EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: MathWizard on January 03, 2023, 01:03:39 pm
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1 of the input path relay coils burned out in my Keithley 197, which is a 5.5digit DMM. The relay was a custom Coto, 3500 Series Low Thermal EMF Reed Relay. And I broke the glass trying to de-pot it, so it's a lost cause.
https://cotorelay.com/wp-content/uploads/2014/09/3500_series_reed_relay_datasheet.pdf (https://cotorelay.com/wp-content/uploads/2014/09/3500_series_reed_relay_datasheet.pdf)
But it's not listed anywhere I've seen, it's a Coto 3500-0021, that used a few mA from a 5V driver, and had a coil resistance of ~2k or something (it might have worked out to 330 or 470 even). I don't mind if I have to fiddle with the driver logic.
I can't find a used 1 or NOS, and IDK what a suitable equivalent would be. I'm not worried about if the pin layout is exactly the same, or the coil current, I can deal with that.
I don't want to pay $50 for 1 relay either.
The relay coil is K101 or 2/3 on the bottom of page 66, the contact/switch parts are near the top of that page
https://www.utwente.nl/en/tnw/slt/documentation/Equipment/GeneralEquipment/keithley197.pdf (https://www.utwente.nl/en/tnw/slt/documentation/Equipment/GeneralEquipment/keithley197.pdf)
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If the depotting went OK and the broken glass is the only problem, one could consider using new contacts and if needed a DIY coil in the old case. Likely the low EMF relay is internally a series connection of 2 equal contacts. This helps with thermal EMF and also maximum voltage.
Single contacts a available from different manufacturers. Chances are one would want low leakage, high voltage (e.g. 200-300 V) ones.
The metal case acts as a shield and it also helps getting away with less current / fewer turns. How much less current is needed than the datasheet nominal amps-turns is hard to tell. It may need an experiment with a test coil to get the right diameter wire.
It still helps to keep the power low and thus use much of the winding space to keep the thermal EMF low.
With the 2 contracts in series much of the thermal EMF compensates and allows for an error quite a bit lower than a single contact.
A small glass diameter helps to have more space for copper windings. So thicker contacts are not a good idea.
If needed one could use temporary more current to engage and than less current to hold (e.g. a series resistor with parallel cap) to reduce the power loss in the relay.
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I haven't looked at it in ages, but I think I threw it out anyways. I've gone on digikey before and tried to narrow down the options, but IDK if it would be Hg wetted, or the capacitance, the static resistance, or the thermal drift, etc.
IDK how much any of that really matters, I just figured some one could suggest a generic relay, with low contact resistance, and be suitable for a DMM. It hasn't been calibrated in years, and has drifting resistors, so maybe any old relay would be fine anyways.
I seem to remember all the ones I was looking at, being like $50-100.
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The relay should be needed for the lower voltage and likely resistance modes to work. I don't need for low resistance, more like Hg wettet or Au contacts. The other point is a high voltage rating.
Low thermal EMF is kind of less important and only effecting the performace that is not that great for the meter anyway.
Using 2 voltage steps ( e.g. 12 V relay getting only 5 V to hold) could reduce the power and this way the thmeral EMF error.
Not many higher voltage relays are available and they are expensive and often without a shield.
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Ok so it's K102, and yeah on voltage mode, the contact resistance won't matter, but watch the voltage rating for when stuff is open.
I did find the relay, 1 of the reed's got pulled out of the glass tube, I didn't heat up the tar or cut enough free. IDK where the other one went yet. The glass seems solid besides the end where maybe a glue plug broke out.
The reed looks goldish on the arm part, but the tip, on both sides, looks more like graphite on paper colour, when it's orthogonal to me, and it looks more gray at a shallow angle. So does that sound like mercury wetted, it could be a heat treating too ?
But the coil is broke and would need some repair. IRC the other 2 relay's measure 470R, and all 3 work with that in LTspice. I probably don't have the mechanical skills or tools to put it back together and work good anyways.
So I'd get have to get 1 with shielding, 5 pins, I know the from factor, ~470R and 5V coil, a pretty high voltage rating, and probably Hg wetted. I'll have a look soon.
I looked around before trying to find what replacement people would use in some of the Fluke meters and stuff of that era, but I never found the right thread, or repair page, with google anyways. No one ever mentioned a part, like if it was some equivalent BJT or something.
I'm just using it for LV stuff anyways, and it's my 2nd best DMM. IDK if replacing some of the old carbon resistors in the signal path, will put it out of calibration. But I'm ordering some anyways, try it and see.
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I contacted Coto, the maker of the original, and here's what they said
Our 3500-0021 is a 1FA, 250V, 0.5A/2.5A, 20Watts, thermal EMF 10.0uV,
Since 2012 our 3500-0021 is not active.
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So they recommeded just some other 3500 series. And it will cost +$35US w/shipping.
To replace the relay for now, I tried a 10-12V relay and that was no good, so I'm about to wire in a switch, and on the coil connections, I'll put a resistor and LED in, to tell me to turn on the SW.
The LCD screen in this is not tightly held to the rubber contact thing. Even with some tape, sometimes I still have to squeeze it tighter to get all the digits to show. Maybe I show try drilling another screw hole, even if I have to drill right through the front panel, and put a nut on, and a little clamp for the LCD.