# Multimeter resistance

1. Oct 20, 2007

Hi I have this lab about exploring rectifier diode and zener diode.

I have this question I have problem to answer.

"You measured the current through the diode (rectifier diode) and voltage accross the diode using the multimeter in the lab.
Use a point in the reverse bias region and the forward bias region, make a reasonable estimate of the resistance of the meter, and compensate the effect of the meter. In what situation would you expect that this meter induced error would be largest
? "

I dont see how to do it.
Thank you
B

2. Oct 23, 2007

### YeMeNi

in foreword baising the meter will induce error but does not effect in measurement.

its need some piuture to explain that i dont have it :)

3. Oct 24, 2007

### Staff: Mentor

You are being asked about the sensing resistor inside the DVM in current measurement mode. The DVM measures current by measuring the voltage drop across this sensing resistor as the current flows through it. Typically larger resistors are used for measuring the lower current ranges, and smaller resistors are switched in for measuring the higher current ranges. Depending on the meter and the range, this series resistor can drop a voltage that then causes some error in the overall measurement.

For example, let's say that you are trying to measure the power input to an IC. You put your DVM in current mode on the 100mA scale, and connect it in series between your 5V power supply and your IC. You measure 30mA, but the power input to the chip is not 5V*30mA. Why? What do you need to do (and how many DVMs does it take) to measure the power input to the IC accurately?

Now, in your problem, assume that the DVM only has one current sensing resistor value (otherwise, you are not given enough information to answer the question). Will you get the biggest voltage drop across that sensing resistor when you have forward bias current flowing through it, or reverse bias current flowing through it? What are the typical forward and reverse current values that you measured in your lab?