Current limiting for LED sources (one for the electronics black belt ninjas)

[Quinten]

I'm working on a little electronics project and need to drive a 3W SMD LED with a car battery.  One that looks like this:

 

 

It has a 800mA forwarding current and a 2.4V forwarding voltage.  After watching a few Youtube videos I've learned that I need to add a current limiter circuit so they don't get obscenely hot but still are as bright as intended.

 

From googling, I found lots of nice current limiting build-up modules, but I need it to be as small as I can get it.  That lead me to the LM317 current limiting circuit

 

And by using the formula

 

 

my resistor needs to be 1.6 Ohm and wired up as follows

 

 

Is that really all there is to it?

[corsechris]

It is, but bear in mind how much power the regulator will dissipate too. (Input voltage - (2.4+1.25)) x 0.8. You may well need to put it on a heatsink, which will bulk it up. You might be able to use the enclosure of the device for that, depending on what it is.

 

Don’t forget the resistor power dissipation either. 

 

The accolades for electronics-fu should go to the blokes that design and manufacture the devices. Many years ago a colleague got stuck in to designing a video amplifier using discrete components. It was a lovely piece of work with impressive specifications. He was understandably miffed when I pointed out it had already been done....you could buy what he had designed in an 8 pin package for about £2, with better performance too.

[Richard (OldStager)]

I use two of SMD ones above ^^,  in series on the back of the Westy, I could have used a regulator but didn't. It was a far neater job to just use a 25w 6 ohm resistor, this is mounted in the airflow under the car and it's heatsink is the chassis. Been working like that for around 8 years so far, never over heated yet.

 

ETA

As Chris has mentioned if using a reg, you will need a decent heatsink, if mounting in an alloy box, the box itself would do, if it's plastic then a multi finned alloy one will suffice.  I may have some knocking about as I used to muck about with stuff like this some years ago.

[Ian Kinder (Bagpuss) - Joint Peak District AO]

It may help if you tell us it's intended purpose? Hence we could then possible suggest an off the shelf item, that's suitable etc.

 

I'm guessing rear fog/brake light?

[Quinten]

I want to keep the project under wraps for now, but the light will not be permanently on, but it can be on for extended periods.  It will be mounted on an aluminium sheet the size of a coaster or so as I can go to a fair size in X and Y axis, but not in Z axis (somehow describing it in H L or D seems wrong).

 

As for an off-the-shelf solution, I've seen

 

 

but the capacitor makes it rather bulky.  Alternatively there is

 

 

which is much flatter but has an output current of 900mA and I probably rather go under the 800mA instead of over (even though it probably wont matter in real life).

 

I should have really done all this investigation before I bought the SMD LEDs, but hey ho, I may even learn something along the way doing it like this...

[Richard (OldStager)]

4 minutes ago, Quinten Uijldert - Webmaster said:

I want to keep the project under wraps for now, but the light will not be permanently on, but it can be on for extended periods.  It will be mounted on an aluminium sheet the size of a coaster or so as I can go to a fair size in X and Y axis, but not in Z axis (somehow describing it in H L or D seems wrong).

 

As for an off-the-shelf solution, I've seen

 

 

but the capacitor makes it rather bulky.  Alternatively there is

 

 

which is much flatter but has an output current of 900mA and I probably rather go under the 800mA instead of over (even though it probably wont matter in real life).

 

I should have really done all this investigation before I bought the SMD LEDs, but hey ho, I may even learn something along the way doing it like this...

A coaster sized heat sink should be sufficient, it's just about surface area and its ability to dissipate the heat. I've been messing with LED's since the 80's ( they were rubbish back then) although many did end up in semiconductor heaven as I played with things. Can't wait to see what it is you are building now....

[Blatman]

Make up light for the sun visor...

[TAFKARM]

59 minutes ago, Blatman said:

Make up light for the sun visor...

Red light...Dutch bloke...I’ve got no ideas. 

[Richard (OldStager)]

17 minutes ago, Rush Motorsport said:

Red light...Dutch bloke...I’ve got no ideas. 

Red lights and Holland?, no idea what you're talking about.... 

[corsechris]

@Quinten Uijldert - Webmaster  the current sense resistor is going to dissipate a fraction over 1W, the regulator dissipation of course depends on the supply voltage. The device itself is rated at a max temp of 125C, so assuming no heatsink, would be good for between 4 and 5W dissipation (23C/W). Bolting it to the alloy ‘coaster’ would help that significantly.

 

Need to know your intended supply voltage really.

[Quinten]

7 minutes ago, corsechris said:

Need to know your intended supply voltage really.

 

Car battery on a running car (up to 15V I believe?).

 

Would it help to reduce the supply voltage to say 5V using a buck (DC-DC) converter?  I've found some really small (flat) ones that could be suitable.

 

[corsechris]

13 minutes ago, Quinten Uijldert - Webmaster said:

 

Car battery on a running car (up to 15V I believe?).

 

Would it help to reduce the supply voltage to say 5V using a buck (DC-DC) converter?  I've found some really small (flat) ones that could be suitable.

 

 

That would work fine. The 'buck' converters are very efficient so heating would be minimal. 5V going in to the current limiter/LED would give you  a Watt of power dissipation in the LM317 so would raise it about 25C over ambient. Fix it to the alloy (using an electrically insulated mounting) for extra safety, but not essential.

[Kit Car Electronics]

I use DCDC converters instead of linear voltage regulators in my Freewheel relay receivers because they are mega efficient. You can even get ones that are footprint identical to the common regulators, like this 3.3V 1A unit. https://uk.farnell.com/xp-power/vr10s3v3/dc-dc-converter-3-3v-1a/dp/3525426

 

I have a little circuit board for mounting it with a TVS protection diode and thermal fuse if needed that I can grab a picture of later

[Quinten]

The more you can throw at me Mike the better

 

Sounds like I'm on the right path now.  The less Voltage I can send into the SMD LED, the better then?  I'm assuming I still need to limit the current with the LM317 but that heat is then becoming less of an issue.

[Kit Car Electronics]

The LED will drop 2.4V, that's fixed, so you just need to drive with a source of at least 2.4V. Then you need to limit the current to 800mA so the LED doesn't burn out (resistor or other current limiter circuit) The LED will generate a lot of heat, nothing you can do about that, but the rest of the heat comes from the power dissipated in the current limiting. So if you drop the voltage close to 2.4V efficiently (eg 3.3V), you have less waste heat to manage as you're only running 0.9V * 0.8A = 0.72W on a resistor