Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[MiDi]

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.

Just scroll down & hit quote on the post you want 

[Atomillo]

Some good and interesting news:

After replacing the leaky relay with the one from the miliamp ranges and recleaning carefully the board, after about 1 hour and a half the current settles down nicely at 32fA (0.032pA) in a very stably manner and can be offset corrected with no problems (I've not performed the current cancellation yet). This is without the BJT protection diodes.

Of course with no relay in the mA range selecting such a range shows only OL. I do not think I will ever use it but knowing it's not "feature complete" leaves me somewhat unhappy... As soon as I can I will order the reed switches posted before, which should be drop in replacement if I manage the extract the existing one without damaging it.

Also I think I've determined conclusively that the protection BJTs add some extra leakage. With them connected the input current rose to about 65fA. Not a huge difference but a noticeable one in this very sensitive meter.

Changing the relays has been a nerve wrecking experience. I don't know why, but solder almost doesn't "stick" to this board. Solder wick didn't work either so in the end I had to "pull" the relay out... The results are NOT pretty and my mental well being was seriously compromised  . Any tips as to why this might be would be very much appreciated since eventually I will have to put the relay back!!! Is this common on old boards like this? Specially the shield pin seemed like it would just not fit. Or I just doing something wrong? The "black" traces are cleaning residue that were properly taken care of after taking the photo.

EDIT: I forgot to mention I've done my first "low current" measurement! Just connecting a 1G resistor to a power supply was able to check the nano amp ranges and the first of the pA ranges. It worked without any issues!! Now I'm looking for a proper triaxial cable and after reading the forums it seems that in order to get a "safe" option paying 120 euros for a Pomona 4725 is the best idea.

[Atomillo]

Fiddling with the current adjustment can get you down to 0.004pA approximately but after that it gets really fiddly and unstable.
I've taken apart the leaky relay and attached in this post. What exactly is the broken brown epoxy and it's ends?

[MiDi]

Changing the relays has been a nerve wrecking experience. I don't know why, but solder almost doesn't "stick" to this board. Solder wick didn't work either so in the end I had to "pull" the relay out... The results are NOT pretty and my mental well being was seriously compromised  . Any tips as to why this might be would be very much appreciated since eventually I will have to put the relay back!!! Is this common on old boards like this? Specially the shield pin seemed like it would just not fit. Or I just doing something wrong? The "black" traces are cleaning residue that were properly taken care of after taking the photo.

EDIT: I forgot to mention I've done my first "low current" measurement! Just connecting a 1G resistor to a power supply was able to check the nano amp ranges and the first of the pA ranges. It worked without any issues!! Now I'm looking for a proper triaxial cable and after reading the forums it seems that in order to get a "safe" option paying 120 euros for a Pomona 4725 is the best idea.

Ok, so I had same issues with my board, thought it was due to the incontinent cap and the electrolyte residues all over the board.
I used good portion of flux and first put good amount of fresh solder on the joints, that helped to clean the board.
The PCB quality is not at a level I know from other instruments of that era, it is quite sensitive to heat.
Which revision of EM board do you have?

The Pomona are not suitable for sensitive measurements as they are not low (tribuelectric) noise.
I recommend to do the 3 lug mod or just file one pin off the 2 lug.
You can build your own triax cables as I described or just get a good deal on used Keitleys or take new Keysight with alligator clips.

Fiddling with the current adjustment can get you down to 0.004pA approximately but after that it gets really fiddly and unstable.
I've taken apart the leaky relay and attached in this post. What exactly is the broken brown epoxy and it's ends?

You could measure the resistance of stripped relay, if you are lucky its the sleeve that is responsible for leakage.
If it is still bad, a thorough cleaning and baking could heal it, but chances are quite low.

There could be another leaky relay, which was masked by the really bad one.
You should check for noise, manual states 1.5fA is normal.

For brown expoxy have no clue, it could just be to hold the relay in place, but I guess it serves a bit more than that.

[Atomillo]

Ok, so I had same issues with my board, thought it was due to the incontinent cap and the electrolyte residues all over the board.
I used good portion of flux and first put good amount of fresh solder on the joints, that helped to clean the board.
The PCB quality is not at a level I know from other instruments of that era, it is quite sensitive to heat.
Which revision of EM board do you have?

I've not yet checked the exact revision but it taking into account it has red relays and protection diodes it must be relatively new. I used flux too as well as adding small drops of solder but sometimes the new solder would have a hard time "mixing" itself with the old one. The result was a joint that had a clear boundary between the new and old solder. I used a temperature of 325C for all of this and I'm really scared about going higher since as you said it seems quite sensitive.

The Pomona are not suitable for sensitive measurements as they are not low (tribuelectric) noise.
I recommend to do the 3 lug mod or just file one pin off the 2 lug.
You can build your own triax cables as I described or just get a good deal on used Keitleys or take new Keysight with alligator clips.

I thought that the only difference between low noise and "normal" cables was that the measurement had to be made in stationary conditions so that the charge generated by triboelectric effect when flexing the cable would not disturb the measurement. I did not even consider low noise cable because they seem to be a lot more expensive.

I do not really feel capable of building my own triaxial cable since I'm quite clunky and the possibility of damaging such an expensive connector is not an alluring one. Filling of one pin on the other hand seems quite easy but while more common in modern equipment 3 lug - alligator cable don't seem to be cheaper than 2 lug, is this incorrect?

You could measure the resistance of stripped relay, if you are lucky its the sleeve that is responsible for leakage.
If it is still bad, a thorough cleaning and baking could heal it, but chances are quite low.

There could be another leaky relay, which was masked by the really bad one.
You should check for noise, manual states 1.5fA is normal.

For brown expoxy have no clue, it could just be to hold the relay in place, but I guess it serves a bit more than that.

I had planned to connect just the reed switch to the electrometer with/without the sleeve after cleaning them both. As you say, I don't expect this to make any improvement but it will be educational for sure.
Btw, after another couple of hours turned on and further attemps at current cancellation: about 3fA. Seeing the "attoamp digit" (and sometimes the first fA digi too) jump like crazy just by moving slowly in the same room is quite the experience.

EDIT: Using quotes sure makes the messages much more readable! Wish I had learned this sooner. Oh well..

[HighVoltage]

The problem is, to fix an Electrometer, you need another Electrometer. 

I still had a very old Keithley 614 and fixed my first 617 with it and found the bad relay.

[Atomillo]

Well, this is the end of the relay adventure...

I tried to measure the input current of only the reed switch and of the reed switch with the protection jacket. Both were fine. Well, this is interesting, I thought...

Then I tried to reinsert the reed with the jacket inside the relay. It proved to be impossible. Because of the indentation in the brown epoxy, the jacket would never cover the entire reed, which I suppose was the cause of the original high leakage. So I tried to carefully break off the brown epoxy. But by doing so, I destroyed the reed switch  .

Now, the first thing I will try is to replace the reed switch with this: https://www.mouser.es/ProductDetail/MEDER-electronic-Standex/KSK-1A66-1015/?qs=KFo7JewZbUHhbHIVxk%2FaJw%3D%3D as suggested by razberik in page 2 of the thread. Just in case it doesn't work (for example, the coil can not close the switch) I want a second option relay, and it really seems like the only thing available is the Standex Meder HI series. Problem is, the HI05-1A66 (5V relay coil voltage) is sold out, and only the HI12-1A85 is available.

While this one has a higher isolation impedance, the coil voltage is 12V. Thus it would seem to not be appropiate. Problem is no matter how much I try, the other suggestions (for example, COTO 7301-05-1000 and everything else listed in the relay replacement guide by MiDi) are not in stock anywhere. Truth be told, after a few hours of searching in Mouser and seeing "out of stock", I'm getting a bit hopeless.

Any help would very much be appreciated. At worse, I can always live without the mA ranges but it's not an entirely satisfactory solution.

[MiDi]

Have the Meder 05 laying around, PM me if you want it.

[Atomillo]

Sent!!

[Kleinstein]

For the read contacts the distance between the coil and the glass does not matter much. The imporant thing is the number of turns and more distance to the glass only makes the turns longer and thus more resistance or a thicker wire needed.

As the coil is still there, one could in theory look for a new contact. Some types are avialable as separate contacts for a relatively low price.
This includes the 1A66 contacts from Meder/Standex (though with not so great leakage specs for the single contracts. Actual performance could still be OK.
TME.eu also has a small coto contact (RI80) with 1 Tohms isolation specs. Reusing the coil one would still have the shield. One would "only" need a kind of low leakage glue.

The Coto 9000 series would be an option too - though a different form factor, but also available with shield.

[Atomillo]

For the read contacts the distance between the coil and the glass does not matter much. The imporant thing is the number of turns and more distance to the glass only makes the turns longer and thus more resistance or a thicker wire needed.

As the coil is still there, one could in theory look for a new contact. Some types are avialable as separate contacts for a relatively low price.
This includes the 1A66 contacts from Meder/Standex (though with not so great leakage specs for the single contracts. Actual performance could still be OK.
TME.eu also has a small coto contact (RI80) with 1 Tohms isolation specs. Reusing the coil one would still have the shield. One would "only" need a kind of low leakage glue.

The Coto 9000 series would be an option too - though a different form factor, but also available with shield.

I was worried that the AT of the turns would not be enought to actuate the switch itselft since I've found no info regarding the relay coil itself.

In the 9000 series I could not find a 5V coil SPST-NO with shield in Mouser, but the reason I didn't consider them in the first place was that the datasheet didn't booast about high insulation and thus I assumed that they would not be suitable (I can live without perfect transient response no problems whatsoever).

[Kleinstein]

I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.

The relay coil looks rather large from the outside diameter, but hard to tell how much of this is isolation / shield.
Separate reed contacts are not very expensive (the coto RI80 is a kind of exception) so a test both for leakage and maybe in the coil would not hurt that much. 

[Hydron]

I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.

I have a new COTO 9002 here that I need to do a leakage check on anyway - would it be useful to bump it up my to-do queue? Would be using a Keithley 237 so I can go up to 1100V bias if needed.

[Atomillo]

I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.

The relay coil looks rather large from the outside diameter, but hard to tell how much of this is isolation / shield.
Separate reed contacts are not very expensive (the coto RI80 is a kind of exception) so a test both for leakage and maybe in the coil would not hurt that much.

Yes, the reed contacts are quite inexpensive. The problem is that I must accumulate something to buy until I get to 50 euro (in the case of Mouser) in order to get the free shipment and thus justify the purchase. I'm trying to plan for some future projects in order to buy something but I've still quite a far ways to go...

Originally the idea was to buy the Pomona cable and along with that anything else. But now I'm not sure about it either! (Also somewhat infuritiangly, Mouser has the cables but not the shielded relay, and TME has the shielded relay in stock but not any cable)

[Atomillo]

I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.

I have a new COTO 9002 here that I need to do a leakage check on anyway - would it be useful to bump it up my to-do queue? Would be using a Keithley 237 so I can go up to 1100V bias if needed.

That would be quite helpful not only for the K617 but because I've learned that shielded low leakage relays are a rarity and have quite a few uses.
It would be desirable to have a modern stocked option available!

[Hydron]

So I went and checked the COTO 9002 I have, at 100V (same as datasheet resistance spec condition):
Either contact to any other pin (other contact, coil or shield) - all measured <100fA @ 100V
Coil to shield - <200fA @ 100V (pic below is of this measurement and shows 150fA, but the offset drifted a few 10s of fA when i checked before and after, so I'd say <200 to be safe)

Pics show the setup inside and outside the shielded test box. The connection to the SMU (a 236 upgraded to 237) is via a DIY breakout box I made up on the right- this is shielded, connected via triax and maintains the guard all the way to the jacks so should not affect the result (and I checked the zero offset current without the DUT before and after the tests to make sure - i did use the suppression function to null out the initial offset but it was only 2 counts, i.e. -20fA). Note that the triax connection from the breakout box to the test box is in parallel with the jacks on the breakout box - I'll try and write up what I built at some point.

All the measurements suggest an insulation resistance WAY higher than datasheet spec - on the order of a peta-ohm!

[Atomillo]

So I went and checked the COTO 9002 I have, at 100V (same as datasheet resistance spec condition):
Either contact to any other pin (other contact, coil or shield) - all measured <100fA @ 100V
Coil to shield - <200fA @ 100V (pic below is of this measurement and shows 150fA, but the offset drifted a few 10s of fA when i checked before and after, so I'd say <200 to be safe)

Pics show the setup inside and outside the shielded test box. The connection to the SMU (a 236 upgraded to 237) is via a DIY breakout box I made up on the right- this is shielded, connected via triax and maintains the guard all the way to the jacks so should not affect the result (and I checked the zero offset current without the DUT before and after the tests to make sure - i did use the suppression function to null out the initial offset but it was only 2 counts, i.e. -20fA). Note that the triax connection from the breakout box to the test box is in parallel with the jacks on the breakout box - I'll try and write up what I built at some point.

All the measurements suggest an insulation resistance WAY higher than datasheet spec - on the order of a peta-ohm!

This is great news not only for the K617 but for any applications that require switching low current levels.

 In page 3 razberik showed very clearly the importance of having a shield in regards to transitory response but at the time no suitable candidate was available.

Many thanks for your measurements and for sharing them with us!!

[r6502]

Hello all,

I also looked the last weeks again for relays, that could be used as replacement for thr high isolated relays fron the K617 and also for the K263.

in the K617  there are 6 of the high isolated relays and in the K263 there are 7. So the price is really fine for this relays. As all original relays I removed from the K617 have verry different leakage, I exchange them all. The 9002-05-00 is the chepest one, and there are about 700 on stock for 6.22€ + local tax(@ Mouser), so I will get them without paying the shipment, even when I order only for the K617.

When I get those, I will also check the leakage for all items with my old K616 - luky to have one in working condition. Has somebody measured the leakage with more the one relay?

The 9002 is also available with an aditional magnetic shield - does this have an advantage in this aplication?

Guido

[Hydron]

While the results I got were rather better than expected, I'd still caution that the SIP style of relay may be a lot less resistant to environmental contamination than the existing style that is in the K617. With the leads so close together (0.1" pitch) any dirt, flux etc is a lot more likely to cause a problem than on the larger original style of relay. That said, they may be more stable in other ways, given that they are fully encapsulated rather than built in the open style of the originals. Also my sample size was one, fingers crossed I had a typical rather than an extra-good one!

As for the magnetic shield, I _think_ this is mainly for applications where a number of relays need to be placed very close to each other - probably not a concern in this use. It also might have the effect of reducing the current required for operation (as it provides a lower reluctance magnetic path).

I'm making the assumption that the co-axial shield has good coverage of the reed (separating it from the coil) - when measuring coil-contact capacitance grounding the shield certainly reduced the numbers I was seeing. Does anyone have an x-ray machine they can check one in?

I look forward to seeing the results using this in a K617/263 - the application I was going to use it for is a bit more forgiving! Some tests to prove that the shield is doing the job would be great too.

[Atomillo]

Hello:

I've got good news. While the Meder 05 from MiDi arrived and I avoided the things I should be studying for finals, I decided to order some of the mentioned relays from TME (I have future plans for them not involting the K617).

When those arrived, I decided to test the COTO 9001-05-00 (this one without a shield but having the correct coil resistance) in the board. It worked without any problem!! The instrument can be zeroe'd no problem, with an stability of about 2 or 4 fA over 1 hour and a half. And as you can see the mA range works also no problem. The reason why the soldering in the teflon post looks so bad is that I've never used flux when soldering and desoldering in fear of contaminaiting the teflon somehow.

There remain however two problems. On the one hand, the protection diodes are way too leaky. Without them, the reading goes down to about 7 fA in 1 or 2 minutes. But with the diodes the current remains at 0.15pA even after about 2 or 3 hours. And secondly, the transformer mounting is broken and somehow decided to mount it with hotglue! I don't really know what to do about it, aside from not touching it.

Thanks again to everyone for your help!!

[Kleinstein]

With the meter partially working, one could test a few low leakage "Diodes". Besides expensive diodes tested for pA range leakage the candidates are:
1) small BJTs like 2N3904 , BC548 using the BE or BC junction
2) small JFETs (e.g. 2N4117/8 ,  could also be SMD version)
3) low leakage diodes. A can't remember a THT version, but just a common SMD one: BAV199:  usually very low leakage, but not tested for low leakage.

The protection diodes are used with low voltage ( 10 mV range)  across, while the usual leakage specs are for a significant voltage (like 20 V or even 100 V). So no need test at such a high votlage, more like some 10 to 100 mV and even than the leakage will likely go down from this.

[Atomillo]

With the meter partially working, one could test a few low leakage "Diodes". Besides expensive diodes tested for pA range leakage the candidates are:
1) small BJTs like 2N3904 , BC548 using the BE or BC junction
2) small JFETs (e.g. 2N4117/8 ,  could also be SMD version)
3) low leakage diodes. A can't remember a THT version, but just a common SMD one: BAV199:  usually very low leakage, but not tested for low leakage.

The protection diodes are used with low voltage ( 10 mV range)  across, while the usual leakage specs are for a significant voltage (like 20 V or even 100 V). So no need test at such a high votlage, more like some 10 to 100 mV and even than the leakage will likely go down from this.

Yes, I plan to replicate some of the excellent measurements of 2x1 of the X Chapters. From that I will select some parts.

[Atomillo]

Well, the Pomona 4725 arrived today and I'm somewhat confused.
Connecting the cable and leaving all the ends in open circuit (of course in a fixed position and allowing lots of time to settle) I get readings of about 1 to 3 nA!

Of course something must no be right. So I clean the output connector of the electrometer as well as the input of the cable. Nothing changes. Then I notice that something in the black isolator where the three cocodrile cables exit seems to be "off". I attach images. It seems as if the isolation between the negative terminals has broken somewhat.

So after cleaning that, the readings go lower, down to 0.150nA / 0.050nA, but are very unstable and constantly jump around. This isn't normal right?

[r6502]

I have a cable from Keithley, and I'm using it always in a shielded case. case is than connected to the outer shielding.

On the Keithley cable with alligator clamps, the shielding of the central wire goes very close to the alligator clip. Is the Pomona cable also shielded till the front?  If not, I can imagine that you will pick up some emissions from the surrounding?!

Guido

[Atomillo]

I was just about to post!!

Shielding is crucial! I drilled two holes in a cookie can and made a connection to the case.

The sheilding DOESNT go all the way to the alligator clips!! It ends at the black stop. This is very noticeable: if you dont insert the entire section of the three wires (even if you just let out 1cm) into the shielded box the electrometer will overflow on it's pA ranges. This is actually better seen by connect an osciloscope to the Preamp Out.

Also noticeable by the reading of the leakage current is any wire touching the metal walls, so another sign of no shielding.

The lowest value I've been able to get is about 87fA, but this is achieved very consistently. Any ideas from where the leakage could come from would very much be appreciated.