odd lpc problem

[Schnoogledeck]

I made an lpc laser with a homemade driver built with an lm1117 (since it has a lower voltage drop than an lm317), but i'm having an issue

My multimeter says that it's running at 385 mA, the highest i can get it to go to, but, with each passing second, it drops down one mA. I've only run it long enough for it to drop down to 375, but it's still kinda weird. This is the second diode i'm using in this build, since i had a similar thing with the first. I could only turn the current up to 380 with my first diode, so i turned the trimmer (potentiometer) a ton, instead of in one turn increments like i was doing before, kinda in irritation, thinking that it wouldn't go further, but when i turned it back on, it was at over 600mA and the diode died after a split second. With this diode, i couldn't get it over 385, which makes no sense to me, and I noticed the gradual decrease in current. The thing is, the laser still looks fine, although this is my first lpc build. it's extremely bright, and the beam is visible, especially at night.

It's running off of 3 AA alkaline batteries, since the voltage drop of the lm1117 is, like, 1.2v. The circuit is an lm1117, a 20ohm trimmer, a 47microfarad capacitor, and a 1ohm resister to measure current. Basically a DDL driver with an lm1117.

I don't want to mess with it anymore for fear of losing another diode

Does anyone have any idea what's causing the gradual decrease in current or why i can't get my potentiometer to go above 380 unless i crank it up a ton? Is it some sort of faulty electircal component?

 

[daguin]

Heat

Peace,
dave

 

[Schnoogledeck]

damn, that was the fastest response i've ever seen...
i was coming back to edit and say that the lm1117 heats up kinda fast... I don't get why, though. if the diode draws 3v, it should be releasing only about 1.5v, which isn't a whole lot, right?
If it's heat, how can i prevent it? the diode is in an aluminum heat sink, and doesn't even feel warm when the lm1117 is gettin' hot.
I don't get why the trimmer would act the way that it is, either

 

[daguin]

The diode takes 3V. The voltage drop of the driver is 1.2V? So you need at least 4.2V for regulation. Are your batteries fresh?

If the driver is heating up quickly with this set-up, something is pulling current.

Peace,
dave

 

[Schnoogledeck]

three fresh alkalines, so that should be more than 4.2v
what could possibly be pulling current, though?

ps, now the initial current is 365 after allowing cooldown time from the last time i turned it on, and it still gradually drops

 

[daguin]

three fresh alkalines, so that should be more than 4.2v
what could possibly be pulling current, though?

There is no way I can say from here. An errant solder bridge. Two conductive surfaces making contact. Wires and or wires to component shorts. A failed/failing component.

Re-check your design and assembly

Peace,
dave

 

[Schnoogledeck]

i figured as much...
I was building it in an altoids tin, and everything is set up, except the problem with the current.
maybe it was that darn potentiometer
it'll be a huge hassle to take everything apart, since everything is in there pretty snug...
maybe i should just bag the whole thing and do something else...
i love lasers, but they've just been drivin me crazy lately

anyway, thanks for the super speedy responses
i'll, uhh.... click on the balance beam below your name

 

[Benm]

You're calculating it wrong: With the DDL circuit, the minimum required voltage over the driver is the dropout voltage (1.2v) PLUS the reference voltage (1.25v). So the minimum required supply voltage is about 3 volts for the diode, plus 1.2, plus 1.25 making a total of 5.45 volts. This is more than 3 alkalines or nimh's will provide, and it is out of regulation.

With a regular LM317 is is worse, since its dropout is 3 volts, plus 1.25 for the reference. In practice they usually work with lower dropout voltages though, which is why the setup does work using a red diode and 2 lithium cells.

 

[Flaminpyro]

If you have a regulated bench supply use that, it sounds like your batteries are sagging...
if you don't have a bench supply then set your current to 120 ma and see if it still saggs if it quits and stays steady then get a larger power source "D" cells any thing that can provide more current than you need then turn your current back to 385ma and see what happens it will stay steady is my guess.
if you had a solder problem it would most likly be constant not changing, could be heat like Dave said !

 

[Schnoogledeck]

You're calculating it wrong: With the DDL circuit, the minimum required voltage over the driver is the dropout voltage (1.2v) PLUS the reference voltage (1.25v).

I thought that the "reference voltage" was there just to insure that the driver still works when the batteries start to lose charge and don't supply as much voltage. If I wasn't supplying enough voltage, then the driver would quit working and the laser wouldn't be functioning at all, right?

I did consider that my batteries were just sagging, but they they were still losing current when i turned it down to 200mA. Maybe I'll have to just turn the current down even more and try again.

 

[HIMNL9]

Have you used only the 20 ohm trimmer ? ..... if yes, the trimmer is dead (or at least half fried), and it's still frying, on the cursor contact point.

trimmers can usually take 1/4 to 1/8 W, the miniature ones also less.

Only wire trimmers can hold like 1W or similar.

If you want a fixed current around 380mA, just use a single 3,3 ohm half watt resistor, better if SMD ..... if you want to change a bit the current, use a resistor with a trimmer in parallel, but keep in mind that you still can fry it, if you use it in the high current zone.

Edit: btw, the dropout of the LM1117 is typically 600 to 800 mV ..... all the ones that i use, are the 600 mV "mid-sized" TO252 SMD ones, these ones i mean:

 

[Schnoogledeck]

ya, i actually do think that it's the trimmer, since it suddenly jumped on my first diode
well, it is a 20Ω one. i'm not sure how much power that it's rated to take, but i guess i could just make a fixed current driver. I have a decent heat sink, so i'll set it to 420.
my lm1117 is of a TO-220 style.

 

[dr-ebert]

Drivers don't stop working when the input voltage is too low (that would require an undervoltage detection capability), they just try to give as much current/voltage as possible. They're not actually regulating any more in this situation, that's why it's called "out of regulation".

The "reference voltage" is the key input to a constant voltage circuit: the output voltage will be 1.25V (the exact value depends on the circuit) above the ref voltage input. In a constant current setup, it's the output current flowing through the sense resistor that's generating this voltage difference.

 

[sciullo779]

i figured as much...
I was building it in an altoids tin, and everything is set up, except the problem with the current.
maybe it was that darn potentiometer
it'll be a huge hassle to take everything apart, since everything is in there pretty snug...
maybe i should just bag the whole thing and do something else...
i love lasers, but they've just been drivin me crazy lately

anyway, thanks for the super speedy responses
i'll, uhh.... click on the balance beam below your name

i built a 16x in an altoids tin, and i wrapped the whole board in electrical tape, but not the heatsink on the lm317 i used, and the heatsink touched my switch that i incorporated frying the diode by shoving 1.45Amps at it, like theyre all saying check everything, because that tin is one giant conductor

 

[Schnoogledeck]

Thanks, dr. e-bert: that makes sense

 

[Schnoogledeck]

It was just my alkalines sagging...
problem solved.