I am doing simple ohm's low using sinometer digital power supply and potentiometer 50 watt when i begin to increase the current the voltage from the power supply decreased that's okay but when i measured the voltage on the potentiometer using my voltmeter it give a value lower than that displayed on the power supply and this value decreased more and more when increasing the current , so what is the reason for this accuracy error ? is it my voltmeter needs the battery to be changed ?could the battery weakness do that?or the error is in the power supply

 

How much voltage and current are you supplying from the power supply?
Did you take into account the resistance of your connecting cables?

 

How much voltage and current are you supplying from the power supply?
Did you take into account the resistance of your connecting cables?

I am feeding more than 2 Amps and 25 volts , if wires resistance were taken into account that should affect also on the power supply digital reading but that didn't happen .

 

Are you simple measuring the voltage drop across the potentiometer?
So you had a 25V supply.. What "reading" did you get when you attempted to measure the voltage on the pot?

 

Measure the voltage across the terminals of the power supply.
Then measure the voltage across the pot.

 

Are you simple measuring the voltage drop across the potentiometer?
So you had a 25V supply.. What "reading" did you get when you attempted to measure the voltage on the pot?

when i began to increase the current the voltage on power supply display drops but slowly may be .1 or .3v and also on the voltmeter but when increasing the current after 1 Amp the vlotmeter reads on the potentiomter 29.5 v but it is still 30.2 on the power supply display .

 

just because the power supply meter reads 30 volte does not mean your getting 30 volts to your load. the resistance of the wires from the supply to the load will drop the load voltage some. measusr as close to the load as possible.

 

What type of meter are you using and what is the rated accuracy of the meter? Do you have any bad connections?

 

What type of meter are you using and what is the rated accuracy of the meter?

http://www.sinometer.com/?Power-Supplies-category186.html

 

I am feeding more than 2 Amps and 25 volts , if wires resistance were taken into account that should affect also on the power supply digital reading but that didn't happen .

Not true. The wire resistance affects only the voltage at the load. The power supply meter measures the voltage before the drop caused by the wire.

 

when i began to increase the current the voltage on power supply display drops but slowly may be .1 or .3v and also on the voltmeter but when increasing the current after 1 Amp the vlotmeter reads on the potentiomter 29.5 v but it is still 30.2 on the power supply display .

Fine. Now try measuring from the + terminal of the power supply to the + terminal of potentiometer. Then do the same measurement between the - terminal pairs.

See if these measurements plus the potentiometer voltage equal the power supply voltage. It *must* be equal, or your measurements are in error.

 

thank you all . it was the wires resistance because it have small area . i used thicker one .

 

thank you all . it was the wires resistance because it have small area . i used thicker one .

There's still a voltage drop in those "thicker" wires. ;-)

 

2 amps and 25 volts is 50 watts. What king of pot are you using to adjust your experimental current?

 

http://www.sinometer.com/?Power-Supplies-category186.html

That link just goes to a page with a bunch of power supplies.

What DMM are you using and what is it's rated accuracy. What is the accuracy of the meter on the PSU for that matter?