Author Topic: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617  (Read 79238 times)

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Offline r6502

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #250 on: October 19, 2021, 07:47:06 pm »
Hello all,

I just dissembled K307, K308, K309, K310, K311 and K312 and measured the resistance between contact - contact, contact - coil and contact to shield. I used my older K616, a shielded box and a laboratory power supply set to 80V output voltage (=max output voltage). The output voltage was filtered with an RC network consisting of 1µF and 20K. The resulting current was measured when it was stable (took something between 5 min and 30 min), then the resistance was calculated.


all values in [Ohm]
Rel.:  cont. - cont.; cont. - coil;    cont. - shield
--------------------------------------------------------
K307:   5.33E+12;       2.00E+12;      2.00E+12;
K308:   1,00E+12;     444.44E+09;    320.00E+09;
K309:  53.33E+09;     800.00E+09;      3.64E+09;
K310:   3,20E+12;       1.00E+12;    800.00E+09;
K311:  11.43E+12;       4.00E+12;      4.00E+12;
K312:   3.20E+12;     533,33E+09;    400,00E+09;


Does somebody have an idea, how the different values of different relays could be explained? Coud it be that it depends on the number of cycles for each relay? the current ratings in this application is not that high I thing.

Kind regards

Guido

Edit: componet names for relays corrected
« Last Edit: October 20, 2021, 10:21:46 am by r6502 »
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Online David Hess

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #251 on: October 19, 2021, 09:20:38 pm »
I would start by assuming surface contamination which varies between relays.

 

Offline r6502

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #252 on: October 20, 2021, 04:13:15 am »
Hello all,

Now I saw, that MIDI had also measuredtnis relays:

Yesterday the Triax accessories arrived (and nearly quadrupled value of unit):
....

relay # |betw. contacts |C-ES long |C-ES short |
K30730515
K308>10040300
K309702070
K310<31010
K311>100100>100
K312>500>100>100

also with totaly different results.

@David:
I cleaned the relays after disassembly very well, and let them dry in the oven, so I would be very sure, that the "low resistance" is not caused by contamination at the outside of the part.

The relays in my K617 are the black/brown tyes, that are filled with a kind of resin arround the reed contact, not the coto typs.

I'm really interested in the cause that the original isolation resistance drops from about 10e-14 Ohm (or higher) to the lower values.

Guido

« Last Edit: October 20, 2021, 10:20:21 am by r6502 »
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Offline r6502

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #253 on: October 20, 2021, 10:42:41 am »
Hello all,

I just found som Informations on "Electrochemical migration (ECM)"
https://en.wikipedia.org/wiki/Electrochemical_migration

Could this be the source of reducing the original isolation resitance of this kind of relays?

Guido
Science can amuse and fascinate us all, but it is engineering that changes the world - - Isaac Asimov
 

Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #254 on: October 20, 2021, 10:53:37 am »
One other possibility for the increased leakage is that the vacuum in the glass has failed.
In order to proof this, one would have to take the relay apart and measure only the vacuum tubes.

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Online Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #255 on: October 20, 2021, 01:47:59 pm »
I've worked on three 617 so far, and on one of these relays were leaky (brown epoxy isolated ones). I've tried my best to restore the original relays (including a very thorough IPA cleaning and a proper +120C vacuum bake for 8 hours) and they still leak, so had to replace the lot. I've used the MEDER HI05-1A66 with good results.

Cheers

Alex

P.S. Now I have better electrometer grade relays to my disposal, however these need more voltage (9V) and would require a modification to the driving circuit.
« Last Edit: October 20, 2021, 01:52:55 pm by Alex Nikitin »
 

Offline r6502

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #256 on: October 20, 2021, 03:39:37 pm »
Hello Alex,

the ons from MEDER do not have the electrostatic shield, so they are no option for me may be used just for testing some stuff because they are relative cheep ...

The Crydom types can not be bought somewhere I think, because of custom made part but interesting ...

Guido








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Online Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #257 on: October 20, 2021, 05:48:50 pm »
I can sell you a set of six, however as I've said, the driving circuit needs to be modified (there is a way to do it without changing the supply voltage, by adding some circuitry). PM me if you are interested.

Cheers

Alex

P.S. - the most critical relay in the 617 is K307 as it will see very high voltages and thus the dielectric absorption becomes an important parameter.  The rest of relays can be replaced by something simpler, I've tried REMtech relays and in all positions but K307 these small relays work OK (again, the driver circuit needs modifications as the coil resistance on REMtech is only 150 Ohm for 5V nominal voltage, too low for CMOS logic gates to drive directly).
« Last Edit: October 20, 2021, 10:49:53 pm by Alex Nikitin »
 

Offline Atomillo

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Hello!

I recently became an owner of a non functional Keithley 617, which showed overflow on all ranges.
After some troubleshooting, I found that the bootstrapped 5 volt supply was missing, because of a cracked trace close to a screw mounting hole (image attached).

The problem now is that even after cleaning both the up and downside of the electrometer board, the electrometer shows a current of up to 0.5pA without anything connected. Furthermore, it seems like this value slowly increases over time and isn't really stable. Zero checking the meter shows a very stable 0.05pA.

I've read the previous replies to this thread but this current seems too high to be due to leaky relays. Is this correct? I've disconnected the input connector but the current didn't improve.

Any help would be much appreciated!

EDIT: I've made input offset adjustment without any issue, it is really stable and now when I zero check I get 0pA. However, the input current adjustment doesn't work at all!
Simply inserting the screw driver into to trimmer screw makes the readings go a bit crazy and turning it doesn't really change anything. Could this potentiometer be toasted or is this normal behavior for such a sensitive meter?

 
« Last Edit: May 25, 2022, 02:49:06 pm by Atomillo »
 

Offline MiDi

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Let the K617 warm up for a couple of hours in the lowest current range (input connector disconnected or protection cap on it).
Apply zero check and zero correct as mentioned on p. 45 manual.
Release zero check, it should only change/drift by couple of fA, otherwise something is fishy.
Switch to highest voltage mode, zero check, the voltage should stay stable, otherwise something is fishy.
Release zero check, the voltage should change slowly, otherwise something is fishy.
If nothing is fishy, you can proceed with input offset & current adjustment see manual p. 131.

As you have read the thread you should know that leaky/high DA reed-relays are common fault.
 

Offline Atomillo

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After couple of hours warm up, zero checking makes it read 0.002pA. After correcting and releasing zero check, the current jumps to about 0.3pA in two seconds or so but stays quite constant there.

I think there is indeed something fishy then. Should I start connecting and disconnecting relays to see if there's anyone leaky?
 

Offline MiDi

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Switch to highest voltage mode, zero check, the voltage should stay stable, otherwise something is fishy.
Release zero check, the voltage should change slowly, otherwise something is fishy.

How does that?

After couple of hours warm up, zero checking makes it read 0.002pA. After correcting and releasing zero check, the current jumps to about 0.3pA in two seconds or so but stays quite constant there.

I think there is indeed something fishy then. Should I start connecting and disconnecting relays to see if there's anyone leaky?

Quite constant means after some minutes settling it changes only couple of fA?

First suspect is K307, it is the only relay that changes between zero check and normal operation (see p. 142)
 

Offline Atomillo

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In the highest voltage range, zero checking makes it read 00.00 no problem, nothing changes.
After releasing it, it slowly climbs up to 1.10V in about 1 minute (slightly less) and then stays constant

Yes, after about 2 minutes it only changes about 10fA.

I will disolder K307 (with all the necessary precautions given the high impedance environment) and report back!
 
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Offline Atomillo

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After desoldering cleaning and letting about 25 minutes for it to settle, the reading is very stable at 0.25pA.
So a bit of an improvement but still far away.

I will now reconnect K307 and try disconnecting K310 and if that doesn't show anything different then K309.

EDIT: K309 is okey, after disconnecting it the reading didn't change. Time for K310
EDIT 2: The same result is obtained with K310.

Thus, the relays all seem perfect!
It now seems to me that the only candidates left are the two transistors connected as protection diodes or the JFET itself. In that case, I would have to do the LMC662 mod. I will first check the transistors though

EDIT 3: Removing the protection transistors makes an inmediate difference. The current now is 0.15pA!!

Now, there only remains one big problem which I don't know how to solve. Simply inserting the screwdriver into the Input Current Adjustment trimpot makes the reading violently change. Turning the trimpot and then stopping doesn't change any of the readings.

I've read the manual but I've not found anywhere a specification of adjustment range. Is this known? Migth be this a current too big to be compensated by the trimmer?
I think
« Last Edit: May 25, 2022, 05:50:24 pm by Atomillo »
 
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Offline MiDi

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Staying at 1.1V in Voltage mode does not seem to be correct, that would mean there is no effective bias current.
If there would be too much input leakage or bias current it would rise to OL nearly immediately.

IIRC trimrange of input bias current was only couple of 10s fA, there should be a post here.
Edit: quick sim gave +-4.9mV, across 250G gives +-20fA.

You removed Q311 (metal case)?
If that changes the current I would guess there is a problem with the bootstrap common (amplifier output).
Could be the rails have a problem, e.g. bad electrolytic caps.

If the bias current adjustment does not change anything at extremes, then there is major fault.

Could be worth checking for readings of known voltages (low current limit) and currents at the input.
« Last Edit: May 25, 2022, 07:35:35 pm by MiDi »
 
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Offline Atomillo

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Yes, I had disconnected Q311. However I've realized that if I put the cover back up and let in turn on a couple of hours the current in all cases (BJT protection diodes or not) rises a bit (in the NO BJT case, up to 0.38pA).

I've rechecked the bootstrapped supplies but they're ok.

I also inject a voltage into the input and it is read correctly.

Moving the input current pot I can see the last digit in the 0.000pA range change consistently (by moving and releasing the screwdriver, the readings move a lot when the screwdriver is close to the trimmer).

Could you explain a bit the expected behaviour in the voltage mode? I've checked I various ranges and it is always at the 1.1something V that it stops rising.

Thanks for all the help so far!!

I've already learned a lot by reading the manual (even if I still have to digest the excellent explanation by TurboTom fully) and examining the construction of the meter. If I could get it working properly it would be the cherry in top.
 

Offline MiDi

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When touching the trimmer with screwdriver it is expected that the reading changes, keep in mind we are at fA level, with a non metallic one (like for oscope probes) it should be less touchy.
The adjustment is an iterative process, cover on or off makes some difference due to changing temperatures, it takes some time for euqilibrium.

In voltage mode with open input there is the effective input capacitance (20pF/<2pF guarded) which gets charged by input bias current.
The voltage settles to bias current times input resistance, with DS values: <5fA x >200TΩ = 1V.
So 1.1V in your case seems ok (have to reject my former statement about that).

But if the bias current in amp mode is correct, than this would give: 1V / 400fA = 2.5TΩ, which is way too low.

Did you perform offset adjustment already?
This has to be done first, second is bias current adjustment.

After that a check of offset in lowest volt mode (Zero check) should give max +-2 least signifcant digits (+-20µV).
 
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Offline Atomillo

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I also tried the oscilloscope thingie (photo attached) but there wasn't any noticeable difference (the numbers still jumped a lot and in order to see anything I had to do the iterative process you describe). Might be it is because the tip still is metallic?

The first thing I did was the offset procedure but looking at the amp ranges. Zero checking the lowest voltage ranges reveals an offset of aprox. 0.2mV however! Correcting that, in the highest voltage range now it reaches 1.89V (and stays very stable there). I understand this would be an imput impedance of about 378T ohms.

One strange thing I've noticed: with the case installed and the BJT removed just after a cold start it reads 0.080pA, but as it warms up this increases. After about one hour it reaches 0.27pA and after an hour and a half it reaches 0.321pA. Opening the case, I can feel the area with all the voltage regulators is warm (NOT hot or burning just warmer than ambient temperature) so I suspect this is the temperature coefficient of the input stage but should it really be this severe?
 

Offline MiDi

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Regarding offset check: Layout flaw
But that should not be an issue for the current range and the bias current observed.

That gets dirty now, you can check bias current with disconnected 250GΩ (bias current compensation).
If that does not do anything than input JFET pair is suspected.
 
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Offline Kleinstein

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The leakage current of semiconductors is very temperature sensitive. A usual number is doubling every 10 K of temperature rise. The main part in question for the leakage is the input JFET.

It really makes sense to keep the input stage part cool or at least not much warmer than needed. A temperature some 10 K above the ambient also has some positive effect: it reduces the relative humidity to about half and this way the chance for surface leakage that can happen above some 50% RH.

In voltage mode the input JFETs may also have quite some drift - how much depends on the luck with matching and some 10 µV/K range would not be so unusual.

Another point to look for is keeping RF sources (e.g. phones) away from the meter. It was designed before mobile phones became popular and has not very much EMI protection.

Even an isolated adjustment tool can effect the readings in the very low current range: the surface can be charged and this can be even worse than a metallic tool. The somewhat slow adjustment procedure should be the least problem. Even with the adjustment not very good, the input current should be stable.

For the protecton BJTs the leakage should not be super critical here, as the diodes see very little voltage. The parasitic capacitance of the BJTs may also help to suppress EMI. So they may also help with the performance and not just add leakage or provide ESD protection.
 
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Offline Atomillo

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I've got bad news I'm afraid...

I disconnected the bias adjustment giga ohm resistor and found no improvement whatsoever.
Getting desperate, I reviewed all the schematics one more time and found one relay that I didn't check: K311.

After disconnecting it, the current plummetted to around 0.012pA and the meter is extremely sensitive to me just moving in the room.
So know I have one of the dreaded leaky relays...

In the second page of this thread math_indy recommends COTO 7301-05-1000 as a substitute. From reading posterior replies, I get the impression that due to the lack of shield the transient response would suffer but I feel a lot more comfortable changing the relays than actually pulling apart the reed switch inside.

Oh, nevermind, Mouser has 0 in stock... Oh boy
 

Offline MiDi

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You could use K310 and loosing 2mA & 20mA range.
You get similar spare parts from Keithley (no direct drop in), look at the replacement overview I posted.
 

Offline HighVoltage

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This video helped me, when I repaired my 617


There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline Atomillo

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You could use K310 and loosing 2mA & 20mA range.
You get similar spare parts from Keithley (no direct drop in), look at the replacement overview I posted.

I'm inclined to lose the highest range in case of not finding a drop in replacement.

As shown in the first photo, the board already seems fragile in some spots and I would not like to stress it further disoldering and resoldering the pins of the HEX inverter. Searching for a Coto 1240-0197 I've found nothing on eBay nor other platforms (aside from very very suspicious clothing stores using photos from previous eBay listings that seem quite eager to get my credit card information).

After that I could just for the fun of it try and replace the reed tube by what was recommended by razberik: https://www.mouser.es/ProductDetail/MEDER-electronic-Standex/KSK-1A66-1015/?qs=KFo7JewZbUHhbHIVxk%2FaJw%3D%3D. Miracuriously in stock!! In case of fail, I just lose a range I would probably never use.

Also the relay suggested by branadic (with or without the homeamde shield) would help reach that all important 50 euro Mouser minimal order price...

Thanks to all the forum for this thread and to MiDi in particular for his great documentation and help. It is truly a goldmine of information and resources!!


 
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Offline Atomillo

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This video helped me, when I repaired my 617



Yes, my first idea was replacing with the COTO  7301-05-1000 and just taking the hit to settling time. Unfortunately it seems to be unavailable from all mayor distributors.

PS: How can I quote from different replies in the same answer? Sorry for double posting but I don't know how to quote without using the "Quote" button that selects all the message.
 


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