I think easy calibration is the way to go for the average bear. What I don't know is the magnitude of the error sources that "come out in the wash" when the Analogic unit is trimmed. That trimming might have compensated for switch resistance, leakage and who knows what else. The unit did step in 10 uV steps on the 10V range. IMO, one of the nice things about the way they did it was being able to go a bit over 1 or 10V; It's very common to need that feature. I like the idea of modules for each decade. They could have Samtec or similar pin headers so you'd plug them together, creating a block.
Doubt it makes much sense to go beyond 1 ppm in terms , as the resistor and reference requirements get way out of hand, not to mention thermal issues. Analogic did make one horrible mistake. They used quadrature switches on the front panel for each decade, that didn't hold up well. The old units can be really unpredictable when setting the voltage, though whatever they read out on the display is what you get. Sometimes you turn it up and it goes down, sometimes down and it goes up. Contact cleaner only helps for a year or two and then becomes ineffective.
In practice, any time you are chasing precision, it is always the detail that you have to be obsessive about. You have to consider all the factors you normally do not have to consider in other designs. If you are using analogue switches you have to look at the switch resistance, the switch-to-switch variations of resistance, the temperature coefficient of the switch resistance, the OFF leakage current through the switch, and the leakage current from the supply rails to both sides of the switch. The currents will probably rise exponentially with temperature, so you have to think of the maximum operating temperature.
Part of the genius of the Kelvin-Varley divider is that the whole divider can be build from the built from one single batch of resistors, all of the same value. If all the resistors have matching temperature coefficients, then errors due to ambient temperature changes cancel out. Hopefully, they will age the same too. In the summing method, you have a very wide range of summing resistors, all that have to stay accurate. Is the 1Mohm resistor temperature characteristics going to match the 1Kohm characteristics? Yes if you were Fluke of 50 years ago, and you make all the resistors from wire of the precisely same composition, and yes if you are HP or Fluke today, and you can make a laser trimmed thick film resistor network, where all the resistors are made from exactly the same deposited film. Probably not if you are a hobbyist getting whatever parts you can find.
I think your maths is a bit off with this one. Top marks for getting the values to work successfully as a tree with each stage having the same impedance as the stage below it. That bit is perfect. Very elegant.
No doubt that was clear as mud!
If the MSD op-amp has a 1K resistor to the summing amp, the others have 10K, 100K, 1M, 10M and 100M for the LSD.
I think easy calibration is the way to go for the average bear. What I don't know is the magnitude of the error sources that "come out in the wash" when the Analogic unit is trimmed. That trimming might have compensated for switch resistance, leakage and who knows what else. The unit did step in 10 uV steps on the 10V range. IMO, one of the nice things about the way they did it was being able to go a bit over 1 or 10V; It's very common to need that feature. I like the idea of modules for each decade.
I stumbled across these switches that look like they might be OK for a Kelvin-Varley divider. 2 pole 11 positions (which I prefer to 10 position).
If the MSD op-amp has a 1K resistor to the summing amp, the others have 10K, 100K, 1M, 10M and 100M for the LSD.
Liked it! Nice design, but isn't the 100M resistor a bit high? Isn't a little bit better to use a 0.1 hamon divider with it´s output a little higher (about 0,1%) followed by a 10M resistor?
I think easy calibration is the way to go for the average bear. What I don't know is the magnitude of the error sources that "come out in the wash" when the Analogic unit is trimmed. That trimming might have compensated for switch resistance, leakage and who knows what else. The unit did step in 10 uV steps on the 10V range. IMO, one of the nice things about the way they did it was being able to go a bit over 1 or 10V; It's very common to need that feature. I like the idea of modules for each decade.
Easy calibration is everything I hope for!
On going a bit over 10V, Richard's design can do that too. If you select the 10V and the 1V outputs, you get 11V. With 6 decades, you can go to 11.1111V.
Good ideas with the modularisation of the design. I'll keep it in mind.
I stumbled across these switches that look like they might be OK for a Kelvin-Varley divider. 2 pole 11 positions (which I prefer to 10 position).
Nice find! But why 11 positions are better? Don't you only need 10 positions for a KVD?
Thank you,
Felipe
This would be way cheaper "How to make custom resistors"
http://www.all-electric.com/schematic/res_trim.htm
This could be a cheap way to make a divider?
Using a file, nail polish and cheap carbon resistors oh and metal film aswell.
There is good tip for tuning filters on that page as well.
Carbon won't get you where you want to go.
Filing won't do anything about tempco or drift. I think there's a table in the Art of Electronics that indicates several percent change in value after soldering, load cycling and other kinds of stress. Don't remember if this was carbon film or carbon comp. The important spec for precision equipment is it's drift (short term, long term) and tempco, not so much the actual value. This is why precision components are sometimes aged by manufacturers. You can cal out any change in value (eg. put a label with 1.010 ohm on it), but you can't compensate for variations, apart from indicating that the instrument has +/-2% tolerance.
I think there might be a job for you in China somewhere where you can use some blue paint to turn carbon film resistors into metal film, although I doubt they actually bother tweaking them to 1%.
Filing removes the lacquer and might make them more susceptible to oxidation, but feel free to try it and find out. It will take a few years to get results about long term stability, though, and you need a stable reference (eg. DMM) that will drift little over that time span.
10mA through 1k is 0.1W dissipation, which for 0.25W resistors will likely result in some heating, since it's fairly close to the max. It would also drop 10V over the resistor, so it's likely a few orders of magnitude off for a buffered voltage source.
The precision wire wound resistors often used for these applications are often physically large compared to their power rating to reduce heating.
I think there might be a job for you in China somewhere where you can use some blue paint to turn carbon film resistors into metal film, although I doubt they actually bother tweaking them to 1%.
Step 2 is you adjust each decade, starting from the second most significant so that "10" is exactly equal to "1" on the range above.
If you are using electronic switches, then to match the ranges, you can switch between the "1" on one range and the 10 on the next lower range at rate of 133Hz (or any frequency that is not a mains harmonic). You will get a square wave out when they do not match, and when you adjust the summing resistor for a match, the amplitude of the squarewave will go to zero. If you built an sensitive AC amplifier, along with a tuned 133Hz bandpass filter to eliminate everything but the 133Hz Ac, and put your multimeter on the output, then you can match the decades with extreme precision without needing anything expensive or precise.
So calibration comes down to matching resistors within a decade, and zeroing an AC value. These are two steps that can be done simply and very accurately without expensive equipment.
Step 2 is you adjust each decade, starting from the second most significant so that "10" is exactly equal to "1" on the range above.
If you are using electronic switches, then to match the ranges, you can switch between the "1" on one range and the 10 on the next lower range at rate of 133Hz (or any frequency that is not a mains harmonic). You will get a square wave out when they do not match, and when you adjust the summing resistor for a match, the amplitude of the squarewave will go to zero. If you built an sensitive AC amplifier, along with a tuned 133Hz bandpass filter to eliminate everything but the 133Hz Ac, and put your multimeter on the output, then you can match the decades with extreme precision without needing anything expensive or precise.
So calibration comes down to matching resistors within a decade, and zeroing an AC value. These are two steps that can be done simply and very accurately without expensive equipment.
Rereading this, I just can't understand how to calibrate the decades. If you sense the output of the summing amplifier, you will get exaclty 0.2 x the AC voltage, not 0! If you remove the summing amplifier and sense the output of the pots, you will get 0 allways! Am I missing something?
One more thing, the first 2 decades need a transistor (something like Art of Electronics figure 4.21) in it's output to source enough current, as the opamps can't source the 10 mA (first decade) or 1 mA (second decade) needed. As the idea is doing as a module, it is needed in every decade.
And instead of using 2 4051, it may be better to use a single 4067. BTW, wht are you connecting the 1V and 0V taps to the X3, X6 and X7 of the second 4051?
Thank you,
Felipe Maimon
If you are going to all the trouble of making a KVD, it is worth trying to source some lower temp coefficient parts for the first decade at least. Perhaps the first two decades.
You really notice the drift of 25ppm parts with 10V or more applied. The trouble is if you are using a KVD and you see the voltage drifting a little, it is easy to loose confidence in the accuracy. If you see it sitting rock solid at the set voltage, then you have confidence. It is really worth investing that extra bit of effort and make something really good.
The negative is that if you buy a small number of 10ppm or better resistors, you definitely cannot select a matching set as well - you will need a trimmer pot on each one.
Now, back to your question on matching, 40ppm is 0.004%. You need one superb set for the first decade. The second can be a little worse. The third worse again. So it comes down to how many resistors do you need to get 11 resistors within 0.004%. The chances are good with 100 resistors. With 150, you probably can do it easily. With 50 0.1%, you may be able to find 11 within 0.01%.
I wouldn't be overly concerned about using all 10K resistors in the KVD which means you just buy lots of the one value. When the 720a was designed, they didn't have the pico-amp fet input opamps that are available now to make a output buffer amplifier. So to minimize the output resistance, they used lower values in the lower decades. They were often relying on very sensitive mechanical galvanometers that can't show lower currents then a few microamps.
That's a good idea. I'll just need 10K and 25K (24.3K + 1K pot) resistors.Quote from: amspireYou cannot put a 10Mohm multimeter across a KVD output - it draws too much current and will change the output voltage, so to use it, you either need a very high impedance input, or you use the KVD to compare with a second voltage, and you are nulling the current between the two.
I won't do that. The output will only go to my Fluke 8840A (>10Gohm) or a LT1050 or similar...
Quote from: amspireThe negative is that if you buy a small number of 10ppm or better resistors, you definitely cannot select a matching set as well - you will need a trimmer pot on each one.
Now, back to your question on matching, 40ppm is 0.004%. You need one superb set for the first decade. The second can be a little worse. The third worse again. So it comes down to how many resistors do you need to get 11 resistors within 0.004%. The chances are good with 100 resistors. With 150, you probably can do it easily. With 50 0.1%, you may be able to find 11 within 0.01%.
These numbers are for 1% resistors, right? I did check Dave's 1% resistor data (400 resistors) and it had 4 sets of 40 ppm. So I assumed (ASS-U-ME?) that 0.1% needed 10 times less resistors to get about the same numbers of sets.
Quote from: amspireI wouldn't be overly concerned about using all 10K resistors in the KVD which means you just buy lots of the one value. When the 720a was designed, they didn't have the pico-amp fet input opamps that are available now to make a output buffer amplifier. So to minimize the output resistance, they used lower values in the lower decades. They were often relying on very sensitive mechanical galvanometers that can't show lower currents then a few microamps.That's a good idea. I'll just need 10K and 25K (24.3K + 1K pot) resistors.
I was pondering the idea of constructing a KVD on the cheap a while ago. My line of thinking was, instead of purchasing expensive resistors, to buy a spool of low tcr wire like evanohm and wind it around a glass rod in order to obtain a whole decade. Then I could tap that at exactly determined intervals to obtain decade divisions.
The best way to tap leads seemed to be to weld them instead of soldering them. Perhaps by discharging a cap through the junction. But I have never tried this so I don't know if it would work in high precision resistors.
Another point I considered was the winding method. Normal helical wining would make the resistor inductive. There are low inoctance windings like bifilar and Ayrton - Perry windings but these are hard to tap precisely because the exact point may not be exposed. So my idea is to wind the wire around two glass rods simultaneously in a figure of eight fashion which would cancel out inductance and at the same time expose the wire enough for easy tapping.