Diode ORing voltage measurement

[vinodquilon]

For the below circuit, I have placed multimeter +ve probe at 'B' wrt GND and measures 4V against the expected value of 0V.

Hence I have terminated the 'B' end with 10K to GND temporarily before taking the measurement and reads 0V.

Why this happens?

 

[ericgibbs]

hi,
Check the reverse leakage current of the D2 diode and input impedance of your multimeter.

E

 

[vinodquilon]

hi,
Check the reverse leakage current of the D2 diode and input impedance of your multimeter.

E

What happens to the anode of D2 while I connect a multimeter to 'B' end wrt GND ?

I have replaced the 10K temporary resistor with 1M, then again 4V measured !

 

[ericgibbs]

What happens to the anode of D2 while I connect a multimeter to 'B' end wrt GND ?

I have replaced the 10K temporary resistor with 1M, then again 4V measured !

hi,
When you connect the multimeter to point 'B' you are providing a path for the reverse leakage current of the diode to flow to 0V.

The leakage current will cause a voltage drop across the internal input resistance of your meter which will display as a voltage, its only nanoamps.

E

 

[vinodquilon]

hi,
When you connect the multimeter to point 'B' you are providing a path for the reverse leakage current of the diode to flow to 0V.

The leakage current will cause a voltage drop across the internal input resistance of your meter which will display as a voltage, its only nanoamps.

E

That is when I connect 1M resistor, V=IR-- a considerable drop occurs and reads as 4V.

In the case of 10K, measures very low as nA * KOhm and will be in MicroVolt range approximately 0V.

Am I right ?

 

[ericgibbs]

But what happens when I connect a 10K ?

Consider 20nanoamps flowing thru a 10K resistor, thats Vdrop = 10000 * .00000002 = 200 microVolts!

You may have a faulty diodes.