in resistors, the voltage drop across them is fixed, and you need to control the current by supplying a correctly-sized resistor, assuming there is enough total supply voltage. Using easy numbers, if you have a 1V LED (voltage drop found in the datasheet) and a 14V aircraft supply, you could wire up to 14 in series at full brightness. If you use less than 14, a resistor will also be needed in series to "make up" the difference as others have described above.
 
Due to the individual voltage drop across each LED, wouldn?t wiring them in series cause each successive LED to be dimmer?

No. Current remains constant in a series circuit.
Do a quick Google search for LED?s wired in series or parallel. A lot of info comes up. Different applications and/or pluses and minuses.

I can?t imagine the OP using a raw LED though.
 
Hmm

My son put together a LED lighted sign. He wired some of them in series and ended up with each successive one being dimmer...
 
My son put together a LED lighted sign. He wired some of them in series and ended up with each successive one being dimmer...

I wrote a really bad explanation an deleted it, but the simple answer is they where not designed for series wiring. If you have the data sheet we can figure it out.
 
If it is a raw LED, it will draw “infinite” current, for a very, very short period of time.

How about a data sheet of what you are using? It would s possible it is more than just an LED, but an LED with current limiting already built in to the circuitry.

I picked a good old fashioned, simple LED. No limiting circuits in it. It is reated for 12V forward voltage. The datasheet states a max current of 100ma and a recommended current of 20ma. I would like the 20 or less, as I think it may be brighter than I desire. Though I will use a PWM dimmer.

I couldn't find any details about the voltage drop. Will I be able to figure that out with a VOM?

Datasheet:
https://vcclite.com/wp-content/uploads/wpallimport/files/files/LTH5MM12VFR4100.pdf

Thanks again for all of the help.

Larry
 
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No. Current remains constant in a series circuit.
Do a quick Google search for LED’s wired in series or parallel. A lot of info comes up. Different applications and/or pluses and minuses.

I can’t imagine the OP using a raw LED though.

I wanted small LEDs with relatively low light output. Everything that I found that had circuitry was pretty high lumen output and would be excessive for a maplight.

I am making something like this:

https://www.aircraftspruce.com/catalog/elpages/led_maplight.php

I enjoy tinkering with the lathe and mill and thought it would be fun to make these. However, I don't have a lot of space to work with so need small LEDs. These raw LEDs are both small and relatively low light output.

Some would call me nuts. The other day, I ordered an RTD thermistor and a chunk of brass and fab'ed up my own oil temp sensor.

Larry
 
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Without having the LED datasheet, everyone's reply is just a guess.
My guess is a 360 ohm, 1/2 watt resistor in series with 4 Red LEDs in series.
That is based on a 14.5 volt supply and 1.8 volt drop across each LED.
 
It seems that the Aircraft Spruce map light already contains a resistor and will operate in a 12 system without any external resistor.
 
Without having the LED datasheet, everyone's reply is just a guess.
My guess is a 360 ohm, 1/2 watt resistor in series with 4 Red LEDs in series.
That is based on a 14.5 volt supply and 1.8 volt drop across each LED.

Datasheet linked three replies up.
 
Oops! Just re-read the data sheet and it has an internal resistor.

So, does that mean I just throw 14vdc at it and it will figure it out? I assume so and why I originally picked a 12v LED. These should be wired in parallel, correct?

Larry
 
LEDs can be purchased for as little as 12 cents each from electronic suppliers such as Mouser or Digikey or Newark or Allied. Shipping will cost more than the parts.
 
LEDs can be purchased for as little as 12 cents each from electronic suppliers such as Mouser or Digikey or Newark or Allied. Shipping will cost more than the parts.

The ACS link is for an example of the final unit that I am making, not the LEDs. I will get the LEDs from Digikey.
 
I picked a good old fashioned, simple LED. No limiting circuits in it. It is reated for 12V forward voltage. The datasheet states a max current of 100ma and a recommended current of 20ma. I would like the 20 or less, as I think it may be brighter than I desire. Though I will use a PWM dimmer.

I couldn't find any details about the voltage drop. Will I be able to figure that out with a VOM?

Datasheet:
https://vcclite.com/wp-content/uploads/wpallimport/files/files/LTH5MM12VFR4100.pdf

Thanks again for all of the help.

Larry

The datasheet says in the title, "Built in Resistor for 12VDC." The datasheet also isn't self-consistent, as at one point it says 13V max, but has a graph for the current going up to 14V. Given that it does already have a resistor built in, you might try buying an extra or two, hooking one up to a 14V supply, and see how it does. If it burns out you'll need a resistor in series.

Since it already has the resistor, you would wire the LEDs in parallel to each other.
 
Ahh .. these are not "good 'ole fashioned" LEDs, they say so in the title:

LTH5MM12V Series 5mm (T-1 3/4) Through Hole LED Built in Resistor for 12VDC.

They have a built-in resistor / current limiter!

These will work fine; Wire them all up in PARALLEL and hook them up to 12V and you'll be good to go. From the dat sheet, a 14V input will result in ~15mA current, well under 20mA spec.

Thanks, Paul from Flyleds; Yes, I was quoting a silicon Vd. I thought LEDs were close-ish, but, clearly off by a factor of 2+.
 
Thanks for the assistance guys and my apologies for requesting it because I didn't read the datasheet very well.

Larry
 
I used Luxeon Star with a lens kit.
https://www.luxeonstar.com/lxhl-md1d-red-luxeon-star-led-44lm
3021 driver. Dims with a simple pot.
15i1bvt.jpg

It is housed in a crude copper cap with a mounting strap soldered to it.
Not sure I would drill the roll bar support again but I doubt the 1/8? hole impacts it much. I left it unpainted. You can do much better.
xbwvp2.jpg

I used a 350ma single unit, 44 lumens. Might give you an idea and this was 2005 technology. Look at your output at 10ma. It?s somehing like 4500mcd. You can convert that to lumens and at a 20 deg angle it is about .43lm
I set this up as a minimum for night flying along with the backlit instruments and an instrument light dimmer from Vans. I put all of this in more than 12years ago and while some commercial products where available, like you, I desired to roll my own.
I have used it rarely as I just don?t fly at night. Works ok.
 
I do understand the desire to build it yourself, and potentially the need for custom fitting. But have you looked around at the dozens (hundreds?) of options that are pre-packaged and ready to install? Even little 'book lights' have potential; all they'd require is a regulated power supply at the same output voltage as the battery pack.


Charlie
edit: Hey Jon, did you run the engineering calcs on the mounting bolts for that light? Are you sure they're strong enough?

;-)
 
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edit: Hey Jon, did you run the engineering calcs on the mounting bolts for that light? Are you sure they're strong enough?

;-)

I think it might be undersized but I'll just have to live with it!
I was going for a "look" and also hoped the bolt would help as a heat sink. Not sure I executed it very well. I am deserving of the teasing for sure.

There are many more off the shelf offerings in LED than there where when I was doing this, 2005'ish. Many of the products today might also be had with internal regulation. Just hook to power and go. So, you make a good point, but hey, you don't need to build an airplane either, nor do you need to forge your own steel.

The Luxeon product I used is discontinued but the same form factor with improved output and efficeincy is cross referenced in their link. I love manufacturers that keep their legacy product data handy and then give you the current alternative. This product was widely used in the performance lighting world in arrays for RGB color changing products. The biggest issue is it is 350ma. That is a lot of heat to dissipate in a small package.
The housing material, mounting strap, and bolt where an attempt to conduct heat away from the junction. Copper conducts heat at about twice that of aluminum, but it isn't used much due to relative cost. I should have used a brass bolt but guess what Inhad laying around? Steel bolts and copper pipe.
In my experiment, if you leave the LED on full output on a hot day, it will regulate and protect itself by slightly lowering the current and output. In practical use, full output is brighter than it needs to be and I just don't use it much as my night flying is little to none.

Again, folks interested in rolling their own can do much better today but I thought I would share what I did.
 
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