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I have a Rapid 85-1901 switching mode 0-20V 0-5A bench power supply:

Without a load attached the current shows 0A no matter how you turn the dial.

After searching how to set the current limit online, a couple of sites state that to set the current limit on a bench power supply you should short the + and - terminals together - for example by joining the leads.

Is this the correct way to set the current limit? Is it safe? Could it fry the supply?

Another user with the same PSU as me claims to have blown his up by following this procedure.

What is the precise procedure to follow to set the current limit? E.G. turning voltage and/or current knobs all the way anti-clockwise before shorting.

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    \$\begingroup\$ Your photo shows an output enable push-toggle button. The procedure is: Turn that off. Leave the voltage limit in place. Short the outputs. Turn it on. Adjust the current limit. Turn it off. Connect to the load. \$\endgroup\$
    – Ben Voigt
    Sep 11, 2023 at 14:27
  • \$\begingroup\$ @BenVoigt That seems to be mostly correct 🙂 For this particular model PSU, 3v is apparently the number to set (although I can’t verify this). Thanks a lot for your help! \$\endgroup\$ Sep 11, 2023 at 20:38
  • \$\begingroup\$ In my instruction, the output is on while you're adjusting the limit, so you can use the display. It's off while connecting/disconnecting leads, so you don't get arcs/bouncing/discharge of capacitance. As a result it shouldn't matter what the voltage dial is set to, because the output starts from 0V and increases only a tiny amount (surely too small for the internal voltmeter to detect) and then the current limit is reached. \$\endgroup\$
    – Ben Voigt
    Sep 11, 2023 at 20:42
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    \$\begingroup\$ The supplies I encountered so far (granted all from reputable high-price brands) showed the current limit either when changing it or while the output was disabled. This procedure sounds like something you'd do on an analog supply - they probably just replaced the dials with fancy digital meters. \$\endgroup\$
    – Arsenal
    Sep 12, 2023 at 11:08

5 Answers 5

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Is this the correct way to set the current limit?

There is really no other way if you don't want to be tweaking the current limit while the load is attached. And that would be not a great idea for nonlinear loads, since a power supply in current-limit mode has high output impedance. So there's no choice as to it being "correct" or not.

Is it safe?

Yes, as long as the wire you use to short the terminals is rated for the full output current of the supply.

Could it fry the supply?

If it fries your supply, that's a good thing! The supply was junk if so, and you can either redesign it to work correctly, or toss out and get a better one.

Older, bigger linear lab supplies are much nicer to work with than the little modern compact ones that switch transformer taps and have fans.

The way I test lab power supplies is by running them overnight at maximum output current into a dead short. A well designed supply will be definitely warm after several hours, but it must work correctly and not suffer any damage afterwards.

If at all possible, get an older linear supply that doesn't have a fan and doesn't switch transformer taps using relays. Those are generally rock-solid, especially from good brands like HP, Kepco, Power Designs, ...

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    \$\begingroup\$ Linear supplies are, of course, less efficient. In a lab environment, robustness may be more important than efficiency. They're also more expensive because of the big transformers and heatsinks. \$\endgroup\$ Sep 12, 2023 at 8:57
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    \$\begingroup\$ @user253751 In a lab environment, robustness is a high requirement for power supplies. The last thing you want is to destroy a project since the supply “ran wild”. I don’t even suggest buying modern cheap linear supplies - they are almost universally “you get what you pay for” e-waste. A 40 year old linear that works is hard to beat. \$\endgroup\$ Sep 12, 2023 at 12:31
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It's recommended to read the manual for the procedure. Did you read the comment in your link that claims the manual says to set the voltage to 3V and then short? I can't confirm it's the same model in the manual, many (including some well-known US names) are re-badged units from one or two Shenzhen-based makers.

Low end switchmode bench supplies are a bit fussier than linear types IME.

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  • \$\begingroup\$ I did look but I wasn’t able to find a copy of the manual. I saw another brand on this PSU while searching but can’t recall it now. \$\endgroup\$ Sep 11, 2023 at 5:18
  • \$\begingroup\$ Any idea why the magic number 3v please, I’m curious! Why not 0.1v? Thanks! \$\endgroup\$ Sep 11, 2023 at 20:23
  • \$\begingroup\$ 3V is probably a recommended maximum if you connect a short-circuit across the supply while it's turned on. That can only have to do with the physical actions making the short-circuit, because the power supply needs to current limit at any time while operating if the load develops a sudden fault. \$\endgroup\$
    – Ben Voigt
    Sep 11, 2023 at 20:44
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    \$\begingroup\$ @BenVoigt I have some bench SMPS for an anodizing bath that cannot be set less than a volt or two. Same manufacturer as OP but massive output power. \$\endgroup\$ Sep 11, 2023 at 22:29
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    \$\begingroup\$ @DannyBeckett You're going to need 'some' recommendation and 0.1v is possibly not enough for a reliable setting if you use longer leads. 3V is an easy value that's safe and high enough. Can you do it just as safely at 5V? Absolutely. Will that matter for your PSU? Most probably not. Will it work with 1.5V if you have short leads? Yes. Writing down 3V as a recommendation is much shorter than putting a whole story out there though and I'd assume the recommended value is something they actually tested to work reliably. A competitor with a PSU of roughly same build could recommend 5V, for example. \$\endgroup\$
    – Mast
    Sep 13, 2023 at 13:05
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Low-cost power supplies like this often have a large capacitor across the output terminals, so if you short-circuit it when the output voltage is high, you'll get a spark as the capacitor discharges, and possibly damage the circuitry. This is the reason why you are advised to turn the voltage down before short-circuiting.

This capacitance also makes the current-limit less useful, as if there is a sudden fault on the circuit being tested, the supply will first dump the capacitor's energy into that circuit (potentially at quite high current), before the limit starts to take effect.

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    \$\begingroup\$ Any bench supply that cannot be short-circuited on the output without damage is junk. A supply that can push 5A into a load can spark even without any capacitor on the output. This is the reason why you are advised to turn the voltage down before short-circuiting. Yeah, that's the "sorry, we don't know what we're doing" "advice" from supply manufacturers that can't make something as simple as a bench supply work well. Excuses excuses :( \$\endgroup\$ Sep 11, 2023 at 14:10
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    \$\begingroup\$ @Kubahasn'tforgottenMonica: Prevention of an arc/spark is desirable to prevent damage to your terminals and test leads. Of course the power supply itself must be able to clamp a sudden excessive current at high voltage -- that's the thing we are worried about happening when the load device faults. \$\endgroup\$
    – Ben Voigt
    Sep 11, 2023 at 20:47
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    \$\begingroup\$ @Kubahasn'tforgottenMonica All that may be true, but it may still be "good enough" for undemanding users. And more significantly, it may be all a user can afford. Given a choice between getting a somewhat fragile PSU and not doing any electronics at all, most of us will go for the cheap crap. Most of my hobbyist electronics back in the day was driven by a wall wart PSU - I'm pretty sure anything is better than that. :) \$\endgroup\$
    – Graham
    Sep 12, 2023 at 7:03
  • \$\begingroup\$ @Graham sure, cheap power supplies exist, but they really shouldn't be marketed as benchtop supplies for labs. Lab supplies are used for its flexibility to connect to many different loads, and must be resilient against all kinds of conditions, as opposed to wall-warts which are supposed to be plugged into one thing for their whole life. \$\endgroup\$ Sep 12, 2023 at 8:58
  • \$\begingroup\$ @Graham This one I got for the best price: free 🙂 I couldn’t agree more, perfect is the enemy of good enough \$\endgroup\$ Sep 12, 2023 at 16:32
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Good ones will happily regulate current even into a short circuit and the current limit you set will not change even if you move up to near max voltage.

One of mine doesn't. Burned some LEDs learning that the hard way.

Same goes for output capacitance, good ones will have very low capacitance after the voltage and current limiting transistors and won't spark or dump lots of current when you short circuit it.

Do some tests on yours and learn how it functions and what quirks or limitations it has.

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The manual for a bench supply can't telepathically know what kinds of leads one is going to have hooked up to it, nor what one will be powering with it. A 5A 20V supply shouldn't be damaged by having the output rapidly shorted or opened, but some test leads could easily suffer at least cosmetic damage if they were used to short the supply when it was set for 5A 20V, and the output of the supply may momentarily exceed the voltage setpoint when the short is removed, possibly damaging whatever it is powering.

If one is using very delicate leads, one might want to avoid directly shorting them at any current limit, but the probability of damage would be a function of the output power produced at the voltage and current-limit maxima, which would be 100 watts for a 20 volt 5 amp supply at maximum settings. If one is operating with the supply set for e.g. 5 volt 100mA, then the maximum power would be 0.5 watts--a level that would be about 1/200 as great, and one which would be unlikely to risk any damage to anything but the most delicate leads. At intermediate settings, beefy leads would be fine with being shorted, but wimpier ones might not.

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