# Is it possible? A voltage source with zero output short ckt current?

1. Apr 10, 2014

### Alkemist

That 2 terminals shows PD of 1 volt on Meter, but when shorted, there is no current shown on Meter?
I checked and rechecked [using different meters, wires, AC, DC options and what not], so I see only V measurement, but no I. Is it possible?
If so, then what is the use of that PD, when we can't have I from it?

The resistance of the wire, which I use for shorting is 0.1 ohm.

Last edited: Apr 10, 2014
2. Apr 10, 2014

### mjhilger

This post has no context to determine an answer. If PD is potential difference and shorting shows no or virtually no current; is it possible? YES!

But what are you referring to with the two terminals? Is it a power supply? Is it a random box?

Power supplies are built to prevent problems from a direct short and fold back so the current might become 0 to prevent harm. Likewise a voltage source with a high resistance in series, say 1 megaohy would read virtually no current (well a microamp, but depends on your test meter as to its sensitivity).

3. Apr 10, 2014

### Staff: Mentor

That implies a very high output impedance for the voltage source. Or an incorrect meter setup. Have you checked the fuse in the current measurement part of your DVM? If you dead shorted the voltage source, there's a good chance you blew out the fuse right away, and that's why you keep seeing no current...

4. Apr 10, 2014

### Alkemist

The fuse is Ok. I'm using same meter for other readings and it shows me the results for current.
I'm curious about the impedance. I think it is the case here. But is that Voltage of any use? Or can we use it in any way?

5. Apr 10, 2014

### Alkemist

Hi MJHILGER, I said 2 terminals, because I don't know the internal of the circuit. For me it is Black box with 2 output terminals. As I said, the resistance I use is 0.1 ohm. So if I the potential difference in those 2 terminals is 1 volt, I was expecting some current to flow through it and see it on meter. But nothing comes up.

6. Apr 10, 2014

### Staff: Mentor

Can you try several different test resistances across the terminals, in series with the current mode measurement of your DVM? We may be able to determine the output resistance of the voltage source that way.

For test resistances, use 1MegOhm, 100kOhm, 10kOhm, 1kOhm, 100 Ohms.

And if you could post a diagram or pictures of your measurement setup, that would help a lot. What are these 2 terminals connected to? Is this some puzzle for a class maybe?

EDIT -- Do you have 2 meters, so you can measure the terminal output voltage at the same time as you measure the current through the test resistor?

7. Apr 11, 2014

### Alkemist

Yes I do have two meters and I'll try them at the same time today.
I do not have different resistances available, but I'll try to use different loads which are available e.g. CFL, or 7W bulb or a small motor and long wires of different sizes.

I'll also post the pic/diagram.

8. Apr 12, 2014

### sophiecentaur

The idea of 'foldback' for PSUs is an old one. Look at this link (and many other Google hits).
It can present the user with confusing behaviour but it's a lot more convenient than a fuse for overload.

9. Apr 12, 2014

### jim hardy

In old days we used a "Weston Standard Cell" for a voltage reference.
It was a nominal 1 volt cell used as a reference , intended to never deliver any current beyond a microamp or two into a null detector.

http://en.wikipedia.org/wiki/Weston_cell
they'd behave rather like you describe. see page 21 of 31 here.

10. Apr 12, 2014

### sophiecentaur

Ahhh. A Voltage 'reference' - not a supply. That makes sense. I read " voltage source" as 'ideal voltage source'.

11. Apr 12, 2014

### jim hardy

Could be OP has run across one. It's likely be in a fine oak box better part of a hundred years old.

12. Apr 12, 2014

### sophiecentaur

We had one at School in 1961. And a polished metre potentiometer plus galvanometer. Griffin and George, no doubt.