LM317 Adjustable Current Application

Thread Starter

knoccy

Joined Oct 6, 2019
5
Hi!

First off, I'm rather new to electronics. I can't completely figure this out :

I have some LEDs and low-power Laser Diodes which I'd like to power with some circuit and not just a Lab Power Supply. I've googled my way as far as finding out that the easiest way to do this would probably be an LM317 used in a circuit similar to the one I attached. I found it in the Datasheet of an LM317T. Other Circuits used for the same purpose looked similar but with minor differences.

As far as I can see, my Output Current = Reference Voltage (1.25V) / R1. So if my Laser Diode needs 250mA, I'd use 2x 10Ω Resistors in parallel. (1.25V/5Ω=0.25A). Correct so far?

This would work, but I want to adjust my Output Current. I've seen People use 100Ω Potentiometers, but I don't need any more than maybe 15Ω, as everything higher than that would give me an Output Current of < 100mA. I don't need that. And I don't want 75% of the Potentiometers Range not to be usable. Adjusting the usable bit of range left would probably be so finicky that I'd fry the Diode.

If I look for 10Ω Potentiometers, I can only find ones for very high Power Applications, and they're usually rather expensive too.

Also, why do a lot of these circuits have multiple resistors in Series (e.g. R1, R2 in the attached picture)?

There must be a better way, right? Or do I just have the wrong approach to this?

This probably isn't a very hard question, but I seem to not understand something here. Any help or ideas would be appreciated! Thank you.
 

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ericgibbs

Joined Jan 29, 2010
8,729
hi kn,
Welcome to AAC.
A constant current for driving the Laser Diode at 250mA would be the best option.
Are you aware of the eye damage hazards when working with lasers.?
E

R1 sets the absolute maximum current and R2 allows some adjustment below maximum current.
 

Thread Starter

knoccy

Joined Oct 6, 2019
5
Thank you!
Yes, I am aware of the eye damage hazards, thanks for your concern.
It would absolutely be the easiest option, but there are different things I want to power with this circuit. It would be pretty annoying to always have to change the Resistors. Is there really no way to make this adjustable? Maybe even an entirely different Circuit?
 

ericgibbs

Joined Jan 29, 2010
8,729
hi,
Variable R2 could be used to make the current adjustable, with that LM317 circuit.
As you can in your diagram the current is adjustable 5mA thru 100mA.
What is you supply voltage to the input of the LM317.?
E
 

Thread Starter

knoccy

Joined Oct 6, 2019
5
The supply voltage can be whatever it needs to be really. I haven't thought about that to be honest, but I have different buck/boost converters as well as other power sources. Anything from 1V to 16V. The Laser Diode needs about 2.5V.
 

Thread Starter

knoccy

Joined Oct 6, 2019
5
Hey,
thanks for all the replies so far, they've been very helpful. But this just leads me back to my initial problem: Where to find a 10Ω Resistor that doesn't cost all that much. Because the only ones I can find are usually rated for 500V and more and cost anywhere from 10-50$. Is there really no cheaper/better alternative?
thx
 

crutschow

Joined Mar 14, 2008
23,317
So use a 100Ω pot.
Below is an LTspice simulation of an LM317 constant-current circuit with a 5Ω fixed resistor in series with a 100Ω pot.
It shows the current for the pot adjusted from 0% (12mA) to 100% (245mA).

Remember, the current is inversely proportional to the total series resistance.

upload_2019-10-6_22-50-24.png
 
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