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When you short circuit a transformer, it heats up and might blow up due to the high current flowing. In lab a transformer or actually multiple transformers got broken this way, one ended up with a hole in it. The teacher explained that it should have better protection inside, like a fuse.

I just wondered, why is it then safe to short circuit a power supply. We use a device that is a voltage source and also a current source and when using it as a current source or setting the current to certain limit, we have to short circuit the supply first. Why is that then safe in comparison to the transformer? Has it only to do with the supply having proper protection build in it compared to the transformers used or is there another reason why it is safe to short circuit a source / power supply and not safe to short circuit a transformer?

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  • \$\begingroup\$ We don't know what you short circuited and how it handles a short circuit. There are power supplies that either damage or don't. There are also transformers that either damage or don't. Both can have extra circuitry to protect from shorting it. \$\endgroup\$
    – Justme
    Dec 17, 2022 at 12:29
  • \$\begingroup\$ <<< The teacher explained that it should have better protection inside, like a fuse >>> Yes, it is a good idea (it is used in some) but the transformer needs moreover to be well "known" ... and well "used". \$\endgroup\$
    – Antonio51
    Dec 17, 2022 at 12:58
  • \$\begingroup\$ Just always think about "how much" energy you have at your disposal ... in a short-circuit, and what it can "do" ... \$\endgroup\$
    – Antonio51
    Dec 17, 2022 at 13:01

3 Answers 3

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Has it only to do with the supply having proper protection build in it compared to the transformers used or is there another reason why it is safe to short circuit a source / power supply and not safe to short circuit a transformer?

Yes, a bench power supply (and even a lot of in-line power supplies) are far more sophisticated than a basic transformer. They can be sophisticated enough to self-protect without destruction.

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  • \$\begingroup\$ Thank you a lot for the reply! : ) \$\endgroup\$
    – sara_
    Dec 17, 2022 at 12:33
  • \$\begingroup\$ If possible, another question about this. In lab, when we dealt with DC, we had a DC supply that could work as a current source and voltage source. For it to work as a current source, with the current controller knob the (maximum) required current can be set by short circuiting it first and the explanation for this short circuit procedure was: 'because current can not flow through air'. I did not understand this sentence... Do you maybe have a explanation for why the short circuit is needed for the DC supply to work as a current source? \$\endgroup\$
    – sara_
    Dec 17, 2022 at 12:53
  • \$\begingroup\$ @sara_ Probably that's because it may be the only way you see the current on the display. Some supplies don't show the maximum current, they just show the currently used current. \$\endgroup\$
    – juhist
    Dec 17, 2022 at 12:54
  • \$\begingroup\$ @sara_ pretty much what the previous commenter said; without a path for the current to flow (aka a short circuit or low value resistor) you cannot be sure what you are setting on the dial. \$\endgroup\$
    – Andy aka
    Dec 17, 2022 at 13:06
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Power supplies are today almost always switched mode power supplies. That's because it's the only possible way to create a power supply with stable voltage as a function of load, low cost, low weight, high efficiency, perfect power factor (sinusoidal current input) and ripple-free output.

Old transformer-based power supplies almost always had huge variation of voltage as a function of load, and if they didn't they just used a linear regulator to dump the excess voltage as heat, becoming extremely inefficient. They weighed a lot for a very low current capability. The power factor was crap due to large capacitor current surges, and if they didn't have a linear regulator then the capacitor ripple was huge.

A switched mode power supply converting AC to low-voltage DC can be for example forward converter or flyback converter.

They create the required voltage by rapidly switching current through an inductor via a MOSFET.

In a forward converter, current through the output inductor is going through the load. At any time, the controller has the option to also supply more current to the inductor via a high-frequency switched-DC transformer.

In a flyback converter, current is alternately flowing from input to inductor (energizing it), or from inductor to load (de-energizing it). The inductor has two isolated windings.

By adjusting the duty cycle, it's possible to adjust the current and voltage through the load. This is the basis of voltage and current limits at most lab power supplies.

To limit voltage, you need to measure it -- nothing else, the rest is in the control algorithm. Same for current, to limit it, you need to measure it, and the rest is in the control algorithm.

It's probably very hard to find a switched mode power supply with absolutely no form of overcurrent protection. It's so cheap to add there that it's almost always there.

Also, do note that short circuiting a SMPS lab power supply isn't advisable. For example, I have a 30V 10A power supply. I have tested short-circuiting it at voltages of below 10V, and at few amperes of current. It creates a magnificent spark show when you remove the short-circuited leads. Not only that, but the metal in the short-circuited leads is damaged, because essentially what you're doing is using the lab power supply as a welding power supply.

I suspect that at 30 volts, and 10 amperes, the spark show would be so huge that you have to wear eye protection or else some of the sparks can fly into your eye.

You don't want to destroy your power supply leads by creating spot welds into them! I have, and won't do it again.

Of course by short circuiting before turning the AC side on, adjusting the current, and then turning the AC side off without removing the leads, and only then removing the leads you can eliminate the spot welds and the spark show.

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  • \$\begingroup\$ Thanks! It's a bit too complicated for me at this point, as I am just starting out learning electrical engineering and electronics. But thank you for replying : ) \$\endgroup\$
    – sara_
    Dec 17, 2022 at 12:32
  • \$\begingroup\$ "or else some of the sparks can fly into your eye" – By "sparks" here, do you mean brief arcs, or tiny bits of molten metal? I wouldn't expect it to be possible for an arc like that to fly into someone's eye. (Of course, arcs can produce UV light, and I don't know how much power is required to make a hazardous amount of it.) \$\endgroup\$ Dec 17, 2022 at 22:01
  • \$\begingroup\$ Well tiny bits of molten metal. I have observed the sparks to fly quite far in that case. I have also observed the arc to create spot-welds in the disconnected leads, so definitely it's melting metal there. \$\endgroup\$
    – juhist
    Dec 18, 2022 at 7:48
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Most bench power supply used in labs have some kind of protection circuit in the form of a current limiter. That is when the output is shorted or just overloaded, the current won't reach levels that can damage the power supply, but the current will be limited to some set level which is somewhat higher than the maximum current the power supply can provide in normal operation.

There are more advanced protection circuits that can reduce the output current wrt the maximum allowable in case of gross overload (these are called protection circuits with current foldback).

This is true also for power supplies that cannot work as current sources.

In addition, power supplies intended to be used not only as voltage sources, but also as current sources have a current limiter whose threshold can be set by the user. In some purely analog supplies this can be done by shorting the output and setting the desired current level using the appropriate knob, usually while watching the current put out by the supply on the built-in ammeter of the instrument.

Modern digitally-controlled bench power supplies, be they linear (not very efficient and heavy, but very quiet) or switch-mode (extremely efficient and lightweight, but producing a noisy output), have the capability to set the current limit using a keyboard or a digital encoder knob on their control panel without shorting the output.

Anyway bench power supplies have multiple levels of protection against abuse. In addition to the current limiter, there is most often an internal fuse on the mains line that protects against gross failures of the internal circuitry (e.g. an internal fault that causes an anomalous current absorption from the mains that can start a fire otherwise). Moreover over-temperature protection is quite common, even in very old analog supplies.

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