Simple LED circuit (I thought)

[audioguru]

With a pretty big REAL aluminum finned heatsink, your LEDs "might" survive 700mA IF you believe the Chinese ad.
Your power supply might be a little high at 13V.
Your LEDs might be on the low end of their range at 3.2V each.
Your 1 ohm resistor might be a little low at 0.95 ohms.
Then the magic (simple) calculation says the current is 3579mA (more than 5 times too high) which will cook the LEDs "well done".

 

[denno]

Thanks for the replies guys.

If I run the LED at just 350mA, do I still need that gigantic heat sink? As I'm mounting them inside a lampshade, I don't have a great deal of room, and also weight could become a problem..

 

[audioguru]

If each LED has a voltage of 3.8V then at 350mA it dissipates 3.8V x 350mA= 1.33W. Its datasheet should rate its thermal resistance from its chip to its case. Then a heatsink can be selected for it.

 

[audioguru]

If each LED has a voltage of 3.8V then at 350mA it dissipates 3.8V x 350mA= 1.33W. Its datasheet should rate its thermal resistance from its chip to its case. Then a heatsink can be selected for it

Why not buy an AC-powered $20.00 LED light bulb instead? I use compact fluorescent light bulbs that cost a little more than one dollar.

 

[denno]

I suppose I just wanted the challenge of making it all myself. I've made some previously, but not with such powerful LEDs, but it's all part of the learning process.

 

[rumpfy]

Denno,
1. It sounds like you are in OZ?
2. In post 17 you said that two of the leds measured 2.5 volt and one measured 7 volt.
I would say that the 7 volt led is stuffed, and the two x 2.5 volt leds are OK.
To test this, you could use 2 LED's on their own and use a 18 or 22 ohm resistor in series with the 12 volt power supply to check they are OK. This value resistor would give about 300 mAmp. Many posts here talk about the need for a heat sink. Use of a heat sink is critical. however, for testing purposes, try to run the two LED's but only for a short time say less than 2 or 3 seconds. Once you confirm they are /are not OK then you can proceed to complete your project.
The heat sink size is easily calculated, and there are many LED threads here.
Hope this helps.

 

[kinarfi]

I know most people don't like the idea of using LEDs without a resistor, but since you have a 12 volt power supply, try 4 LEDs in series, no resistor and see if that gives you enough light for your project. I speak from experience, I have a light on my bench that is 4 in series, on a fan cooled heat sink and they have lasted for months so far, I also use the same set up on my off road vehicle, putting 6 string of 4 tied directly to battery power via a switch. Haven't burned an LED yet and I have run them for hours at a time.

 

[audioguru]

I know most people don't like the idea of using LEDs without a resistor, but since you have a 12 volt power supply, try 4 LEDs in series, no resistor and see if that gives you enough light for your project. I speak from experience, I have a light on my bench that is 4 in series, on a fan cooled heat sink and they have lasted for months so far, I also use the same set up on my off road vehicle, putting 6 string of 4 tied directly to battery power via a switch. Haven't burned an LED yet and I have run them for hours at a time.

Your LEDs survived due to pure luck.
But we do not design circuits using luck. Instead we use theory.

We know that any LED could have a voltage within a range of voltages as shown on its datasheet and we don't know the actual voltage of our LEDs unless we test each one.
We also know that a "12V" battery is rarely 12.0V. It is probably 13.8V or might even be charging at 14.4V.

Then we take all the "worst case" numbers, crunch them and use extra voltage headroom then calculate a suitable current-limiting resistor for the string of LEDs.

 

[kinarfi]

"Your LEDs survived due to pure luck.
But we do not design circuits using luck. Instead we use theory."

Theory - each led is rated at 3 watts, for ~ 3.6 v, 4 X 3.6 v = 14.2v
Measured voltage of vehicle, 14.2v with engine running.
Minus losses in wires. Actually tests, current was less than max specs, 700 ma and way under with out engine running.

"We know that any LED could have a voltage within a range of voltages as shown on its datasheet and we don't know the actual voltage of our LEDs unless we test each one.
We also know that a "12V" battery is rarely 12.0V. It is probably 13.8V or might even be charging at 14.4V."

"Then we take all the "worst case" numbers, crunch them and use extra voltage headroom then calculate a suitable current-limiting resistor for the string of LEDs."
Head room voltage is 0.0000 Maybe even less than zero, You tell me watt size resistor is needed.
This is NOT luck. If you'll admit it, It's PROOF that you don't always need a limiting resistor!
Someone needs to get out of his box of rules

 

[audioguru]

Have you ever tried to buy some "3.6V" LEDs? They will laugh at you because some will be 3.2V and others will be 4V. They might not even have any that are 3.6V.
If you connect four 3.2V LEDs in series (total is 12.8V) and connect them to your 14.2V vehicle then they will smoke and burn!

 

[Diver300]

With discussions like these going on, and the most popular calculator not taking any account of tolerances, I've put this calculator online:-

https://www.mtrak.co.uk/led_calculator.html

It takes account of how much the current will vary with supply and LED voltage changes, and resistor tolerance.

 

[Diver300]

"Your LEDs survived due to pure luck.
Actually tests, current was less than max specs, 700 ma and way under with out engine running.

The circuit you have will be very, very sensitive to supply voltage variation.

You might want to bear in mind that car manufacturers generally rate equipment in cars to 9 - 16 V for continuous running, 28 V for 1 minute, and a load dump surge of 30 - 60 V for around 1/4 second. Of course there is the reverse polarity test, with -14 V as well.

LEDs without other protection components would probably be die on many of those conditions, and would just not light at all at 9 V.

 

[kinarfi]

Some good values to try:

As supply voltage:
For molex: 5, 7 and 12 volts
Batteries: 1.5 and 9 volts

As led forward voltages:
Red and green: 2 volts
Blue and white: 3.0 - 3.5 volts

Led current:
20mA will work for most regular leds.
Superbright leds can go from 30mA up to several amps.

Copied randomly from the internet,
If you had one blue or white LED, 3.0 - 3.5 volt powered by 2 1.5 batteries, what resistor would you use, do you see any resistors in these cheap 9 LED, 3 AAA battery flashlights.
Why is so hard to believe / accept that 4 bright white LED in series with no resistor won't work on a car?

 

[Diver300]

Copied randomly from the internet,
If you had one blue or white LED, 3.0 - 3.5 volt powered by 2 1.5 batteries, what resistor would you use, do you see any resistors in these cheap 9 LED, 3 AAA battery flashlights.
Why is so hard to believe / accept that 4 bright white LED in series with no resistor won't work on a car?

Of course 4 LEDs can work in a car, but it is relying on a luck to work, and as such is poor design.

The current is not controlled, and the brightness varies a lot when the supply voltage changes due to the alternator running or not. It would only take a little more voltage variation for the LEDs to turn off completely or to blow. Variation of the LED voltage due to manufacturing differences or temperature will also add to the uncertainty in the current.

Admittedly high power white LEDs have quite a lot of internal resistance, which helps to stabilise the current, and that is probably why it works at all. However the current variation is too large for the design to be taken seriously, or to be recommended for others to try.

The LED calculator is partly there to demonstrate how much the current will vary with tolerances in components. If you select the E6 range of resistors, which gives a 20% tolerance, no design is possible unless there is a very wide range of currents allowed. By relying on the internal resistance of the LEDs, you are in that sort of situation.

 

[audioguru]

I bought a very nice but cheap Chinese flashlight that has 24 white LEDs in parallel, no current-limiting resistor. I am amazed that all LEDs appear to have the same brightness so the factory must employ little girls to sort them. It is powered from three very old fashioned "Super Heavy Duty" AAA cells.
If I replace these old cells with modern alkaline cells then I expect the LEDs to blow one-at-a-time like fireworks, or maybe just one big BANG!.

 

[alec_t]

It has been suggested in another thread that each LED may be made with some internal resistance to aid current -balancing in parallel arrays.

 

[kinarfi]

I bought a very nice but cheap Chinese flashlight that has 24 white LEDs in parallel, no current-limiting resistor. I am amazed that all LEDs appear to have the same brightness so the factory must employ little girls to sort them. It is powered from three very old fashioned "Super Heavy Duty" AAA cells.
If I replace these old cells with modern alkaline cells then I expect the LEDs to blow one-at-a-time like fireworks, or maybe just one big BANG!.

Replace them with the best alkaline batteries you can find and it will keep right on working, then maybe you'll see that LEDs have a much wider base of operation than you know.

Of course 4 LEDs can work in a car, but it is relying on a luck to work, and as such is poor design.
Admittedly high power white LEDs have quite a lot of internal resistance, which helps to stabilise the current, and that is probably why it works at all. However the current variation is too large for the design to be taken seriously, or to be recommended for others to try.
.

It's actually a very good design if you expect and accept the fact that the the light intensity will change, when driving, you need the brighter light, when stopped, engine off, you benefit by having less current draw with some decrease in light, but still plenty of light, considering I have 24 (6 strings of 4) LEDs in each of 2 lights and NO failures. I also have a light bar that works off of a SMPS and the light is constant, but draws more current when the engine is off plus the disadvantage of this is it is more complicated and more things to go wrong.

 

[kinarfi]

It has been suggested in another thread that each LED may be made with some internal resistance to aid current -balancing in parallel arrays.

In that other thread, was it pointed out that the internal resistance would be a source of extra heat and become a grave disadvantage and in general, serve no purpose?

 

[audioguru]

When an LED heats then its forward voltage drops which increases its current which increases its heat which increases its current which increases its heat .... Ka-boom!
Unless you regulate the current.

 

[Jaguarjoe]

I don't need the LEDs to be pushed to their maximums

'cmon, man, no guts no glory!

When Bugatti was building racing cars 100 yrs ago, he was questioned about the lack of brakes on his speedsters. His reply:"I build cars to go not to stop!"