How quickly will the current limiter of EP-613 power supply limit current? - Page 1
Forum section: electronics / beginners

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: October 01, 2023, 07:44:11 am
content: I have a PCB with white phosphor SMD LEDs on it. I suspect that the LED chips have inside four series junctions giving a forward voltage of ~12 V but I don't know for sure. I couldn't get them to light with 3 V. I don't know what their rated current is.I have an old Manson EP-613 bench power supply and I can set the voltage to 12 V and the current to limit at ~0.01 A.Suppose I was wrong and the chips actually had a forward voltage of 6 V. My question is, would you expect the bench power supply to limit the current quickly enough to avert damage to the LEDs?When I try shorting the leads, I sometimes see the current spike to >0.6 A before the power supply limits it but maybe it would be more reasonable with a sensible load rather than a dead short.

author: m k
author_rank: Super Contributor
timestamp: October 09, 2023, 02:58:57 pm
content: You have a sort of a wrong question.It's of course a right one for right situation, but also very difficult one, as you have noted.Your more right, and easier, question would be closer to LED.Regular PSU current limiting is generally slow.It can be occasionally fast, but as you have seen it can't be guaranteed without special precautions.So you don't know LED's threshold voltage, or maximum current.D there means Diode, means also that above threshold voltage it's close to a short cut, and so its current must be limited externally.Many small LEDs can stand 10mA current.So take something like that as a goal, but use limiting resistor instead of current limiting of a PSU.http://wiki.sunfounder.cc/index.php?title=RGB_LED_Module

author: Terry Bites
author_rank: Super Contributor
timestamp: October 09, 2023, 04:01:38 pm
content: General purpose PSU current limitting is normaly slow. It will have been designed to ingnore fast transients.If you need fast OC protection you can brew your own limiter or use electronic circuit breaker ics. LTC1153 et al.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: October 15, 2023, 10:27:02 pm
content: Thanks. Not sure why I didn't think to do that (just use a resistor). The other issue I face though, is that I don't know the polarity and I might go over the maximum reverse voltage. I couldn't get them to light at 12 V, though I confirmed that the unit does work when connected to mains so the LEDs aren't broken (yet).

author: srb1954
author_rank: Super Contributor
timestamp: October 16, 2023, 01:25:16 am
content: Quote from: seanspotatobusiness on October 01, 2023, 07:44:11 amMany bench supplies have a reasonably large electrolytic capacitor directly across the O/P and this, when charged up to your selected 12V O/P voltage, can supply quite a high peak current when the O/P is shorted or connected to a low voltage device like an LED. The peak current supplied would largely be determined by the electrolytic capacitor ESR and any wiring resistance and can easily be of the order that you are seeing in your measurements.This current spike is independent of the current limit set on the power supply and so you can't prevent damage to small LEDs etc. by just turning down the bench PSU current limit. You need some external resistance or other circuitry to limit the peak current to the LED.