understanding how DMM works in diode and resistance modes

[m4rtin]

My digital multimeter supports the diode test mode. According to manual, it is very easy to use:

1. Connect the red test lead to "VΩmA" jack and the black one to the "COM" jack (the red lead has a positive polarity "+".).
2. Set the rotary switch in the " " position.
3. Connect the red test lead to the anode of the diode to be tested and the black test lead to the cathode of the diode. The approx. forward voltage drop of the diode will be displayed. If the connection is reversed, the display will merely show a "1".

However, I'm confused why resistance mode has five different "modes"(2M, 200k, 20k, 2k and 200 ohm) while diode mode does not. Also, how does the DMM know, that now it is in the 2k resistance test mode and not in diode test mode when the switch position is the same. Or is the amount of current sent out by the DMM the same both in 2k resistance test mode and in diode test mode?

[bd139]

In resistance / ohms mode the meter is a constant current generator. The meter applies say 1mA through the terminals and measures the voltage difference.

If there is a 2K resistor in there then that gives 2V (ohms law) which is actually what the meter uses internally for this range.

Now when you do the same with a diode, due to the V-I curve of the diode a voltage is developed across it, usually around 0.5-0.7V so it measures that. 1mA happens to be a convenient amount that turns on all diodes and doesn't blow them up. in 200R mode, that would need to be 10mA (will kill some small signal diodes). In 20K mode, that would need to be 100uA which may not turn on some big diodes.

Basically it's always measuring voltage. When it's in ohms mode it applies a current as well.

[Kleinstein]

In diode mode the meter reads the voltage. With 1 mA current for the 2 Kohms range the same number also represents the resistance in KOhms. So the meter does not need to know - the user decides whether to read the number as voltage on a diode or resistance.

[jeroen79]

However, I'm confused why resistance mode has five different "modes"(2M, 200k, 20k, 2k and 200 ohm) while diode mode does not.

The voltage drop for diodes will not come in as wide arange as resistors.
.6-.7V for Silicon diodes, a bit less for Germanium or Schottky diodes.

LEDs will range from a bit under 2V to over 3V.
Many multimeters will not be able to turn on LEDs, some only red LEDs and some all.
Zenerdiodes come with many reverse voltage drops.

The diodemode on a multimeter is usually just for the simple diodes in rectifiers, logic, etc. to tell you if the diode is good or not.
For testing LEDs and Zeners you can better use different equipment where you can choose higher voltages and different currents.

Also, how does the DMM know, that now it is in the 2k resistance test mode and not in diode test mode when the switch position is the same. Or is the amount of current sent out by the DMM the same both in 2k resistance test mode and in diode test mode?

With this meter there is only one mode under this switch position.
The 2k resistance mode IS the diode mode.

Other meters may have a separate diode mode (under a different switch position or shifted) that will behave different.

If you want to get to know your multimeter better you should get a second one.
Connect them together to measure eachother:
How much current produces the resistance or diode mode?
How much voltage produces the resistance or diode mode when there is only the high resistance of the other meter conencted?
What is the resiatnce of a meter in voltage mode?
And what is the resistance between the common and current jacks?