# Question about measuring the internal resistance of power supplies

1. Mar 13, 2018

### yubson

In measuring the internal resistance of a power supply, why is a resistor with low resistance used?

2. Mar 13, 2018

### phinds

Who says that it is? You need to be more specific in your question. I think you have misunderstood something.

3. Mar 13, 2018

### Merlin3189

I don't know exactly how you are using the low resistance here, but speaking generally,
the internal resistance of a power supply is a low value to give good load regulation, so to measure it by measuring the change in voltage at varying loads, you will see the biggest voltage drop when you draw the maximum current. That means using the lowest load resistance that will not overload the supply.

4. Mar 13, 2018

### lekh2003

Are you talking about an ammeter? Could you be more clear?

5. Mar 14, 2018

### davenn

no

he's referring to the actual internal resistance
We say a battery has an internal resistance, the value of which is determined by the chemical composition etc
it is what limits the short circuit current value

from Wiki

how to measure internal resistance of a battery ..........

I never really hear anyone talking about the internal resistance of a non-battery PSU ... I would assume it would also have one

ahhhh here's some info.......

Dave

6. Mar 14, 2018

### sophiecentaur

I think you are just asking why they use a low resistance load on the power supply. The reason is that the voltage drop with a high current is greater and easy to measure. If you know the Voltage lost (open circuit volts minus test volts) and the current being supplied, it will tell you the value of the Series Resistance that's in the box.
It's only an approximate answer because many power supplies will not have an unvarying internal resistance.

7. Mar 14, 2018

### David Lewis

Some power supplies have virtually no internal impedance within their design operating range. The voltage stays the same no matter the output current.

8. Mar 14, 2018

### sophiecentaur

For the regulation to be stable, a PSU would need a finite internal resistance, I think but that could be very low.

9. Mar 14, 2018

### davenn

yes, that will be the case for specifically designed constant voltage or constant current supplies

but for general supplies, internal ( output ) resistance is a factor and is discussed in the link I provided

Dave