questions about LED's (help: electrical engineers)

[3100 MPFI]

I need some help with figuring out some of the specs for LED's that are to be run in parallel.

I have figured out that the voltage drop across the LED is only 2.2 volts for a series of 32 bulbs, but I am having some issue finding out what the desired LED current in MILLIAMPS should be. I only get from 20-30 mA for basic, and >200 mA for upper requirements.

Just playing around with it, I figured out the resistors for the 20 mA, and for the 200 mA, but to do this correctly, I really need a guiding hand and some insite. If I just go from 20 mA to 30 mA, it is totally different resistors, etc.

Can anyone help me with this?

[CobraCDR]

This might help

The resistance of the led's is governed by the following equation:

Rs=(Vinput - Vled)/Iled Where: Vinput = source voltage
Vled = Activation voltage of the led (0.7 typical)
Iled = Led current draw
OHM's law will also help you:

Voltage = Current X Resistance or V = I*R

Resistor Networks are governed by the following equations:

Resistors in Series: Rt = R1+R2+R3 where: Rt is total resistance and r1-r3 are resistors connected in series

Two Resistors in Parallel: Rt = (R1XR2)/(R1+R2)

More than two Resistors in Parallel: Rt = 1/((1/R1)+(1/R2)+(1/Rn))

Using these simple equations you should be able to figure out based on the led's you are using (which also have their own resistance which you have to calculate) what voltage, resistors and current source you will need.

You are on the right track with putting them in parallel as according to this equation for a voltage divider:

Vout= Vin X (R2/(R1+R2)) in series voltage is divided between resistance (LED's) but in parallel the voltage on each led will remain constant but the current will be divided. If you look at the box the leds came in it will tell you the minimum activation voltage, current draw and resistance.

Hope this helps Tat. Do yourself a favor and go to Radio Shack and pick up a Engineers Mini Notebook for lik 2 bucks called (Formulas, Tables and Basic Circuits. It is by Forrest M. Mims and covers basic circuit design and theory.

Ken

[3100 MPFI]

Thanks kenny. I would have asked you if you were on AIM, but you never log on.

I will check into that. The thing that is really giving me an issue is that I can only figure this up to 32 LED's, and I am going to use more than that. As to how many, I don't know yet. It could be a few as 32, or as much as 200.

Check your PM's, BTW.

[Juglenaut]

It would be easier if you are running that many to invest into some LED driver IC's.

Awe crap and I went to school for the jargon and I could have spent 2 bucks!

I am wondering if you could reflect some Analog circuit anylisis, and predermine a 4 stage signal amp with a gain of 12 total with the second stage at 1.8 gain and the last stage unity. What is Z minimum.

If I were running that many I would setup a charge pump with reactive gain and a setpoint regulator to keep everything stable at that point you could use 12 volt or 5 volt LED's, a charge pump will regulate total current capacity and the regulator will keep voltage in check after the pump.

[CobraCDR]

It would be easier if you are running that many to invest into some LED driver IC's.

Awe crap and I went to school for the jargon and I could have spent 2 bucks!

I am wondering if you could reflect some Analog circuit anylisis, and predermine a 4 stage signal amp with a gain of 12 total with the second stage at 1.8 gain and the last stage unity. What is Z minimum.

If I were running that many I would setup a charge pump with reactive gain and a setpoint regulator to keep everything stable at that point you could use 12 volt or 5 volt LED's, a charge pump will regulate total current capacity and the regulator will keep voltage in check after the pump.

I'm not an EE douche, I'm a ME Not saying the mini engineer's handbooks are a replacement for formal education but for simple theory and electronics for those of us without electrical backgrounds it gives you a path to go down. Looks like he has your answer there tat

[Superdave]

I'd just hook it all up with a decent current limiting resistor and put an ammeter in series. Play with the resistor values till you get the brightness you want.

If you want to get into adding regulators you may as well build a true dedicated power supply. Probably alot easier to build it all around a simple ATX computer supply..


It's all too easy to get lost in the math, just expierement till you get the desired results.

[IsaacHayes]

Standard 5mm 20ma leds? http://led.linear1.org/led.wiz Do a search in google for led calculator. There are several that can show you how to wire them the most efficient way in series and parallel so that you are not dropping a lot of wattage in the resistor.

Basically you want to keep the amount of voltage dropped in the resistor very low or you'll burn it all up as heat and have to get a huge wattage rated resistor. So you want to use the leds in series as much as possible.

For longest life I would drive them at 20ma. 30ma some leds can be driven too but with 5mm they do not like overdrive and the heat can shorten their lives a lot especially if you are using china no name leds. You can get some super cheap decent very bright china leds though for penies per peice in bulk though. http://www.besthongkong.com/ for example. Good selection and cheap incase you blow some. They seem to have high power leds now too.

I'm assuming these leds are red or old school yellow green if the voltage forward is 2.2v? If so 7 in series would light them up without a resistor and be safe for car voltage while the car is running. Or 6 with a resistor for a little more brightness. Then just make multiple clusters of these for as many leds as you have. Keep in mind car voltage can have huge spikes and negative voltage which can knock out leds since alternator voltage is very dirty and you got counter EMF from relays/etc in the car. A diode would help protect them some and would give you .7voltage drop too.

5mm stuff is pretty basic so do a quick search like I said and you should be good. I don't do much 5mm leds I do the big ones some that run 1 or more amps of current per led!!! Here are some past modifications I've done:

http://isaachayes.home.mchsi.com/leds/2C-Mags/

http://isaachayes.home.mchsi.com/leds/TriLux/

EDIT: Wait, what is your power source? Is not for a car?? Need to know what your power source is the voltage and if it's current limited.

[Juglenaut]

With a reactive charge pump input voltage is of no concern, that is if it is under 24 volt and over 5 volt.
If you have a matched series parallel you can drive a bank with one resistor if needed, however if one goes dead the current rises. By matched I mean finding the Beta of every LED you have and put them in parallel and series one resistor to all inputs. Current in this case will divide by each paralleled, if you limit the current then you can subtract each from the source amps. Example Current limited to 2 amps minus 80mA (.080 A) for every LED in a parallel, this way you don't have to use a resistor per LED, however it is up to the person.

Zin is in most cases higher impedance than Zout. Inputs are in most cases higher resistance than outputs. Matching a RCA resistance to the input will yeild a 10% gain in EFF.

[Kardain]

Um... a series of 32 bulbs is going to require quite a bit of juice. best bet is to use that link previously mentioned and run in series/parallel:

http://led.linear1.org/led.wiz

that site is my bible

If so 7 in series would light them up without a resistor and be safe for car voltage while the car is running. Or 6 with a resistor for a little more brightness.

I always set the input voltage as 13.8 rather than 12 as that will be w/in tolerance for the spikes.

So far, of my own vehicle and some customers' vehicles - (its a side hobby of mine), none have blown due to overvoltage (of approx 1000 bulbs)

[IsaacHayes]

My car has ran 13.8 to 14.4 volts before. I usually just put in 15v.

[Kardain]

that works too. My experience is that usually LED spec sheets have an operating range (example 1.5 - 2.5 volts). Use the midpoint of that range and something over 12v as the source voltage, and depending on however many LED's in the series, they will soak the spike. that's the reason I use 13.8

It also allows for underpowering (when the car is off) w/out dimming out the LED's too much.

Most vehicle alternators will spike out at around 14.5 - 14.4 on initial startup, then taper down to 14-ish - again, depending on year/make/model/brand of alt/alignment of planets/etc.