Output short circuit keeps killing lm350t - Page 1
Forum section: electronics / beginners

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 23, 2023, 06:14:36 am
content: As the title states I've killed 2 x lm350's from shorting the outputs. I have the normal adjust - output and output - input protection diodes. The only thing I've done differently to the lm317 which copes just fine with output shorts is I added protection diodes from ground - input and ground - output. The output - input diode was dead as well as the lm350t(I guess it died then output went through to input somehow?).I have 5uf tantalum on the adjust pin and 5uf tantalum on output and both capacitors were fine. The lm350 also copes fine if I reverse polarity on the inputs(doide gets hot though). Output was under 10v when the lm350's died too.

author: magic
author_rank: Super Contributor
timestamp: April 23, 2023, 06:42:19 am
content: Getting lost a little. A schematic maybe?The output was shorted to ground, right?

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 23, 2023, 07:36:41 am
content: Quote from: magic on April 23, 2023, 06:42:19 amYes I shorted output to ground and 2 x lm350's died. I'm using the same circuit as this diagram except with extra diodes between ground+ input as well as another diode between ground + output. One of the lm350's also had a SS36 shottky diode in series with input which didn't save it either.

author: floobydust
author_rank: Super Contributor
timestamp: April 23, 2023, 07:40:59 am
content: It sounds like you have batteries involved? Otherwise, there's no way to roast an input-output diode.I thought the LM350 is a bit over-rated, good for only 25W and not as tough as an LM317 with higher voltage input.

author: Damianos
author_rank: Frequent Contributor
timestamp: April 23, 2023, 08:03:47 am
content: What is connected at the input? Levels? Characteristics of "normal" load (except the short circuit)?Reasons for this regulator to fail include: high voltage, reverse polarity, high current at adjustment pin, not an "original part". It is protected for over current, over temperature, over power. I have used the "K" version (metal can) and never had a fail, even stressing it to the limits (except of voltage level and polarity).

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 23, 2023, 08:25:34 am
content: Quote from: Damianos on April 23, 2023, 08:03:47 amBoth times it was powered from a 12v atx pc supply(which has never caused any issues for the lm317).

author: magic
author_rank: Super Contributor
timestamp: April 23, 2023, 09:01:59 am
content: Quote from: bobbydazzler on April 23, 2023, 07:36:41 amAnd you say that D1 on this schematic was blown?This can only happen if the input voltage goes above 100V (or whatever is the rating of the diode you use) or the input becomes lower than the output and there is some energy source on the output (other PSU, large capacitors, battery) and the "input" side consumes a large current through the diode (again, above the maximum rating of the component chosen).

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 23, 2023, 10:12:19 am
content: Yes D1 was blown, I was using a 4007 so I think 1000v reverse voltage. Is it possible my power supply cut out the input voltage when I shorted the outputs? The lm317 can cope with this fine(2.2amps max) but the lm350 can pull 4.5amps plus so maybe input went to ground as I was shorting output? I was using my pc when it happened and it had no issues staying powered on.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 23, 2023, 02:31:55 pm
content: Are these “real” LM350’s from a reputable distributor? My first thought is they may well be bargain parts from unknown sources.

author: Damianos
author_rank: Frequent Contributor
timestamp: April 23, 2023, 03:09:40 pm
content: Quote from: bobbydazzler on April 23, 2023, 10:12:19 amIf that happened then the voltage difference would be zero. The blown D1 indicates that the real circuit is not what you describe. Have you the real schematic and any photo of the implementation?

author: floobydust
author_rank: Super Contributor
timestamp: April 23, 2023, 04:27:46 pm
content: The problems aren't making much sense with what we've got to go on.OP if you're generating 1kV spikes from a 12V rail on an ATX PSU also powering your PC...Having long wiring and lead inductance, or a PC PSU with toast output capacitors could do that.The LM350T must have an input cap >0.22uF film if far away from the source.I've given 1N4007 over 1kV and they just avalanche but do not short unless something nearby was arc welding. So it the diode junky or damaged from soldering etc.Since a shorted LM317/LM350 usually smokes the potentiometer, I've learned to add a series small fuse or PTC polyfuse to prevent that. Especially if you are using an expensive multiturn pot.

author: Jwillis
author_rank: Super Contributor
timestamp: April 25, 2023, 12:07:34 am
content: Quote from: bobbydazzler on April 23, 2023, 08:25:34 amI don't normally dead short regulators intentionally to check if the current limiting works.I've been running a few experiments with the LM317, LM338, and LM350K in an attempt to replicate a destructive situation. I am getting some interesting current spikes when preforming dead shorts at the outputs. Tested at a 5A input and a 10A input at 12v. On the LM317 I don't see the degree of current spike as the LM338 and LM350. Current spiking on output is a little more than the current supplied on the later 2 . As much as 11A when supplying 10A from supply. Albeit the spikes are only a few microseconds. Did several tests on each regulator to confirm findings. So far, non of the regulators have failed.Just out of curiosity. What is the current supplied on the 12V rail of your ATX PSU ?

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 25, 2023, 10:08:17 am
content: 45.8A listed for the 12v rail on my power supply, but I was using really thin 28awg cable connected to a molex adapter to power the lm350 so any current spike should've come with a lot of vdroop. I short circuited the 1st lm350 on accident and the 2nd time I did it on purpose. I forgot to mention in my first post I also had a 100uf electrolyte on input too, I guess that could've supplied the surge current.

author: Jwillis
author_rank: Super Contributor
timestamp: April 25, 2023, 09:19:15 pm
content: Quote from: bobbydazzler on April 25, 2023, 10:08:17 amWow, OK. That's a lot. So based on the findings in my experiment, you could predict, that when "Dead" shorting the output, the current passing across the pass transistor inside the regulator can be up to 45.8A . Even in an event of only a few microseconds that could be potentially destructive.On all Linear regulators the output current is determined by it's input current minus circuit losses.In most laboratory conditions the maximum current supplied is around 10A or less. So in most conditions this kind of destruction would be rare and overlooked.I will continue the experiment with a higher current to see what happens.I the mean time try putting a resistor before the the input of regulator. A 3 ohm to start. This will limit the maximum current to 4A and should dampen the current spiking on a dead short.I'm not saying that this is the problem but it sure looks like a potential problem.Good luck.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 26, 2023, 07:30:52 am
content: I tried again this time powered from a buck converter with current limiting feature(though it does have a 470uf output cap). The lm350 survived output short circuit fine until I raised the current limit to 4.5A, then it died again.

author: Jwillis
author_rank: Super Contributor
timestamp: April 26, 2023, 08:01:29 am
content: Quote from: bobbydazzler on April 26, 2023, 07:30:52 amAnd you had the protection diode in place?My tests only had a 1uf cap on the output and a 0.1uF on the input. 470uF is kind of excessive.Starting to look like you had two main problems. To much current at the input and to much capacitance at the output.

author: Damianos
author_rank: Frequent Contributor
timestamp: April 26, 2023, 09:51:26 am
content: Quote from: bobbydazzler on April 26, 2023, 07:30:52 amIt seems that it is not an LM350! By a genuine regulator and try again.The mystery of the burned diode remains! An output short circuit biases it in reverse, so there is not current...

author: Jwillis
author_rank: Super Contributor
timestamp: April 26, 2023, 10:12:15 pm
content: Quote from: Damianos on April 26, 2023, 09:51:26 amQuote from: bobbydazzler on April 26, 2023, 07:30:52 amFinished the experiment. Supplied circuit with a car battery (well over 50A) with a 470uF cap at the output of LM350. I could not destroy it even with repeated shorting. I'm in agreement with Damianos that your LM350 is either not what its labelled or its complete crap . Did they fall out the back of some unmarked van? Where did you get them?

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 27, 2023, 08:47:32 am
content: The 470uf was on the output of the buck regulator that was supplying power to the lm350t, so basically on the input to the lm350t. I also had 5uf tantalums on input + 1.5uf mlcc's. For output capacitors the one I killed yesterday had 5uf tantalum on the outputs and the other ones I killed all had less than 10uf tantalums on output.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 27, 2023, 11:51:13 am
content: I was using 235ohm resistance between output and adjust so maybe that wasn't drawing the minimum load current of 2ma, and that caused weird oscillations?

author: Jwillis
author_rank: Super Contributor
timestamp: April 27, 2023, 11:00:20 pm
content: Quote from: bobbydazzler on April 27, 2023, 11:51:13 amAll I did was the basic setup with a 0.1uF at the input ,1uF at the out put, a 260 Ohm from input to adjust, and a 10K poteniometer from adjust to ground. Found on page 8 of data sheet .https://www.ti.com/lit/ds/symlink/lm350-n.pdf?HQS=dis-mous-null-mousermode-dsf-pf-null-wwe&ts=1682582399625&ref_url=https%253A%252F%252Fwww.mouser.dk%252FNo protection diodes were used in experiment.Come to think of it I did have a similar problem with a TL783 regulator when the resistor from output to adjust was to low . Changed from the suggested 82 Ohm to 120 Ohm and never had a problem again.Could very well be that the adjustment pin is getting a voltage spike.