Electronics > Beginners

Multimeter input protection - what are these BJTs doing?

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dobszi14:
Hello guys!

Recently I watched this EEVblog episode, about multimeter input protection:

Since that, I tried to figure out, what's the purpose of Q1 and Q2 transistors. (on page 58 in the service manual)
Some kind of voltage/current limiter? I also tried to do a simulation in LTspice, but I still don't have a clue.

If you have any ideas, please share them!
Thanks!

Wytnucls:
Overvoltage protection, usually on the microcontroller and ADC input lines (transistors wired as 2 diodes back to back).
The UNI-T UT71 series has the same configuration (10 transistors):

dobszi14:
Thanks for the answer!  :-+ How can I calculate the voltage limit, where the protection is "activated"? So which parameter of the transistor is needed to calculate it? For example using this: http://www.mouser.com/ds/2/149/2N3904-82270.pdf

Thank you!

plesa:
In case you will use 2N3904 the voltage will be somewhere between 8.8V-9.2V. The upper 2N3904 behaves more like zener diode ( 8.1V-8.5V) and lower like diode (0.7V).
For exact voltage you will needs to measure exact transistors.

SeanB:
Base emitter reverse breakdown, typically in the region of 6-10V. They are used because the junction has low capacitance and very low reverse leakage. Important in a high impedance circuit.

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