I have a dozen or so dc-dc modules, adjustable from 0.75 to 5.5 volts at 6 amps (if my memory is correct), adjusted by external resistor. I'm wondering how hard it would be to use them as a constant current source for a couple 3w LED's. Here is the datasheet. I want to use these to drive the LED's from a 12 volt regulated source, at higher efficiency than using a resistor or linear regulator, and cheaper than having to buy LED drivers. I'm guessing a simple voltage divider between the output and adjustment pin would do the job. Or would I be better just using a trimmer pot and setting the voltage to wherever the LED's draw their rated current? Will an LED's resistance change during use that would cause them to take more power from a set voltage? I expect them to get quite warm at times. They're going to be headlights on my electric bike, so they will see temperature variations during use, heating up while at red lights and cooling off while riding. They will be mounted to aluminum heatsinks using thermal paste mixed 50/50 with epoxy in a thin layer.

 

3W LEDs absolutely must be driven from a constant-current source; not a fixed voltage.

The feedback loop in a constant-current power supply is radically different than the feedback loop in a constant-voltage supply. Besides, by definition, a constant-current supply must have a current-sensor; a constant voltage supply does not...

 

3W LEDs absolutely must be driven from a constant-current source; not a fixed voltage.

Thanks, I've only ever powered small indicator LEDs from constant voltage, I'm still a bit new to the high power ones. So constant current it is, any way to use these converters? If not I may just set to 4v and calculate a current limiting resistor, from 4v the power lost won't be much compared to 12v, and I have a bunch of 5 watt 0.28 ohm resistors I could use. My ebike has a 60v battery and I have a 60-12v 5 amp power supply that puts out 12.2v steady on my multimeter to power all lights and other features. I'm already at 3 amps, so I need to be as efficient as possible when I'm adding lights. Plus that power adds up over the hour or so of riding I can get on a charge.

 

Running a high-power LED with a series resistor with only 0.7V of drop in the resistor is a far cry from a proper constant-current source. Only slightly better than no-resistor at all...

 

Running a high-power LED with a series resistor with only 0.7V of drop in the resistor is a far cry from a proper constant-current source. Only slightly better than no-resistor at all...

However it is an answer to being able to use the devices he has on hand.

 

Whats the minimum voltage drop? I would be fine with running the LED's around 2 watts if that makes a difference. I could adjust to 5.5v but the converter won't go any higher. Still my preference is to wire these converters as constant current using a voltage divider.

 

Whats the minimum voltage drop? I would be fine with running the LED's around 2 watts if that makes a difference. I could adjust to 5.5v but the converter won't go any higher. Still my preference is to wire these converters as constant current using a voltage divider.

Explain more clearly what you mean by all the above.

We are all beginners in electronics and have absolutely no idea what you mean by "Whats the minimum voltage drop?" and "Still my preference is to wire these converters as constant current using a voltage divider. "
You must be an electronics expert to be able to wire DC to DC converters into constant current devices by using a voltage divider.
This is all way over my capability.

 

OK, I have looked over the DC converters you have and to make them a constant current source would be difficult. There are other paths to making constant current sources that are much less painless than trying to adapt these converters.

Below is a simple constant current switching regulator. It has a voltage limit set by zener D1. The current output is set by R8 (Iout = 1.2/R8). The values indicated for L1 and C4 were for my application. Look at the data sheet to fine tune for your application.

 

Simply turn the converter to 5.5v

The output voltage of the Austin MicroLynx TM II 12V
SIP can be programmed to any voltage from 0.75Vdc to 5.5Vdc by connecting a resistor

and connect 8 x 5 watt 0.28 ohm resistors to a single 3watt LED.
Reduce the voltage and remove one resistor at a time until you leave 1 resistor and the brightness you want.

 

OK, I have looked over the DC converters you have and to make them a constant current source would be difficult. There are other paths to making constant current sources that are much less painless than trying to adapt these converters.

Below is a simple constant current switching regulator. It has a voltage limit set by zener D1. The current output is set by R8 (Iout = 1.2/R8). The values indicated for L1 and C4 were for my application. Look at the data sheet to fine tune for your application.

View attachment 90704

Thanks for the info. Out of curiosity, how much power is the zener rated for? with my LED's the supply would need to put out at least 5 or so amps.

Simply turn the converter to 5.5v

The output voltage of the Austin MicroLynx TM II 12V
SIP can be programmed to any voltage from 0.75Vdc to 5.5Vdc by connecting a resistor

and connect 8 x 5 watt 0.28 ohm resistors to a single 3watt LED.
Reduce the voltage and remove one resistor at a time until you leave 1 resistor and the brightness you want.

I think this is going to be what I do. I'm going to run the LED's at a bit lower than their rated power as well so I have a safety margin.

 

If the module had a remote voltage sense line you could use that to generate a constant current through an added series resistor, but it's not apparent that you can do that from the spec sheet. For example 0.75V across 0.15Ω would give 5A.
You would have to look at the module and see if you can spot where the voltage sense point for the output is located, disconnect it from the output, and run it to the remote resistor.

 

If the module had a remote voltage sense line you could use that to generate a constant current through an added series resistor, but it's not apparent that you can do that from the spec sheet. For example 0.75V across 0.15Ω would give 5A.
You would have to look at the module and see if you can spot where the voltage sense point for the output is located, disconnect it from the output, and run it to the remote resistor.

What about using the sequencing pin? If I read correctly it allows the voltage to be varied without changing the resistor value.

 

You must have a resistor in series with the output of the DC to DC box of tricks.
The output is just like connecting to a 5v or 6v battery. The current will be up to 5amps without a "regulator." The resistor is a REGULATOR. It "regulates" or "controls" or "limits" or "reduces" the current to a value you have worked out mathematically by doing a 5 year Maters of Electronics course.

 

The resistor will also waste power as heat. If I can get the dc-dc converter to regulate current it will not waste as much power. But I'm probably going to just set the regulator a volt above the LED and use a resistor to drop a bit and keep the LED running at the correct current.

 

YOU CAN'T DRIVE A LED FROM THE DCC BOX OF TRICKS WITHOUT A little, long thing with coloured stripes on it. I have forgotten what you call it.

 

What about using the sequencing pin? If I read correctly it allows the voltage to be varied without changing the resistor value.

That point is for connecting a resistor to ground to vary the voltage.
To control the voltage across an external resistor in series with the load requires an external voltage sense line.

 

YOU CAN'T DRIVE A LED FROM THE DCC BOX OF TRICKS WITHOUT A little, long thing with coloured stripes on it. I have forgotten what you call it.

The resistor will also waste power as heat. If I can get the dc-dc converter to regulate current it will not waste as much power. But I'm probably going to just set the regulator a volt above the LED and use a resistor to drop a bit and keep the LED running at the correct current.

Set regulator to 5.5v and use resistor do drop a bit. The LED I'm using has a voltage drop of 3v. That's the plan as of now. Because there isn't much power being dropped on the resistor I'm going to run the LED at 30% less than it is rated for.

 

That point is for connecting a resistor to ground to vary the voltage.
To control the voltage across an external resistor in series with the load requires an external voltage sense line.

OK thanks, I figured it was worth a shot.