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.
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
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.
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?
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.