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overvoltage protection circuit for measuring instrument (electrometer)

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zeeshan728

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Hi,
I am measuring current across a resistor by connecting an electrometer across it.i want to make a overvoltage protection circuit for the electrometer that as the voltage across the resistor increases say 5 volts protection circuit supress the overvoltage and not direct the overvoltage to the electrometer.
Kindly give your suggestions. thanks
 

In this schematic , resistor protects measured circuit against overcurrent
Zeners protect measuring device.
Calculate R so that zeners do not get overloaded.
Check the measuring error caused by added circuit.
In case DC-meter only one zener needed.

Meter_Protect.jpg
 

hi, kak 111
thanks for your reply..
Actually the whole system will be running on dc. It means only one zener diode should be used.and also there is one shunt resistor of 50 ohm in the circuit.so the value of R should be small than shunt resistor.how much small ???
kindly have a look at the circuit for better idea.and give ur comments thankscircuit.png
 

That is just as i think the circuit.
R is there for current limiter to zeners.
Worst case is , if 50 ohm current mesuring resistor burn off,
in that case added R must protect zeners and that way meter too.
Other hand , high value R means more error to measuring device. ( series to meter )

KAK
 

Hi,
thanks,
i got your point.so what value of resistor R you suggests around 5 ohm or less.???
and only one zener will be ok ?
thanks
 

If only DC is used in circuit then one zener is Ok.
Must check the situation when meter is connected in wrong polarity ( Vzener is in that case 0.7V )

Value R must calculate with max voltage value over shunt resistor ie. when shunt is burned off.
another needed value is zener voltage = max. voltage over meter
and zener Imax. or zener Pmax.
Measure error can then be calculated with meter resistance value ( in case DC ).
 
Last edited:
thanks alot for your precious comments
 

What is the maximum voltage the zener can protect. Can the zener tolerate 600V DC.
 

Do you know how a zener works?

When the voltage across the zener rises higher that the rated voltage the current through the zener starts to increase rapidly, if you have a resistor in series with it then this current through the resistor will create a voltage drop so that the zener keeps across it the rated voltage.
If there is no resistor then the zener will be burned very quickly by excessive current.

If you have used a proper resistor that can drop the excessive voltage while the zener is within specs then there is no problem.
 

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