What defines a "load switch" on a power supply? - Page 1
Forum section: electronics / beginners

author: FenderBender
author_rank: Super Contributor
timestamp: February 23, 2013, 03:45:55 am
content: I guess this is a rather simple question. Just saw in the below thread about power supplies, there was a bit of talk about the presence and lack of load switches on some bench power supplies.Is a load switch literally just a switch right before the output terminals? Is it anything more complicated for that?I swear I saw somewhere that there's a certain right way to do it so that you don't kill the circuitry behind it, though that might be complete BS.Thanks.

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2013, 05:18:51 am
content: On my TTi power supply the output load switchramps up the voltage slowly and does not overshoot.Seethis threadwhere I switched on the output with the power supply set to 10 V and a 5 mm red LED connected to the terminals, and didn't blow up the LED.

author: FenderBender
author_rank: Super Contributor
timestamp: February 23, 2013, 05:45:07 am
content: Ah, I see. Do you have to use the load switch every time you want to activate the power?

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2013, 06:00:04 am
content: Yes, it's basically an output on/off switch. You set the power supply to the correct settings and then turn on the output. It saves you having to connect and disconnect wires when you are working on something.

author: BravoV
author_rank: Super Contributor
timestamp: February 23, 2013, 06:18:07 am
content: Quote from: IanB on February 23, 2013, 06:00:04 amOther factors especially at quality and complex psu, it needs time to settle down from cold boot.By switching the main power on and offf just to control the output is not the best way.

author: fenclu
author_rank: Regular Contributor
timestamp: February 23, 2013, 10:09:59 am
content: What do you think about "soft" load switches? I plan to put one into my next PSU (you press a tact-switch and an MCU turns on a MOSFET which pulls the output - to ground). I know it's not as straight forward as a toggle switch but I wanted to do sth different.

author: c4757p
author_rank: Super Contributor
timestamp: February 23, 2013, 03:08:57 pm
content: So you are shorting out the output? You will have a lot of heat. And why would use tact switch + MCU + FET when a toggle switch does just fine? The only reason I can think of is if you want a computer controlled supply. In that case go for it, but if it is standalone I don't see the need for anything digital at all.

author: fenclu
author_rank: Regular Contributor
timestamp: February 24, 2013, 08:49:03 am
content: I wanted to add some extra features like turning off the output when left on for too long. I thought about using a relay but I"m scared of the relay bounce which would give it pretty awful turn-on performance (but also a pornographic click every time I turn it on)... And no I'm not shorting the output. That's a standard use for a MOSFET. Output negative terminal (let's call it GND1) is pulled to ground.

author: EEVblog
author_rank: Unknown
timestamp: February 24, 2013, 09:06:00 am
content: There are three types of load switches:1) A simple on/off switch on the load output2) A soft power switch which controls a relay on the utput3) A soft power switch which electronically controls the output to ensure that it ramps at a certain rate or whatever. Often it won't even disconnect the output, it just ramps down to zero volts.#1 and #2 are essentially identical.I'm not a big fan of #3, and I prefer #1, because often when testing gear I'll want to simulate real-world plugging in of the power to the device under test, i.e. contact bounce etc. With a real power switch I can "fiddle" with the switch and make it do some horrible things to my DUTDave.

author: fenclu
author_rank: Regular Contributor
timestamp: February 24, 2013, 10:57:28 am
content: I've got a crazy idea. I know you experienced designers are gonna laugh at me but I don't care. I intend to put an MCU cotrolled relay (with a soft switch) in series with a toggle switch. I can use the soft switch (with the toggle switch ON) and when I want to fiddle with it I'm just gonna play with the toggle switch.

author: burro
author_rank: Contributor
timestamp: February 24, 2013, 04:33:43 pm
content: Quote from: EEVblog on February 24, 2013, 09:06:00 amHi DaveSo are you saying that it is "OK" if a power supply overshoots? or that you prefer the #1 option to simulate other real life conditions like contact bounce etc. and it should not overshoot even with the a normal #1 load switch?. (sorry if this is a stupid question, I am newbie)

author: IanB
author_rank: Super Contributor
timestamp: February 24, 2013, 04:49:01 pm
content: If I am buying an expensive and high quality lab power supply,it had better notovershoot on switch on.If I happen to want to subject my device under test to abnormal power conditions I will test it with something designed to do that in a realistic manner, not with an expensive but faulty piece of lab equipment.

author: BravoV
author_rank: Super Contributor
timestamp: February 24, 2013, 05:08:16 pm
content: Quote from: IanB on February 24, 2013, 04:49:01 pm+1, I'm with IanB.

author: M. András
author_rank: Super Contributor
timestamp: February 24, 2013, 08:07:43 pm
content: Quote from: EEVblog on February 24, 2013, 09:06:00 amif the regulator itself have an output disabled function, which category would be suitable?

author: FenderBender
author_rank: Super Contributor
timestamp: February 24, 2013, 10:38:23 pm
content: So I watched Dave's soft latching video. I've gathered some good information from it. I do want to use a toggle switch on the little supply I'm building.I was thinking that I would put a capacitor in parallel with the load after the switch so that when you flick the switch the capacitor has to charge up first, hopefully reducing overshoot.I feel that there has to be a better way than that. I haven't done anything really with filter design. I may review TAOE before I go to bed tonight, but I'm just curious with a low pass filter, does the series resistance mess with the output?

author: EEVblog
author_rank: Unknown
timestamp: February 25, 2013, 08:14:11 am
content: Quote from: burro on February 24, 2013, 04:33:43 pmNo, it won't overshoot, I don't know wheer you got the idea that a load switch would cause a PSU to overshoot? Any load switch should be outside the feedback loop, so the (low impedance) regulated output is always there on the other side of the switch waiting to be delivered when you close the switch. If anything, there is a small sag when the load connects, never an overshoot.Dave.

author: ConKbot
author_rank: Super Contributor
timestamp: February 25, 2013, 05:22:11 pm
content: Quote from: IanB on February 23, 2013, 05:18:51 amThats quite impressive, Ive had a xantrex dual bench supply blow out a cree XP-G with the start up surge from it being set ~10V with a 1A current limit. Never had any problems with luminus SST-90's though (go figure, its hard to blow out a 9A nominal LED :p)

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2013, 03:04:41 am
content: Quote from: ConKbot on February 25, 2013, 05:22:11 pmYes. There are engineers who know how to design stuff, and engineers who think they know how to design stuff. TTi has certainly had staff of the former kind.