diode tester - silly question

[whiz115]

Hi!

up to now i had 2 multimeters (one analog and one digital) and i was naive enough to believe whatever my digital multimeter said... today i bought one more digital and while i was playing with some components i noticed that my digital mutimeters disagree about the voltage drop on my diodes!

the cheaper one shows most of my rectifying diodes (1n4007,1n539) more close to realistic values 0,6/0,7V the other one shows lower values like 0,5/0,4V but the funny thing was when i did the following!

i connected the two digital multimeters togather and i regulated the first one at resistance measuring function (so it can output voltage from the probes) and the other one at voltage measuring function, when i was measuring the voltage comming from the first multimeter it was about 2.88V and when i was putting a diode between the first pair of probes i could see a voltage drop close to the non realistic values my new multimeter showed on diode testing!!! in fact the voltage drop was about 100-150mV lower depending the diode!

what's going on? ghosts?



my excuse: i'm new on electronics...newbie..bla bla bla...

 

[dknguyen]

Different Bias currents? The more expensive one would be expected to have lower bias currents I would think = lower voltage drop (ideal diodes have a constant volage drop with current, real diodes don't) Also, it sounds like you were measuring one DMM with another DMM? You can't really do that since the DMM needs to have a much higher input impedance than the circuit impedances in order to have minimal impact on the circuit. Two DMMs- both very high, probably comparable input impedances = a lot of inaccuracy.

 

[whiz115]

which one should i believe? which is the real voltage drop? i have just tested a 1N539 with the new one and says "564".. 0,5V right?! and the old one says
"774"...

 

[dknguyen]

They both are. The voltage drop just varies with the load current.

Second graph on page 2:
https://www.electro-tech-online.com/custompdfs/2008/01/ds30096.pdf

 

[Boncuk]

Hi whiz and dknguyen,

sorry for the misspelling. However editing is impossible today. (Software error?)

Regards

Hans

 

[whiz115]

ok i think i got my answer... thanks dknguyen for helping me! i couldn't imagine because i never had the chance to see it somewhere happening.

"the voltage drop increases slightly as the current increases so, for example, a silicon rectifier diode might have a forward voltage drop of 1 volt when 1 Amp of current is flowing through it."

and now what? what's the importance of the diode taster? it tells me that
at 2.88V/1mA specific diodes have 0.5V drop? but as battery runs out this will change!!!

 

[dknguyen]

It tells you the diode is working? It's also used to test transistors. It also tells you around the voltage the diode will turn on at (assuming that the DMM bias current used to measure it is insignificant compared to the current it will conduct in your application- just not he voltage that will actually be across it once the current has ramped up to steady-state. Just one of those things.

 

[Dean Huster]

The whole thing reminds me of the old adage: I you own a clock, you know the time. If you own two clocks, you're never sure.

Or something like that!

Dean