Understanding current regulation

[icwiz]

Hello Everyone,


I've successfully made a postive voltage regulator with the LM317, for a red 200mw diode.

I am a tad bit confused about something, and I was hoping someone here would have the knowledge to clear it up.

When I was testing for voltage, I noticed that as I increased the current, the voltage went up slowly. I was never able to reach 130ma for the diode, because at the point the voltage was 2.9v. The datasheet for the diode said it wants 2.5.

I basically made the senkat driver and supplied it with a 9v battery.

Am I doing something wrong? Or did I get it right and im just scared?

Thanks a bunch

--ICWiz

 

[Cyparagon]

A schematic and/or picture would help.

 

[icwiz]

A schematic and/or picture would help.

My apologies, I thought since its such a common circuit on the site, no one would need me to post a schematic lol.

I have the same circuit, except I used a 25 ohm pot. It does seem to give me the right power...im just confused about the voltage.

I am feeding it 9V.

I hope that explains it better

--ICWiz

 

[Cyparagon]

That is a current regulator, not a voltage regulator. 2.9V sounds fine. Keep going. You might want to add a resistor in series with your pot. Otherwise, one twist too far, and POOF.

 

[Bionic-Badger]

The datasheet's 2.5V is the voltage for running the diode at its suggested ratings. However, we run these diodes at higher currents than their ratings, and the voltage goes up accordingly. The LM317 circuit adjusts the voltage as needed when it's used for current regulation.

Remember that diodes drop their forward voltage and pass current, and that's pretty much it. You might have problems if you're forcing the voltage across the diode to a particular value, say with a direct connection to a battery, but for circuits that are only doing current regulation, the forward drop of the diode determines what the voltage between either end of the diode.

 

[icwiz]

The datasheet's 2.5V is the voltage for running the diode at its suggested ratings. However, we run these diodes at higher currents than their ratings, and the voltage goes up accordingly. The LM317 circuit adjusts the voltage as needed when it's used for current regulation.

Remember that diodes drop their forward voltage and pass current, and that's pretty much it. You might have problems if you're forcing the voltage across the diode to a particular value, say with a direct connection to a battery, but for circuits that are only doing current regulation, the forward drop of the diode determines what the voltage between either end of the diode.

Ahh, I see, That clears a lot up. I guess my understanding of what it means for the diode to "drop their forward voltage" is iffy. Do you know of what I can read up on to better understand this?

Thank you very much for the info

 

[icwiz]

I just created another driver circuit exactly as in the diagram. When i turn it up to 126ma, the voltage is 3.3V. My input is 9V. The 3.3v scares me. I am using a test load of 4 diodes and a 1ohm resistor.

Is this safe for my LD? i dont wish to blow it

 

[BShanahan14rulz]

What kind of diodes are you using for your test load? If you look at their datasheets, they will have a graph that will tell you what the Vf of each diode should be at 126mA. This is usually an average value that may or may not have some wiggle room. It sounds like each one is dropping about 0.8V, which is about normal.

Keep in mind that if your lm317 is dropping a lot of voltage, you might want to heatsink it.

 

[Bionic-Badger]

3.3V is perfectly fine and the LM317 adjusts to what is needed anyway. Just make sure the diode is heatsinked. You should also reduce the amount of voltage you're feeding the LM317 like Bshana mentioned. The LM317 drops about 3V or so; 7V would be better than 9V so that it doesn't to eat that extra 2V as heat.