Is Protection diode needed in LM317 current source mode ? - Page 1
Forum section: electronics / beginners

author: basis-double-57
author_rank: Newbie
timestamp: September 22, 2022, 03:47:11 am
content: Hello everyone, I am building a current source for very expensive 340nm UV LEDs based on classic LM317 circuit (just one resistor between output and adjust).My nood question - Since there is a resistor (R_adj for current setting) between the output pin and the output capacitor, is reversion protection diode between output and input still needed ?What is even maximum reverse voltage/current that LM317 can handle ? I could not find that anywhere...Thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2022, 04:00:26 am
content: Is there any potential failure mode that would result in the output being energized and current flowing backwards? With an LED driver I don't quite see how. If there were, whether the resistor would be sufficient protection depends on the resistor value, the possible fault voltage and so on. I don't know the maximum reverse current for the LM317, but from what I understand just charging a large-ish supply capacitor can do it in, but not small spikes or surges. I did just build a low-noise LED driver using this method and I didn't include a reverse bypass diode.

author: basis-double-57
author_rank: Newbie
timestamp: September 22, 2022, 04:06:04 am
content: I just found Onsemi LM317 datasheet that states:"protection diodes for output voltages in excess of 25 V or high capacitance values (Co > 25 uF, CAdj > 10 uF)"My output voltage will be around 21V max, and output capacitance 1 uF, so I guess I will be fine without diode ?

author: golden_labels
author_rank: Super Contributor
timestamp: September 22, 2022, 05:24:56 am
content: You wrote, that you’re going to power “very expensive 340nm UV LEDs”. If you spent money on those, does it really matter if you use a cheap diode for protection?Aside from output side generating energy — which is not your case — the diode is there to protect LM317 when input goes low and can sink current. The regulator should withstand short events of this kind, as the output will be quickly brought low by the load. But if you have a large output capacitor, that capacitor may store enough energy to create a situation in which LM317 output is powered and the input is lower than the output.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2022, 07:18:13 am
content: Quote from: basis-double-57 on September 22, 2022, 03:47:11 amThat's the real problem, the datasheet sucks. In some regulators, you can find hints about maximum tolerated reverse current pulse (and its duration) at least in the "application hints" section's text, but I don't see it in LM317. And it really ought to be listed in "absolute maximum ratings".Quote from: basis-double-57 on September 22, 2022, 04:06:04 amIf you use LM317 from ON SemiBTW, what's the point of an output capacitor in a constant current LED driver?Quote from: golden_labels on September 22, 2022, 05:24:56 amThere is some truth to this. I have added a seemingly unnecessary reverse protection diode to a regulator once simply because I didn't want to deal with fixing the powered circuit should something unexpected go wrong.

author: golden_labels
author_rank: Super Contributor
timestamp: September 22, 2022, 08:12:16 am
content: To understand better, why the protection diode may be needed even if the load can’t provide energy, seeLM317 die shots and reverse engineering(thread on the forum). The massive area at the top is primarily the output NPN transistor (Q26 in the schematic around the middle of the page). Connected directly between VINand VOUT, with just tiny resistances to even current from each sub-transistor.Now imagine you have an output capacitor charged to 20V and the input voltage drops to 0V (and the input can sink current). You end up in -20V across the CE and some positive voltage across BC. This creates a situation a bit like Q26 was upside-down and conducting, with nothing(1)to limit the current. “A bit like”, beause real BJTs are not exactly symmetrical. While they all consist of NPN or PNP layers that work the same both directions, they are adjusted to prefer one direction. The other direction is unspecified and performs poorly. But well enough that you do not want to reverse bias this transistor.(1)The 100mΩ R26 does not provide any real resistance.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2022, 01:33:51 pm
content: Sorry, I was wrong. National datasheet gives numbers and they are the same as ON semi:Quote