I hope I’m not just thinking past something trivial but I can’t make sense of this. In this video, https://m.youtube.com/watch?v=XDf2nhfxVzg , variable voltages from .1-20V are used and when he shorts out the wires, they all produce the same amperage. How does this work?

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    \$\begingroup\$ he is using a power supply that has current limiting circuitry ... looks like the current limit is set to 150 A .... the power supply reduces the output voltage automatically \$\endgroup\$
    – jsotola
    Jan 28 '20 at 6:46

In the video, the power supply can provide a maximum of 150 amperes.

If the resistance of the short is low enough, then more than 150 amperes would flow.

The cables used in the video are very thick and so have a very low resistance, and would allow more than 150 amperes of current to flow.

But, the power supply can only provide 150 amperes so you only ever see it putting out 150 amperes.

Take a couple of examples:

Set the power supply to 1 volt. Assume a cable resistance of 0.005 ohms. That would be a current of 200 amperes. The power supply can only deliver 150 amperes, so that's all we see in the video.

Now set the power supply to 20 volts, but keep the cables with 0.005 ohms. Ohm's law says you should get a current of 4000 amperes. The power supply can still only deliver 150 amperes, so that's all you will get.


I didn't watch the video, but the way lab power supplies work is that you set the voltage you want (maybe with no load), and the power supply will supply any current up to some set maximum at that voltage. If the load gets too low resistance, the power supply goes into a kind of constant-current mode (often pretty crummy in terms of stability compared to the voltage setting). Not all power supplies are happy working into a short some need a volt or two output to work properly (some switching supplies or those simple hobbyist ones based on an LM317 without negative bias)

So to summarize: You have two knobs. One sets the maximum voltage if the current is less than the maximum current setpoint. This is normally your regulated voltage for a circuit like 3.3VDC. The second knob sets the maximum current if the voltage is less than the maximum voltage setpoint. You normally set this to something that won't damage wiring or whatever, perhaps 100mA for a given circuit.

So, when open the meter reads 3.30V & 0mA. When you put a 50 ohm resistor on the supply, the meter reads 3.30V & 66mA. When you put a 10 ohm resistor across the supply the meter reads 1.0V and 100mA. Short it and it reads 0.0V and 100mA.


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