A reminder on bench PSU memory settings. - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 01:09:00 pm
content: I fried several components a few weeks back.The reason is... I was using a bench supply I hadn't used in a while and completely forgot that it resets back to "Memory 1" when you power it on.I had set memory 1 to 15V 3Amps. The circuit I connected and switched on was 3.3V 100mA.... or... was.Best Practice: ALWAYS check the setting before you hit "ON"Best Practice: Set Memory 1 or the default power on parameters to something minimal, like 3.3V 50mA depending on your usual circuits.It's usually more forgiving to send a 15V circuit 3.3V, than it is the other way around.

author: pqass
author_rank: Super Contributor
timestamp: January 20, 2023, 03:48:02 pm
content: No, it's not recommended to use your bench supply mains power switch to gate power to your test circuit. All sorts of nasties can be on the output before it stabilizes; including: noise, glitches, overshoot, ramp-up delays. I'd try to get a bench supply with an output on/off button. But lacking that, use a multi-pole switch (with caps to ground on each output pole). That way, you can synchronize power on/off from several supplies if necessary.See attachment for the power-on profile of my HP-E3611A (upper) and XANTREX XT7-6 (lower) supplies (outputs are loaded with a 56R resistor). Notice the power-on glitch (upper) and the weirdness before the regulator stabilizes (lower).

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 04:13:18 pm
content: That's not what I meant.Say you are working on the project with the PSU set to 3.3V 100mA, you might even have that saved in Memory 2. After an hour you turn it off (load off and mains off) to go make dinner.Later you return, power the mains on and hit "LOAD ON" on the PSU without checking.In the case of the Tenma it defaults back to M1 settings, not last used. Cost me a £10 DAC.The RDTech supplies return to last used, although I think you can force them to reset to memory.

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 04:15:25 pm
content: I suppose it's a breach of commandment no. 1 Thy shalt check thou voltages.

author: pqass
author_rank: Super Contributor
timestamp: January 20, 2023, 04:31:32 pm
content: I can't relate as the power supplies I own have indefinite memory retention; just don't bump the knobs.

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 04:51:29 pm
content: Quote from: pqass on January 20, 2023, 04:31:32 pmYou hope

author: wasedadoc
author_rank: Super Contributor
timestamp: January 20, 2023, 05:47:24 pm
content: Quote from: paulca on January 20, 2023, 04:15:25 pmBack to school for you! You have "thy" and "thou" swapped.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 20, 2023, 05:54:32 pm
content: Don't feel bad about yourself.If anyone is routinely active on the bench, I could bet good money that at least once that person has fried a circuit, a piece of equipment or both because of a distraction or lack of knowledge.You lick your wounds, repair what got broken, and most importantly, learn from the mistake and apply the corrective actions.A story of mine:Back in the early 1980s, at the company I worked, we had in the laboratory some R&S signal generators that our head office in Germany had sent us directly. These were exclusively 220VAC units, no range switching. All the other equipment within laboratory was 120VAC.The original DIN plug on the R&S units had been replaced by the common NEMA 1-15P plug, because.....simplicity. All of the other equipment used it, so why not? In hindsight it was a stupid simplification, but hindsight is always 20/20.To ensure that the R&S devices were fed correctly, some of the NEMA 1-15R receptacles were wired for 220VAC and painted bright red. Those who worked at the lab knew the unwritten rule that those receptacles were to be used solely for the R&S equipment.All was fine until a new lab assistant was told to connect and prepare a bench with some equipment. He did, but upon searching for empty power receptacles, the only one he found was -you guessed it- the red ones. He proceeded to plug both an expensive Hewlett-Packard spectrum analyzer and a Keithley voltmeter. As the equipment required some time to stabilize, he proceeded to turn them on.Boom!Murphy's law being what it is, the only person who didn't know shit about the red receptacles was the one tasked to connect some expensive equipment near them. That person was me. To say that I was utterly embarrassed would be an understatement.But again in hindsight, The person who originally asked to remove the DIN connectors was at fault. If he didn't want to use the European receptacles because of a lack of availability, at least he should have replaced it with a proper NEMA plug, like the 6-15P or the 6-30P.

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 06:11:57 pm
content: Curiosity can kill however. My "big one" was wondering what the little sucker was for on the back of a CRT tube I had opened aged 15. I'm still here to tell you 400VDC is not in anyway pleasant and you do get thrown, high and far and shake like a leaf for minutes afterwards. I am lucky. Lucky to be alive. That right there is considered a fatal shock by any merit.You mentioned mains. I felt obliged.On mains and related to your story. I have seen too many people using off-grid solar systems at mains voltage via inverters ... that are not "grid tied" or in anyway referenced or synced with their actual mains. That's a big flash of sparks at the best waiting to happens. That and people fitting common positive solar systems to common negative van electrics and wondering why it went wrong. I think electrics and electronics is becoming a bit too open to tinkering and so many mistakes are beign... but not all.Don't play with mains. Buy something that makes it idiot proof, work with that.This is why local authorities INSIST in "certified professionals", even the cowboys.Mains kills.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: January 20, 2023, 07:21:49 pm
content: My PSU story isn't about damage happening, but resulted in a lot of unnecessary work. My PSU doesn't have memory, but I was using itto power a project that only needed 100mA of current, so I had theCC set to 100mA. Fast forward to the next day when I powered my9 volt label maker with the same PSU. The machine wouldn't print and kept telling me to check the batteries. Well, I tore the machine apart to find out why it wouldn't print. After about 20 minutes of poking around, it hit me that the CC was set, (yeah, the CC indicator was lit...duhhhh).

author: KaneTW
author_rank: Frequent Contributor
timestamp: January 20, 2023, 07:28:45 pm
content: Quote from: paulca on January 20, 2023, 06:11:57 pmNo? It's perfectly safe as long as it's done properly (sufficiently low loop impedance, good grounding, etc). I feel like this statement is missing some crucial qualifier (e.g. connecting an off-grid system to the grid or similar).

author: paulca
author_rank: Super Contributor
timestamp: January 20, 2023, 07:41:31 pm
content: Quote from: KaneTW on January 20, 2023, 07:28:45 pmIt would be the elephant in the room yes.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 20, 2023, 08:49:14 pm
content: I always turn the power supply on first. Test the voltage before connecting it to the load. Many power supplies can be turned on and set the voltage and current limit before turning on the output.