Can I add 2 Current Sources together?

[Ocelaris]

I would like to have 4 350ma capable LEDs running my brake lights, one current source running 200mA for the running lights, and then go to 350mA for when I hit the brakes. The problem is that I have 2 positive inputs +12v, and one common ground... Both +12v inputs stay on so normally a smaller filament is lit, and when the second input is activated a second filament is lit, so both are lit at the same time... that's the problem I think... If they were mutually exclusive I could use a 200 and 350ma current source, but I don't know how to turn one off if the other is on?

So I was thinking of using two constant current regulators (possibly the switching regulators I am working on in the projects forum) in parallel, or simply a LM317 in constant Current mode...

Does that make sense, Like 150+200= 350mA? Or would that mess with the current sensing of the chip?

I could do some finagled relay setup, where the first current source is turned off when the second one is triggered, but I'd rather use something solid state if possible to not have any moving parts, and to utilize the speed of the LEDs.

Thanks for the help, Bill

 

[Aily Sajjad]

You can use two different current sources with diodes at output of every current source (capable of operating at circuit current). But this is practicable only if the output voltage at the current sources behave similarly for load changes (when both operating).

Aily

 

[hjames]

If this is going into a car, you have other issues that need to be dealt with - namely load-dump and double-battery jump starts (most electronics that plug into a car are usually rated, and will survive for 40-70V transients...).

In any case, the sum of all the currents going into a node equals zero - so if you have two current sources going into a node, and one net leading out, then the current going out is going to be the sum of the two going in.

The hitch is that these refer to ideal current sources - real current sources have limits to their operation, and occasionally they don't do too well when the voltage across them is < some value, and most definitely don't do too well when the voltage across them is negative... The LM317 in particular has a ~1.6V dropout in addition to the 1.2V sense, which means it will need~2.8V of overhead. Red LEDs of that wattage are going to be somewhere between 2.5-3V which means that the total voltage in series is going to be 10V - 12V, which doesn't leave enough headroom for the LM317... If you get rid of one of the LEDs, then it might work with the LM317.

 

[Ocelaris]

It's actually going into a motorcycle, but same basic problems, if not worse since it's a much smaller system amperage wise...

I can drop one of the LEDs, no problem... Or do two parallel strings of 2 even... If everything goes according to plan, I'll have a switching buck regulator made up, and use those instead of the LM317s... The Vf of the switching regulator is like 2mV or something much smaller if I recall...

But it would be a good idea to diode isolate both sources in any circumstance, lose half a volt or so with an 1N4001 but if it helps, can't hurt. If they're driving the same load, shouldn't it be fine? i.e. the voltage should be similar since it's the same load... I just don't know if when the 200ma is running, and I put the 150ma source "AND" (logic and) the two, if the 200ma is going to scale back it's voltage... since they're sharing the same ground. Would I have to diode isolate the grounds as well?

Thanks! at least I can give it a whirl on a breadboard and see...

 

[Analog]

You can use PWM to vary the brightness very easily.

 

[Roff]

You can use PWM to vary the brightness very easily.

And that's how it's done in the factory LED taillights/brakelights that I've noticed. You can tell by scanning your eyes rapidly from side to side while looking in the general direction of the vehicle. It works best at night.

 

[Analog]

And that's how it's done in the factory LED taillights/brakelights that I've noticed. You can tell by scanning your eyes rapidly from side to side while looking in the general direction of the vehicle. It works best at night.

Yes! I can really see it with Cadillacs for some reason - perhaps they use a low enough frequency to be able to "catch" with the human eye?

 

[Ocelaris]

I had heard that the PWM had set off some people's radar detectors! But we don't have them in VA... Cadillac may be an innovator with LEDs, but you're right, the frequency is wayyy too low, very buggy... Never see that with the Nissans or Toyota Prius... they're just really bright, almost hurts.