Diode resistance

Discussion in 'General Electronics Discussion' started by vick5821, Feb 28, 2012.

1. vick5821

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0
Jan 22, 2012
Hey guys, what's actually diode resistance means ? And how to measure it ? Anode of the multimeter to the cathode of diode ?

Thank you

2. jackorocko

1,284
1
Apr 4, 2010
A diodes resistance is not linear. As a forward biased diode has very little resistance as opposed to a diode that is reversed bias(within max limits of the device), as long as you are talking about normal diodes. Things change when you start talking about other types of diodes.

Last edited: Feb 28, 2012
3. vick5821

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Jan 22, 2012
I am using an analog multimeter, why the black lead is connected to the anode of diode ? Why in this way ?

4. vick5821

700
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Jan 22, 2012

If the terminals of the transistor is not know, how do I determine which is the base..I assume I never know about anything in transistor

5. jackorocko

1,284
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Apr 4, 2010
Well a diodes resistance will change with temperature as well. So the best way to know the resistance is to take the Vfd and divide it by the current. Ohms law!

1,284
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Apr 4, 2010

7. jackorocko

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Apr 4, 2010
Last edited: Feb 28, 2012
8. duke37

5,364
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Jan 9, 2011
On the resistance ranges, an analog meter normally gives a positive voltage on the black lead and a negative voltage on the red lead. You can check this with another meter.

A digital meter normally measures the voltage across a diode when a small current is passed through it. Germanium or Schottky 200mV, silicon 600 to 700mV.

Resistance is not a good terminology for diode characteristics.

9. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010
Some diodes (particularly Zener diodes) have this as one of their published characteristics.

It is non-linear, so it is always defined as a fixed point (usually a current) and at a particular temperature. At this current the resistance is defined as being dv/dr.

And you can't measure it in a meaningful way on a multimeter.

You do it be measuring the voltage drop at two currents (preferably 2 currents that are only slightly different) and then using r = (v1 - v2)/(i1 - i2) as an approximation. he measurements have to be done quickly to prevent heating of the junction to become significant (which it will do more rapidly at higher currents and at lower differences in current)

10. vick5821

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Jan 22, 2012
How do you verify this ?

11. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Connect one multimeter's leads (with the multimeter on a resistance range) to another multimeter's leads (on a voltage range) and check the polarity

12. vick5821

700
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Jan 22, 2012
Testing a diode with a multimeter

The techniques used for each type of meter are very different so they are treated separately:

Diodes
a = anode
k = cathode

Testing a diode with a DIGITAL multimeter

Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol.
Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).
Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).
Testing a diode with an ANALOGUE multimeter

Set the analogue multimeter to a low value resistance range such as × 10.
It is essential to note that the polarity of analogue multimeter leads is reversed on the resistance ranges, so the black lead is positive (+) and the red lead is negative (-)! This is unfortunate, but it is due to the way the meter works.
Connect the black (+) lead to anode and the red (-) to the cathode. The diode should conduct and the meter will display a low resistance (the exact value is not relevant).
Reverse the connections. The diode should NOT conduct this way so the meter will show infinite resistance (on the left of the scale).

Why is this so ? How's the current flow ?