Help with LM317 regulator current limit indicator - Page 1
Forum section: electronics / projects

author: diydidi
author_rank: Contributor
timestamp: February 06, 2016, 06:37:26 pm
content: Hi Guys.I build the attached circuit. Instead os the LM317 I used a LDO LM1086 regulator. It has the same pinouts, and works a treat.My question relates to the LED in my circuit. I added it from memory from an old school power supply I built which added an LED in the collector of the transistor to indicate a "current limit" state.Problem is, that the LED changes the limiting current slightly, and the rate of current limit drastically. The LED also lights up very early, long before the calculated limit value.Is there a better arrangement that I can use here. I know that the datasheets don't show the LED, but would be awesome to have some form of indication.

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 07:03:54 pm
content: HiOk, let me be the first to admit that I have made a few mistakes in hand drawn schematics .... It happens.The way you have the circuit drawn, the ground pin of the regulator goes to the top of the LED. As things change with the LED, the regulator output voltage changes. That's probably not what you wanted. The top of the LED probably should go to the unregulated supply through a resistor.A more subtle problem is that anything you do with a regulator like a 317 to sense current on it's output will degrade the regulation of the supply. As current goes up, your output drops. You no longer have a regulated voltage source. There are a variety of ways to get around this. Most of them involve something other than the 317 or produce other issues (like stability or transient performance) as a side effect.Here's a suggestion:Move the current sense to the *input* of the 317. You will need to bypass after your sense resistor. The same 0.68 ohms and a transistor base / emitter junction will work. You will need a PNP rather than an NPN transistor. The LED and current limit resistor go in the collector of the transistor and hook to ground.Bob

author: Seekonk
author_rank: Super Contributor
timestamp: February 06, 2016, 08:38:41 pm
content: Your circuit is not that bad when used at higher voltages to charge batteries. The LED bumps up the minimum voltage an additional 2V. Previous suggestion has problems too as it will light all the time. Just use a second transistor to light the LED

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 08:50:57 pm
content: HiOk, 0.68 ohms between emitter and base on the PNP. At 1A through the resistor, you get 0.68 volts. The transistor turns on. Put a half amp through the 0.68 ohms and you get 0.34 V, the transistor does not turn on. When the transistor is on, current goes from emitter (the bulk supply pin) through the collector. From there it runs through the limiting resistor to the LED and then to ground.It's essentially the same way a normal current limit works on a lot of IC's out there *except* it's ahead of the IC rather than after it. A protection resistor in series with the transistor base is a pretty good idea, something around 1K ohm works fine.Bob

author: Seekonk
author_rank: Super Contributor
timestamp: February 06, 2016, 10:22:24 pm
content: Another option would be to have the transistor drive an opto isolator in series with LED with an appropriate current limiting resistor to the raw DC. The value should limit the current to 20ma. Then the isolated transistor of the opto can shunt the pot to reduce voltage. That would eliminate the second transistor and have a positive indication of limiting with no interaction.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2016, 11:18:24 am
content: Quote from: uncle_bob on February 06, 2016, 07:03:54 pmThat's not a problem here as the sense resistor is in series with the negative side and the LM317 regulates the positive rail.

author: uncle_bob
author_rank: Unknown
timestamp: February 07, 2016, 01:29:49 pm
content: Quote from: Hero999 on February 07, 2016, 11:18:24 amHiAnd the sense resistor is between the input bypass cap and the ground. It's carrying loop current from the output so it just may be an issue.Bob

author: diydidi
author_rank: Contributor
timestamp: February 07, 2016, 01:36:55 pm
content: Thanks guys for the input.I found a solution!!

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 07, 2016, 03:58:10 pm
content: Here's a solution that monitors the voltage across the output and adjust pins to determine when the LM317 goes into current limit.The problem with it is that the necessary parts are two or three times the price of the LM317.