# Does voltage limit current?

When I short the negative and positive wires on my DC lab power supply, at ~1V the supply shows it's max current output of 5A. but when I turn down the voltage, at some point the output ampere decreases.

I was wondering, if there is a physical limit, which affects the maximum amount of ampere that can flow on low voltage or if this is just from my power supply?

What I also recognized, was that if I put a resistor in between the contacts, the current seems to drop much earlier while decreasing the voltage. So I tought, that the resistance might affect this effect?

• Are you familiar with Ohm's Law? – Eugene Sh. Jan 24 at 20:50
• The behavior of your power supply when it is shorted is determined entirely by the design of your power supply, and you haven't told us anything about that. – Elliot Alderson Jan 24 at 20:51
• @Eugene Sh. Sadly not yet. I'm fairly new to this topic and it is something I didn't found a direct anwser to... – TheEquah Jan 24 at 21:03
• Well, then you should probably familiarize yourself with it. It is very simple (in it's final form) and serves a basis to a large subset of electrical engineering. – Eugene Sh. Jan 24 at 21:04
• @Eugene Sh. Thank you for mentioning Ohm's law. After taking a quick look at it, it makes things a lot more clear. – TheEquah Jan 24 at 21:33

At the very theoretical end: no, it's limited by other factors.

It's possible to have a superconductor carrying a current; this would then have zero voltage across it. However each superconductor has a critical current density above which magnetic field effects cause it to stop superconducting, at which point it usually self-destructs.

More prosaically, spot welders can achieve thousands of amps at about one volt.

• ahh, so in theory (and maybe superconductors), the low voltage does not limit the current capability ...which would anwser my question. – TheEquah Jan 24 at 21:30

So I thought, that the resistance might affect this effect?

Absolutely.

The resistance of a 'short-circuit', including the contact resistance from the supply terminals to the wire, is not zero, but finite.

If the supply is being asked to deliver (say) 1v to a 0.1ohm resistance, then ideally it would take 10A. If the supply limits at 5A, then the output voltage would fall to 0.5v so that's all it's delivering. Reducing the requested voltage to (say) 0.2v would result in only 2A flowing, which it would deliver happily.

Congrats on knowing the difference between effect and affect.

• "Congrats on knowing the difference between effect and affect." I know they're quite commonly confused, but that's a bit condescending! – NMF Jan 25 at 0:13

0 Ohms only exists on paper or by assumptions when higher loads can neglect the effects of current limiting. We call this real resistance or effective series resistance ESR and it exists everywhere in caps, batteries and even the AC grid and EE lab supplies.

The error is due to the limited feedback gain to correct the error to make the loop stable. The loss is initially defined by a series pass transistor Ron then reduced by its feedback loop gain by sensing the error voltage.

Normally I define ESR as a percent of rated max load which is the inverse of load regulation drop error. On the AC grid, a transformer impedance is rated the same as Zpu or per unit of rated load impedance.

So if a car battery can crank 1000 Amps while dropping to the test voltage from 12.5V to 7.5V or a 5V drop, we can say its ESR is about 5V/1000A = 5mΩ

So we expect the load resistance of that battery test level to be 7.5V/1000A = 7.5mΩ for the same result as the CCA crank spec. Usually, the start is much higher than 20mΩ in warm weather with less friction so the voltage drop is less.

You lab supply may have a current limiter or not, but it's ESR or Zout or source impedance at DC is defined by that incremental drop in voltage with a drop in short circuit current. or ESR=ΔV/ΔI which is some % of the min load R at max power with Vo/Imax=Rmin.

A good design is 1~2%. A great design is better, and not so good worse.

Here is another similar query.

Its good to know the limits of our test equipment, so keep exploring. Of course, there is reactive storage energy in the output cap, so peak power currents and durations will be higher. But don't overdo it with pulsed inductive loads putting energy back into unknown protected sources.

I always am aware of the ESR of all components and it varies widely and with frequency as well. It can be used for a transistor or diode or LED saturation and is inversely related to the Power capacity of the part.

Often it has different names like ESR for dielectrics and Rs for conductors, and DCR for inductors or RdsOn for FETs and Rce for BJT's.

Yet no one can make true 0Ω but get pretty close with cryogenic superconductors for MRI's.