Keithley 177 4-wire mod - Page 1
Forum section: electronics / testgear

author: Roo
author_rank: Contributor
timestamp: July 12, 2017, 10:35:09 am
content: I recently found a Keithley 177 in a sorry state, but following Dave's video on the unit I followed his advice and cleaned the switches. I now have a pretty nice little meter, which is pretty accurate and precise for a fraction of the cost of other 4-1/2 digit meters. (£14.95 !!)The only feature I would like on it would be 4-wire resistance mode. As the meter will read down to mOhms it really deserves this extra boost of accuracy gained from 4-wire connection.Initially all I was thinking was to add a separate internal constant current source and then use it on Volts to get the readings. I then realised there is a perfectly good constant current source inside already, so why not re-wire this (via some switches - plus the addition of a couple of source connectors) to provide true 4-wire connection.To be honest I am slightly struggling with working out where the low-side connections need to be made. Obviously the current must only flow through the test resistor, and not through the sense leads, otherwise there is not point.Would anyone have any suggestions on this?You can find schematics in a post below.Thanks, from a new EEVblogger in the UK :-)

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 11:16:50 am
content: I do not have the board layout in front of me, but I would expect there to already be separate traces for the current source and sense that are connected near the input terminals. So it might just be a case of cutting those traces and leading them to separate jacks. Again, I do not have the circuit layout in front of me, so I could be wrong.If you want people to help you, then it pays to make life for them as easy as possible. For example, link to any relevant information instead of telling them to go find it themselves.You may end up with volts being on the sense jack instead of the more common combination of force and volts jacks, you might want some way of switching back to 2-wire for convenience, and you may need to provide separate input protection for the newly created jacks. Generally input protection for the terminals only used for resistance measurement might be a bit lower (for example only up to 300 V, while the volts jacks are protected up to 1000 V).

author: Roo
author_rank: Contributor
timestamp: July 12, 2017, 12:10:30 pm
content: I have attached the best schematics I have. One is the official Keithley one from their web-site 177.png (not clear) the other is from another contributor, however it is only a partial diagram 177 2.png.I have only included the part 177,png that shows the current source and terminals etc. If anyone needs more, such as the PSU, then let me knowIn the manual it describes the current source as "floating", although I don't see that really in the schematic.To answer your question about switching. Yes, my intention is to have a switch that can be set for 1) 2-wire 2) 4-wire constant 3) 4-wire pulsed (momentary) to avoid thermal effects. This last one may not be necessary bearing in mind the sensitivity of the 177. I need to walk before I run :-)Sorry for being lazy with the schematics alm.

author: Roo
author_rank: Contributor
timestamp: July 12, 2017, 03:18:15 pm
content: Schematics (failed first time)

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 13, 2017, 10:42:29 pm
content: Ad usum:http://www.tamoggemon.com/test/2017/K177.zipWant any more pages? Chant a devils song, ideally to tamhan at tamoggemon roundthing come.

author: Roo
author_rank: Contributor
timestamp: July 14, 2017, 10:17:26 pm
content: Great, thanks Tamhan. A lot clearer.I just need to work out where the ground side should run to.The high side for the force I think should run from the ohm switch terminal 3. (disconnected from the terminal of course). The low side of the force is what I am unsure about....Anyone care to have a look and see what they think?? Will I need to sever all the ground cons from the current source and run them to the low force connector? (which I will add to the front panel, along with the high force connector).

author: Vtile
author_rank: Super Contributor
timestamp: July 15, 2017, 11:26:56 pm
content: I assume you can analyse the current, voltage and resistances from picture below. Just checking.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 16, 2017, 01:22:20 pm
content: Hello,sorry for my slow response. I am a bit caught up in other files, but am watching this thread...

author: Roo
author_rank: Contributor
timestamp: July 18, 2017, 03:59:22 pm
content: Thanks for giving me the impetus to exercise my rusty ohm's law for parallel/series circuits.Having done some calculations I can now appreciate the benefits of the Kelvin/4-wire connection method. So long as the internal impedance of the volt meter is high then the current, and therefore the volt drop, in the sense wires is negligible. The Keithley 177 has a 10M input impedance therefore the total loss in the cabling will in practical terms be around the same as the accuracy of the voltmeter for resistances in the 0.005 - 0.1 Ohm ranges. Anything higher and they can be considered irrelevant.I am about to make some tests on the 177 itself and see if my calculations match reality.I will report back with my findings.Roo

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 10:26:05 am
content: Last night I did some work and got the 4-wire mod working. It seems to work well and all the advantages are there. Just need to add the new 4mm shrouded sockets and the front panel switch now. Just waiting for them to be delivered.I have made videos of the mod and will be posting this on my Youtube channel once it is complete. In the mean time, here are the schematics of the mod.Any comments or advice will be gratefully received.Now off to design and make some Kelvin-clips!

author: MosherIV
author_rank: Super Contributor
timestamp: July 19, 2017, 11:54:28 am
content: Nice mod.I got some cheap Kelvin clamps from Amazon. The seller I used no longer has them but other do.Any cheap ones like the ones I got will work.Here my thread on how I made mine :https://www.eevblog.com/forum/projects/diy-4wire-probes/msg1245690/#msg1245690

author: alm
author_rank: Super Contributor
timestamp: July 19, 2017, 12:17:06 pm
content: Note that Kelvin clips are mostly a convenience thing (granted, of you want to measure an 0603 resistor, they become pretty much essential). It works just as well to use four separate wires and connect the sense wires at the points you want to measure resistance between and the force wires further away from the device under test.Quote from: Roo on July 19, 2017, 10:26:05 amDid I see it correctly that your sense and force low terminals are basically shorted? I can not see from the schematic if there are easy traces to cut to isolate the ground connection of the Ohms current source and connect it to a separate jack (or to ground via the 2w/4w switch). That would require study of the PCB layout together with the schematic. You can test if it is effective to put small value resistors, say 1 Ohm, in series with all four wires. In a proper four fire measurement, the effect of these resistors should be negligible.

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 01:15:33 pm
content: Interestingly I did ask that question and got really no reply.I think the current source really should float entirely from the sense lo. As you say, the ground points of the source should be routed directly to that point, however I am unsure how they can share the same PSU in that case. Maybe I will also require another +-15V psu just for the current source.....or a battery?

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 01:20:23 pm
content: PS - Good tip with the 1R resistors. I will try that.

author: alm
author_rank: Super Contributor
timestamp: July 19, 2017, 01:48:27 pm
content: Quote from: Roo on July 19, 2017, 01:15:33 pmIt is hard to tell without knowing the PCB layout. For example, if there is a solid ground plane, then isolation becomes tricky. I do not think that the power supply is an issue: between sense low and force low should be < 1 V, so should be well within the common mode range of U107. I think you would have to isolate all the ground nodes around the Ohms current source. How easy this is depends on the PCB layout. It would definitely require testing.

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 01:54:28 pm
content: OK - Thanks for that alm. I will have a sniff around the PCB tonight. Unfortunately there is no silk-screening on it so you have to work out which component is which one by one.I don't really understand how the constant current circuit works, nor the purpose of the reference.Surely if I connected all the grounds together and took them to the source lo, there would the be no 0v connection to the PSU. Or am I being thick?? (likely)

author: alm
author_rank: Super Contributor
timestamp: July 19, 2017, 02:34:13 pm
content: Looking at the block diagram I see some connections not visible on the schematic. The current source is coupled to both the reference used for the ADC and the chopper amplifier. I am not sure if there is an easy way to disconnect the current source from the common circuit ground. I think you might have to design a different current source for this to fully work.

author: MosherIV
author_rank: Super Contributor
timestamp: July 19, 2017, 02:37:37 pm
content: QuoteYou have a preceice measurement instrument here. I would not mess with the grounding, they may have been seperated for a reason - ie to keep noise low in the analogue low voltage section.I suggest you try out the 1 ohm resistor and see how accurate it works.

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 02:38:21 pm
content: I may revert back to my original plan and that is to add my own constant current source, from a battery....like Garry Lacomte's design (below).It may be a lot easier than chopping the 177 into bits!

author: Roo
author_rank: Contributor
timestamp: July 19, 2017, 02:41:00 pm
content: I am inclined to agree MosherIV. I might test with the resistors first. If it is accurate (enough) I will call it a day. If not I might try the battery powered CCS.

author: Roo
author_rank: Contributor
timestamp: July 24, 2017, 11:24:15 am
content: After much poking and prodding I decided the 4-wire mod was not really feasible without major re-wiring and I think that would compromise the normal operation of an otherwise good meter. The only thing I have done is to update the terminals to the shrouded type. This is for obvious safety reasons and so I can use all my existing connection leads and probes. photo's attached.Nice, small, simple meter for general bench use. And cheap!Now I am on the look-out for a 4-wire meter.....deeper pockets required!Thanks for all the advice.Roo

author: MosherIV
author_rank: Super Contributor
timestamp: July 24, 2017, 03:16:44 pm
content: Nice jobKeep an eye out for TTI 1905a and Schlumberger 7150 (normal or plus modal) both come up frequently on ebay in UKI have both, got the 7150+ for £60HPs can be had but they go for more, everyone is looking for HP, Keithley and FlukeHappy hunting

author: macboy
author_rank: Super Contributor
timestamp: July 24, 2017, 03:22:41 pm
content: Quote from: MosherIV on July 24, 2017, 03:16:44 pmTTi 1905a is a nice 5.5d meter, but it is manual ranging and does not have 4 wire resistance.