basic electronics lab experiment

why multimeter give different forward resistance values of silicon diode at different resistance scales of a digital multimeter???

The circuit inside a DVM uses the same battery with resistances in series to get the different resistance ranges, so as you change range the current flowing out of the probes changes. As diodes drop different voltages dependent on what current flows through them, the meter calculates these different voltages as different external resistors.
To prove this effect to yourself, connect a battery and a resistor(10k -100K ohms) to a diode and measure the voltage across the diode. If its the battery voltage then reverse the diode connections, the result should be between .5 and .8. of a volt. Calculate the current I = (Vbatt - V measured) / resistor used. Change the resistor and repeat.
Frank

ok.at various resistance values voltage drop is different but how same diode have different resistance values at different resistance scale???????????
let suppose i will set my multimeter's nobe to diode testing and take out the forward resistance of diode (i-e, 540 ohm) and after that i will set DMs nobe at different ratings(like 1 kohm, 20 Kohm, 1 Mohm, 1 Gohm and so on) diode will give different resistance values...why?????????????i dont understand yet...

Resistances are "measured" by detecting the current that flows through the device under test. The diode resistance is not a constant but depends on this current since the diode is a non-linear device. More than that, this current is determined also by the internal resistance of the multimeter, which is different for different scales.

It's because a diode is not a resistor. When current passes through a diode, it causes a voltage drop across it and a multimeter shows that as the equivalent resistance. But the equivalent resistance is highly non-linear, which means that the voltage drop is not proportional to the current. Conversely, the current is not proportional to the voltage.

For example, a diode may have a voltage drop of 0.6V when you pass a current of 1mA through it. That is seen by a meter as a resistance of 600 ohms. If you pass 2mA through a 600-ohm resistor, the voltage drop will be 1.2V. But if you pass 2mA through the same diode as before, the voltage drop may be only 0.62V, and it will be seen by the meter as a 310-ohm resistor.

A multimeter, whether digital or analog, puts out different currents at different resistance ranges. Since the diode does not behave like a linear resistor, the meter shows different equivalent resistances for the same diode.

ok ok....it is because diode have a negative temperature coefficient

---------- Post added at 17:27 ---------- Previous post was at 17:24 ----------

thax everyone for helping me.................:smile:

No! This has nothing to do with its temperature coefficient which is an entirely different matter.

ok then tell me that why diode resistance is not constant?????:-?

Because a diode is NOT a resistor. Because that is how a diode behaves. Why do you seem to think that it should have a constant resistance? This is not the place to write a long presentation of the physics behind semiconductors and P-N junctions. If you want to know more about the properties of a diode, read some elementary books on electronics or search for such information on the net.

i was stucked at some point..........anyways thax for the advice.

How Ohm Meters Work

An Ohmmeter was designed for measuring resistors.

When using an ohmmeter, you are actually measuring the current through the resistor. By virtue of Ohm's Law and a known voltage source the scale displays the resistance of the resistor. Different currents are used for different ranges so you can measure a wide range of resistances. This is not a problem for measuring resistors because they are linear devices - their resistance is independent of the current passing through them.

Diodes on the other hand are not linear devices. Their resistance changes with the amount of current passing through them, so changing the range on the ohmmeter will display a different resistance.

Beenish, one additional remark (I forgot to mention in my former posting#4):

Multimeter measurements of the diode resistance gives you some rough information about the static dc resistance only.
Don't forget that for each non-linear device you have to distinguish between the static and the dynamic (differential, small signal) resistance.

than k you!!!now i understan...........:smile: