solved: What happens if you ignore the LM317's minimum load? (ie. relay/sw out) - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 12:50:09 pm
content: What happens if you ignore the LM317's minimum load? For example you have a relay or switch on the output and the feedback network is high-ish impedance. I don't really care about voltage accuracy.This is in an application where LM317's are providing standby power for sometime akin to "hot swapping" (one rail falls, a relay switches power sources, and for 25ms several paralleled LM317s provides up to an amp each)My impression from the transient response graphs in the datasheet is that it's probably fine.edit: I just read on stackoverflow that the output voltage can triple

author: SCSKITS
author_rank: Regular Contributor
timestamp: April 14, 2025, 12:58:20 pm
content: If you use the recommended resistors to set the voltage, they should satisfy the minimum load requirement.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 14, 2025, 08:12:39 pm
content: Quote from: incf on April 14, 2025, 12:50:09 pmYes, the LM317 will act as a current source/resistor to main supply, so VOUT will start to ramp the output capacitor, if there is no other load.You could clamp that with a Zener, or if the change over time is short enough, a large enough CL will limit the dV.

author: bdunham7
author_rank: Super Contributor
timestamp: April 14, 2025, 08:44:11 pm
content: Quote from: incf on April 14, 2025, 12:50:09 pmIt's an individual characteristic so you'd have to test each one to find out what you can get away with. The TI datasheet shows a typical minimum of 3.5mA and a max (guaranteed) of 10mA. That's for a 40V differential without a temperature specfied and in your case you probably can get away with even less per regulator, but using 3 in parallel that might get a little weird.If you don't have enough load, the voltage will start to float up towards Vin. But if you only have a small capacitance on the output, then when you switch in your 3.2A load that voltage will drop instantly. Note that the LM338 has a guaranteed minimum load of only 5mA and that is specified for 35V differential and over the entire temperature range. This means if low quiescent current is important the LM338 looks like the better option if you want to design something that doesn't depend on tweaking or characterization of components.I'm curious about one thing--how will your standby protection circuit be switched in to take over the power for 25ms? Is it always connected, is there a diode, what?

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 09:45:36 pm
content: A diode is used.

author: bdunham7
author_rank: Super Contributor
timestamp: April 14, 2025, 09:50:19 pm
content: Quote from: incf on April 14, 2025, 09:45:36 pmThen during normal operation if the output voltage starts to creep up because you don't have enough load current, then before it gets very high the diode will conduct and allow a small amount of power to go to your load.

author: langwadt
author_rank: Super Contributor
timestamp: April 14, 2025, 10:06:10 pm
content: do you need the protection? how about a simple emitter follower as booster?

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 10:46:13 pm
content: Quote from: bdunham7 on April 14, 2025, 09:50:19 pmI suppose it is best if I respect the minimum load requirements. Although, what you describe would be perfectly acceptable.Quote from: langwadt on April 14, 2025, 10:06:10 pmI don't quite understand - are you saying to "ditch" the LM317 and replace it with a emitter follower?

author: langwadt
author_rank: Super Contributor
timestamp: April 14, 2025, 11:32:52 pm
content: Quote from: incf on April 14, 2025, 10:46:13 pmpossibly more related to your other thread about getting 3A, stick a emitter follower on the output of the LM317, so the LM317 doesn't have to handle the current