Multimeter input protection - what are these BJTs doing?

[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.

[retiredcaps]

Since that, I tried to figure out, what's the purpose of Q1 and Q2 transistors. (on page 58 in the service manual)

Page 2-4 in the same service manual also explains what Q1 and Q2 do.

2-4. Input Overload Protection

Transistors Q1, Q2, and Q1 provide additional overload protection for the millivolt and ohms functions. If sufficient overload voltage is present, the transistors turn on and connect that input to common through limiting resistors R2 and RT1, thereby protecting the circuitry in U1. A clamp circuit (CR6 and Q15) connected to the volt/ohms/diode-test input through Z1 and C3 provides similar protection for the volts/ohms/diode-test input.

[dobszi14]

Many thanks guys! Now it's all clear 

[bhishmar]

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!

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.

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.

This is an old topic, &  above two posters have reasonably answered  "dobszi14"'s question. 
But let me fill in the blanks & add some more meat into the answer for the benefit of other searchers.


Q1 & Q2 used here with their base & collector shorted, is effectively a  back-to-back bipolar zener-diode, used for clamping/limiting  a bi-polar input signal.

This has several advantages over standard back-to-back zener-diodes.

• Less Capacitance than a standard zener-diode as pointed out by Sean.
• Faster response/switching  time vs zener-diode [ns vs μs/ms] (a zener switching is very slow)
• Knee (Vi-characteristic) of this configuration is sharper than the reverse-bias V-I charac of a zener.  So before breakdown happens (input-voltage < limit-value condition), the leakage current is very low (neglible pa/nA).  So it does not create inaccuracy in μA current measurement by stealing some of the "current-shunt resistor" current, when used in such configuration.

This transistor configuration is non-standard, and is not usually explained in most standard electronics text-books, so when you comes across it, for the first time, it puzzles & surprises you, as me.  I encountered such a configuration first in the feedback circuit of a precision amplitude sinewave generator,in an avionics circuit, in my work. it was using BJT 2N2369. It was having a VEBO spec of 4.5V, but when reverse biased, with a current-limiting resistor, it always breakdown at 6.9V, acting as a fast zener.  I initially thought this was a queer behaviour of 2N2369 alone. 

I found it difficult to  find a similar circuit behavior referred or explained in any electronics textbooks.  But after some research & testing found that this was standard behavior of any BJT-transistor. But the breakdown voltage value may not be same, & is influenced by the doping of the base-emitter junction.  But it will be around 6V.

Bear in mind, this is a non-standard use of a BJT.  The breakdown process is avalanche breakdown effect, not zener break down effect, accounting for faster switching time, I think.  And also the current-limited avalanche breakdown, does not damage or degrade the BJT-device, which is difficult to comprehend initially.

So as long u are not very specific about the value of the breakdown voltage to millivolts or 100's of millivolts-level, then you can safely use such a configuration to replace a zener.
Hope this helps others.