DDL Driver Current Question

I'm new to building lasers, and this is my very first build.

I'm making a small laser burner out of a diode I pulled out of an old, broken 16x DVD burner.

So I constructed my driver and test load according to the instructions on this site, which I found extremely helpful. The only way my driver differs is that I used two different value resistors in parallel to give me a max current of about 300 mA, and a 200 ohm multiturn POT.

When I tested out my driver, I find that the current jumps up in a series of "steps" instead of a steady increase like I expected.

For example, at a certain point, the current is at about 180 mA. I turn the POT a quarter turn and the current stays exactly the same. I turn it up a tiny bit more, and it jumps to ~290 mA. That appears to be the maximum, as I calculated.

I double checked to make sure my circuit was wired exactly according to the diagram, and it was.

I thought the LM317 was defective so I swapped it out with a new one, but I'm still having this problem.

So is this sort of thing normal for this type of driver circuit? It's not going to be a problem in this build, but it's really bugging me.:undecided:

Edit: I thought it might have been the test load, so I hooked it up to the actual diode. The current still behaves in this manner.

There is only the pot left to check. Did you attach the DMM to the pot and look if the resistance is changing smoothly ?
I also think the range of the 200 Ohm pot is too big. Combining with the fixed resistor I think 20Ohm is enough.

Blord said:

There is only the pot left to check. Did you attach the DMM to the pot and look if the resistance is changing smoothly ?
I also think the range of the 200 Ohm pot is too big. Combining with the fixed resistor I think 20Ohm is enough.

Click to expand...

Ah, thanks.

I checked, and it was the POT. It jumps down to ~nothing from ~3.2 ohms if you so much as touch it.

I didn't have any lower value POTS at the time, and I figured 200 ohms would be fine.

JimBolder said:

It jumps down to ~nothing from ~3.2 ohms if you so much as touch it.

Click to expand...

I suppose you mean open, as "nothing", not 0 ohm ..... in this case, it's probably cooked (too much current fried the carbon track in the contact point where it go open circuit)

HIMNL9 said:

I suppose you mean open, as "nothing", not 0 ohm ..... in this case, it's probably cooked (too much current fried the carbon track in the contact point where it go open circuit)

Click to expand...

Yeah, I think it's open.

I don't understand how it would fry that easily, since it's a 1/4 watt POT and I've never gone over 300mA on this particular circuit.

I ramped up ludicrous current up to 3 amps on a previous prototype circuit I built, and the same exact potentiometer from the same manufacturer didn't burn out on me.

Maybe this one was just defective or something? I'm going to be getting a few 50 ohm ones to replace this one soon anyway.

300mA at 1,25V is 369mW, and 1/4W is 250mW ..... it may not fry immediately, but in time, it's easy that it overheat in the contact point and slowly fry completely the contact spot.

To get 300mA, the total resistance needs to be about 4.2Ω and needs to be able to handle at least 0.38W. If all the power is concentrated in 2% of the total length of resistance strip in the pot, the 1/4W rating may not be enough.

So if I use a lower value 1/4W pot such as 50 ohms, it will be less likely to burn out, right?

Yes, I would think so. But make most of the resistance fixed if you can.

^ as example ..... if you need 300mA, then place a resistor that gives you at least 250mA in parallel with the trimmer, so the trimmer needs to regulate only the last 50mA (if you still need to regulate it ..... otherwise, if you only need a fixed current, just use a fixed resistor of the needed value and power)

HIMNL9 said:

^ as example ..... if you need 300mA, then place a resistor that gives you at least 250mA in parallel with the trimmer, so the trimmer needs to regulate only the last 50mA (if you still need to regulate it ..... otherwise, if you only need a fixed current, just use a fixed resistor of the needed value and power)

Click to expand...

So you're talking about having a fixed resistor in parallel with the POT apart from the other fixed resistor already there to set a maximum current?

I'm sorry, I'm not exactly an electrical major. :cryyy:

And yeah, I kinda wanted to be able to regulate the current between ~150-300mA for experiments and such.

What other fixed resistor ?

I mean something like this:

Ah, okay.

Thanks, guys.