DIY resistor standard.

[chriswood1900]

I am in the process of creating some test gear for calibration purposes, as a starting point I thought I would try a prototype 10K resistor standard and I would like others to critique it so I can refine my methods before making more of different values.
I have used a standard small RS aluminium box, some CPC gold plated on brass terminals. The 2 bus wires are conductors from 2.5mm twin and earth cable stripped and cleaned. The 2 resistors are Vishay PTF surplus 5K 0.005% 5ppm in series soldered with 60/40 leaded solder.
• So how can I do it better for a reasonable budget?
• Does anyone know a source of info on the best way to create accurate resistor/capacitor/inductor standards?
• The use values shown on the label are from the Vishay datasheet when the resistors are used as standardised resistors.
• How sould I age the resistors?
• The total resistance shown on an uncalibrated 4.5 digit meter is 10,000K @20°C
I look forward to your comments.
Chris

[Herald1360]

Should that be 10,000R? Or 10.000K?

Or is the meter a continental type?

[mhennessy]

Very nice

As you may already know, those two 0.005% resistors should give an even better tolerance in combination. If you had many more, you could really go to town with them! E.g., five in parallel would give 1k with 0.002%, assuming a Guassian distribution. It's a useful technique to get good performance from reasonably-priced parts.

The Vishay PTF series are quite reasonably priced, even from the likes of RS. Certainly a lot cheaper than the usual "bulk metal foil" precision types. Looking more closely, the best they do (in tolerance terms) appears to be "only" 0.01%, so I'm wondering if 0.005% is a typo? Or perhaps from a discontinued range?

In terms of ageing, I don't know what to expect from resistors. I do know that they can suffer from thermal hysteresis, which can make a mockery of attempts to carefully select them before assembly. So if you're not heating up the resistors - which you won't be, as DMMs only put out tiny amounts of energy when measuring resistance - I wouldn't expect to see much change over time.

Watch out for thermocouple effects...

This can all be a bit of a rabbit hole, BTW. Have you encountered "Volt nuts" and "Time nuts"? I've tried to avoid falling into the trap, but it's easy to lose your footing and slip. There's quite a lot on the EEVBlog forum about this sort of stuff - it's really quite intimidating to see the gear some of these guys have in their workshops!

[chriswood1900]

Oops 10.000k, funnily enough I have been typing up documents for Germany and using the , as a decimal point.
Thanks
Chris

[The Philpott]

Assuming they are for their own use, have you considered putting multiple values in the same enclosure? 0.005% is pretty tight, how much better did you want to get? I was given some 0.001% resistors to complete a project, which i considered to be weapons grade precision!

The resistors aren't under under any mechanical stress in your unit; looks good to me.

I am not convinced you need to age the resistors. My voltage reference chip was aged before i bought it, but i would have thought your resistors will be right on the bubble from day 1 in a range of 10-25 Centigrade. I guess it all comes back to what you want the standard for and how accurate you need it to be.

There is the long term possibility of zinc pest with die-cast boxes of this type, but one hopes modern alloys have tied down problems such as corrosion from trace lead content.

[chriswood1900]

Hi Mark
Thanks for the advice, yes I have encountered Volt Nuts etc but it tends to leave me wondering if I should be gold plating everything. As regards the resistors they were old stock, on the packet indicated the value and tolerance which is what I'm relying on. I was aware that putting together multiples to achieve better tolerances but I was after 10k and had reasonable 2 x 5K resistors so it seemed the obvious choice.
The things I was wondering were around actual construction and type of solder I used. Based on this first one I was also going to try some other values like 1K, 100K and 1M.
Chris

[The Philpott]

60/40 solder, no problem.

I suppose one could argue that the resistors are hanging in the air, and could be sitting flat on a pierced non-conductive board instead, but this is nit-picking really.. the assembly (presumably) isn't going up in the space shuttle.

The precautions i would use on these high accuracy resistors is use snipe pliers across the wires to suck heat away from the joint which is being soldered, and don't introduce a kink in the wire too close to the resistor.

[chriswood1900]

No its definitely not destined for the space shuttle, I did use my old Radio Shack heat shunt for soldering and I tried not to bend it near the body and avoided sharp bends anywhere.
Chris

[chriswood1900]

I have made up some more resistance standards as per my earlier design, if anyone else wants to copy them I include a picture of the stages and the parts used.
The finished range of values I intend are 100Ω, 1KΩ, 10KΩ, 100KΩand 1MΩ. To construct them I used -the RS Ali box (517-3377) Terminals CPC (CN00622) and RS (645-7834) and the labels were made using laser printable plastic film from Greenstiks .
The two bus-bars were made from freshly stripped and cleaned 2.5mm coper from mains cable and the resistors were sought out over a period of time from Ebay, radio shows and RS. You could choose the resistors according to your needs ie, selected 1%, or 0.1%, I used mainly 0.01% Vishay foil and a Caddock 1mΩ.
I designed the label layout on paper in MS Word against the box size and moved things until they were right and printed the labels onto a single sheet of film. Then I marked out a box using graph paper and transferred the dimensions onto the box and centre punched the hoes drilling them with a battery drill starting with 2.5mm and finishing with a 12mm step drill. Next clean the box in clean soapy water to remove residue cut and applied the labels. Cut out the holes in the labels with a scalpel and fitted the terminals. Soldered the two bus-bars between them lengthways and also tinned the area where I was going to solder the resistor. When soldering the resistors I did one leg at a time protecting the resistor with a heat shunt and let it cool before doing the other leg.
Initial results with a 4.5 digit DMM look good and I will also measure with a 6.5 digit once I get access to a calibrated one.

My next project will be a Hamon divider.

Links.
https://uk.rs-online.com
http://cpc.farnell.com/unbranded/st-...als/dp/CN00622
https://www.gmcrafts.co.uk/product/m...-plastic-film/