Protecting LEDs from overcurrent in a car?

[Andy1845c]

I have been thinking about this for a few weeks now, and I haven't come up with any good ideas/solutions....

As we all know, voltage in a car can be 11. some odd volts with the engine off, or 13.8 volts or so with the engine running.
I am working on a project that powers leds from a cars electrical system. I have found a resistor value that keeps current in check at 13.8v and is only slightly dimmer at 12v.

But I am worried about the voltage going over 13.8 volts. I don't know how likly this is, but it will burn all my LEDs out in short order if it does. Thats not somthing I really want to happen.

I wish I could use a regulated 10v supply, but I don't think this is going to be possible with the voltage in the 12v area. I have read about low dropout regulators, but if i'm understanding correctly, they need to be at close to max current to exibit the low drop out voltage.

I know there is going to be no win/win perfect solution, but is there any clever way of protecting things?

 

[bananasiong]

See the attachment, the dropout voltage is increased if the load current is increased, and maximum up to 0.5 V only. You're just running LEDs right? So the current drawn is not that high.

 

[Sceadwian]

What about an automatic buck/boost converter?

 

[Oznog]

What's the ratio of the LEDs forward voltage to the source voltage?

Once a ballast resistor is used, essentially current goes up with the diff of LED forward voltage to the source voltage, rather than exponentially. Since you say the intensity doesn't drop that much when you go from 13.8V to 12V, it won't increase a tremendous amount going higher either.

Going 2x the rated current for brief periods like during spikes you get when you run the starter will generally not be a problem.

 

[Optikon]

Put a 10ma constant current diode in series.. this will keep current to max of 10mA regardless of battery voltage.

 

[phalanx]

In terms of size and ease of use, Zetex makes two constant current buck converters which are perfect for your application. The ZXLD1350 has a 350mA limit and the ZXLD1360 has a 1A limit. I've used both extensively and they are a piece of cake to work with. They are dimmable too!

 

[ThermalRunaway]

Put a 10ma constant current diode in series.. this will keep current to max of 10mA regardless of battery voltage.

That's a suggestion I'd not heard of Optikon. I'm guessing you'd still use a current limiting resistor though with the diode being an addition? I'll have to do some googling on this...

Brian

 

[Optikon]

That's a suggestion I'd not heard of Optikon. I'm guessing you'd still use a current limiting resistor though with the diode being an addition? I'll have to do some googling on this...

Brian

Look at Central semi CCLM series... if you dont want to buy one, you can make one easily from a JFET & resistor in source. Thats how they are made anyhow just sorted for value due to large IDSS spread on Jfets.

 

[Hero999]

You could even build a simple constant current switching supply with a couple of transistors.
https://www.romanblack.com/smps/a04.htm
If the open circuit voltage isn't an issue then remove the Zener.

 

[gramo]

Here's a simple solution, use a zener diode, you'll have to play around with the circuit to suit your application (as the resistances/current requirements would be different).

I tried to make the circuit as generic as possible - it might just suit your app straight away?

Here is a video simulation of the circuit

 

[Andy1845c]

Thank you everyone for the suggestions, I have been googling more about them all day

I've never heard of a constant current diode. I did some reading on them but didn't find much info. From what I did find, it looks like 10mA is about all you can run though one? I am looking for around 40mA for each string of 4 leds.

I did look into buck/boost converters. Havn't seen the perfect one yet though I looked into buckPuck LED drivers, but most are constant current, and for 72 strings of leds, I don't know how to make use of one.
or any constant current device for that matter., unless I used 72 of them, and I don't have room for that.

gramo - Your zener diode circuit has me interested, yet scratching my head. I don't understand how it works. I had though about a way to use a zener, but couldn't figure out how, or if it was even possible. I'm gonna sleep on it and look at it more tomarrow.

I found this site: https://www.muzique.com/schem/lm317.htm
It says to make sure input on a lm317 is atleast 1.5v greater then output. I though you needed closer to a 3v seperation? Who is right? If I could get away with 1.5v, I could possibly regulate the supply at 9.5v. But that also puts me very close to the total forward voltage of the leds.

 

[gramo]

gramo - Your zener diode circuit has me interested, yet scratching my head. I don't understand how it works. I had though about a way to use a zener, but couldn't figure out how, or if it was even possible. I'm gonna sleep on it and look at it more tomarrow.

Put simply I guess, the zener will try its best too change its resistance to keep its rated voltage. That said, you need to keep a few things in mind, mainly nominal current and maximum power ratings of your zener.

Designing a circuit that is protected by a zener can be a little mind boggling, but just remember that the current limiting resistor is going to act like a voltage divider with the load. Perhaps this video will explain it more;

Click here to watch the example

I know that I said 90.1 milliohm instead of 90.1 milliamp but you get the jist of it

 

[RODALCO]

Design your LED circuit for the highest possible voltage. In a car trhat is 14.4 Volts max.

Run the LED at about 18 mA's and put a small diode (1N914) antiparralell with it.
Just the simple series resistor is fine, depends which colour LED you use.
Say for 3.3 Volts blue LED at 14.4 Volts @ 18 mA's.
(14.4-3.3) / 0.018 = 616 hm:

A 620 or 680hm: resistor 1/4 or 1/2 Watt will do.


The hash from the ignition coil or fan motor, is usually the LED killer because off short spikes of opposite polarity.

If you do that your LED's will last forever.

edited to insert calculations.

 

[Andy1845c]

Thanks for the video, Gramo Thats pretty cool. What is that sim program? It does help me understand better how it works. I think if I get time tonight or this weekend i'll make the circuit and play with it.

 

[audioguru]

You said 40mA. What is the absolute max current allowed for your LEDs? Most 5mm LEDs are 30mA max when cool.

You said 9V. What is the max forward voltage for each LED and how many will be in series? LEDs have a range of forward voltage and if yours is 3.9V max and you have 3 in series then they will need 11.7V minimum.

Some LM317's stop regulating when their input voltage is less than 2.5V more than their output voltage. At 40mA, a typical LM317 stops regulating when its input voltage is less than 1.75V more than its output voltage. You don't know if your LM317 will be typical or if it will need the max voltage.

You can make an LM317 regulate the current at 25ma by connecting a 62 ohm resistor from OUT to ADJ and taking the regulated 25mA from ADJ. The minimum input needs 1.25V more than a voltage regulator so if the input drops to 11V then the output voltage minimum will be 7.25V.

 

[Andy1845c]

Hi AudioGuru,

Thanks for the reply.

These LEDs have a rated max forward current of 70mA, but don't get alot brighter at 70 vs. 40mA. I like to keep them in the 35-40 mA range, as that seems to give me plenty of light and seems like it will be easier on the LEDs.

The average forward voltage is 2.1v. I wanted to go with 4 in series. 3 in series would be okay too.

I have so many strings of leds in mind, that constant current seems impractical. I would rather have a stable voltage and just use resistors.

Some LM317's stop regulating when their input voltage is less than 2.5V more than their output voltage. At 40mA, a typical LM317 stops regulating when its input voltage is less than 1.75V more than its output voltage. You don't know if your LM317 will be typical or if it will need the max voltage.

That answers a question I had Thanks

I wish I could get a nice stable 9.5 - 10v. Then i could use 4 in series and have 1/4 watt resistors on each string that would stay cool. If I did the math right, at 14v the resistor would be dissapating .22w and I assume would be very warm. Since this is going to be in a car, I am trying to keep in mind that its a 120 degrees in a car in the summer sun, so nothing is going to cool that well on those days.