What does this circuit do?

[simce]

I've found that in many DVM's this circuit is placed between input terminals, but i can not figure what does this circuit do? It is some kind of protection, but how it works?

 

[vdaniel]

This circuit works like a simple diode connected in series with a Zener diode.
The back biased b-e junction works like Zener with about 6V breakdown voltage and forward biased one like simple diode with about 1V dropout. So the whole circuit is a sort of bipolar voltage clamp (or transient suppressor) for about 7V.

Varuzhan

 

[simce]

That was my first thougt, and to be sure i made this circuit in circuit maker to be sure. But this circuit does nothing on input voltage ramp from 0-1kV! (resistor is placed in series with V source and this circuit)!

 

[IanP]

Circuit maker is a simulation lab, and may not include this fact, that in BJT the (+/-) BE breakdown voltage is as low as 6V.
This circuit, as stated by VDANIEL, is +/- ≈7V clamping device..
Regards,
IanP

 

[artem]

This circuit works like a simple diode connected in series with a Zener diode.
The back biased b-e junction works like Zener with about 6V breakdown voltage and forward biased one like simple diode with about 1V dropout. So the whole circuit is a sort of bipolar voltage clamp (or transient suppressor) for about 7V.

Varuzhan

I think that forward biased trans is used for temperature compensation of backbiased zener alike trans.

 

[Borber]

Actually this circuit is equivalent of two zener diode connected antiseries. It works for positive voltage so as for negative.

 

[simce]

I don't think that temperature compensation is needed here.
Anyhow i think that this is bipolar clamping device as IanP and vdaniel said before.

 

[michel95]

It is a clamping device with low leakage current.

 

[vdaniel]

Really, though beeing mainly a clamp, it is the same time enough good temperature compensated Zener too.

Varuzhan

 

[power-twq]

actually, this circuit are two zener diode in serise back to back,

I think it is used to suppress over voltage spikes.

best regards

I've found that in many DVM's this circuit is placed between input terminals, but i can not figure what does this circuit do? It is some kind of protection, but how it works?

 

[Jack// ani]

Hi,

I’ve a doubt….here

Why does the reverse biased b-e junction, works like a zener diode? Why not the transistor will get damaged if applying reverse bias on b-e junction?

Thanks

 

[michel95]

This circuit is explained in the service manual of the Fluke 77 multimeter. It serves as back-to-back zener diodes which limit the input voltage to between 7 and 9V.