Products > Test Equipment
Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
snik:
--- Quote from: ZhuraYuk on May 16, 2024, 12:31:50 pm ---
Anyway, I was able to remove old reed switches from the relays. Some are easily sliding out, some require desoldering the relay and poling the switch with tweezers.
Replaced only first 4 relays and now input current goes to 5fA in 5-10 seconds after switching to amps. Easily can be compensated to zero current.
The original look of the board was preserved.
--- End quote ---
Nice work.
Which Reed Relays do you use ?
ZhuraYuk:
--- Quote from: snik on May 16, 2024, 12:46:23 pm ---Nice work.
Which Reed Relays do you use ?
--- End quote ---
Initially I used MEDER KSK1A85-2030 because they are larger than original and rated for 1kV. However after breaking two because of too hard leads I switched to MEDER KSK-1A66/3-1015 which are same size as original ones and found no differences. So I installed KSK-1A66 in both K617 and all were fine.
I assume that epoxy filling is just to hold reed inside and to avoid glass breaking when forming the leads. The epoxy absorbs water with time thus leading to high leakage.
Also insulation tube around switch seems to be made of PTFE it holds 250 degree C just fine, so I reused it as well.
ZhuraYuk:
Did you know that triaxial connectors with sapphire insulators existed? These were some experimental stuff that was supplied together with only one soviet electrometer V7E-42 (В7Є-42). And even there in documentation for this electrometer it is not explained what is their purpose, they were not used in electrometer itself but supplied as spare parts for some undocumented switch box filled with sapphire insulators relays. I guess they were there for connection with western electrometers like K617.
ZhuraYuk:
After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.
David Hess:
--- Quote from: ZhuraYuk on May 22, 2024, 10:48:52 pm ---After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.
--- End quote ---
An LMC6082 would have lower offset and drift.
Are there schematics showing what you did?
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