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Overcurrent Protection on 6485

2502, 6485, 6487 - Support
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Overcurrent Protection on 6485

Post by cheeverbean » September 18th, 2014, 12:31 pm

Dear all,

I have a question about the 6845 - that I can't find the answer to in the manual. Let's suppose I'm measuring a 0.5 nA current using the 2 nA range with auto-ranging disabled and then the current suddenly jumps to 5 uA but the ammeter is still on the 2 nA range (since auto-ranging is disabled). Is such a current jump potentially damaging to the -? How does the ammeter protect itself from over-current on a specific range (i.e. if the current is suddenly 5 uA on the 2 nA range)?


Dale C
Keithley Applications
Keithley Applications
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Re: Overcurrent Protection on 6485

Post by Dale C » September 19th, 2014, 8:34 am

The nature of a feedback ammeter allows it to not necessarily need true current limiting on the input.
If you force too much current into one of the ranges the preamp in the feedback loop will run up to the rail and will essentially lose control of the loop.
This means that the input of the ammeter will no longer look low impedance like it does in normal operation. The voltage will start to climb at the input with the increase in input impedance and the input voltage clamps will turn on if the voltage gets too far away from ground.
If the voltage limit or compliance of whatever is sourcing the current is low then no damage should be done.

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