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What does this circuit do?

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simce

simce

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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?
 

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

    simce

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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)!
 

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
 

    simce

    simce

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vdaniel said:
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
Click to expand...

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

    simce

    simce

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

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.
 

It is a clamping device with low leakage current.
 

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

Varuzhan
 

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

I think it is used to suppress over voltage spikes.

best regards




simce said:
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?
Click to expand...
 

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
 

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.
 

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