Resistance source?

[Brutte]

Very interesting. 1,2,4 and there you stopped. Lets proceed: how do you plan to buy 2^19? That is 524288R. I am afraid power-of-two decade box cannot be made with 20 x E24 resistors. BTW, anyone wishes to count how many 1% E24 resistors are needed to match 2^19=524288R to within 1%??

Since we are dealing with software here, and can calibrate, there is a very easy way to make a "almost power of two" decade box.
(..)
1, 1.8, 3.3, 6.2, 12, 22, 43, 82, 160, 300, 560, 1K1, 2K0, 3K9, 7K5, 13K, 24K, 47K, 91K, 180K, 330K, 620K,  1.2M, 2.2M, 4.3M, 8.2M.

Ok, so how do you plan to set 1R4 to 1% with those values?
No, I do not get it. Schematics please.
As mentioned earlier, with arbitrary size of voids the task is trivial: 1R + 1M, one relay and whole 1R:1M range is "covered", to 1%, as long as you are limited in selection to those several values.

So could you narrow down your requirements, the goal is to:
 a) cover whole range, with any value from the range, to 1%,
 b) cover selected 123 values to 1%,
 c) cover unspecified count of randomly distributed values but not using more than 26 relays and 34 m of wire,
 d) use up to 26 relays and E24 1% resistors ?
 ...
 z) Use up to 26 of E24 1% resistors and unspecified count of relays?
??

[1audio]

I have a JRL automatic resistance standard that does the 1 Ohm to 1 Meg at .01%. However it may be overkill. Fluke made a similar box. There are occasionally boards from instruments on ebay with relays and resistors.

One approach I have seen for calibration is essentially a dynamic load with voltage and current sensors that adjusts to the target equivalent resistance. http://exodus.poly.edu/~kurt/manuals/manuals/Other/VALHALLA%202724A%20Operation.pdf

For calibration you do one approach, for tuning circuits you need to know how the circuit reacts to parasitics before to try or it may all be a waste of time.

[Fungus]

1, 1.8, 3.3, 6.2, 12, 22, 43, 82, 160, 300, 560, 1K1, 2K0, 3K9, 7K5, 13K, 24K, 47K, 91K, 180K, 330K, 620K,  1.2M, 2.2M, 4.3M, 8.2M.

Ok, so how do you plan to set 1R4 to 1% with those values?

I don't think it was ever claimed that that scale was 1%, it's just a reasonable scale that could be made with standard values.

(...with 2-3% accuracy? Something like that, I haven't looked at all combinations)

If you want exact values you could do it with standard resistors and a file for manual adjustment.

(is that an advantage? Why do you need whole numbers? Maybe your circuit really needs 13.8 Ohms, not 14  )

[casinada]

https://www.eevblog.com/forum/blog/eevblog-544-fluke-5450a-resistance-calibrator-teardown/msg323986/#msg323986
 

[technogeeky]

A lot of this thread is dedicated to making something that I'll just call a "precision resistor substitution box". In other words, what we all imagine but at high precision. However, I think the OP has stumbled onto an interesting question, namely:

Why aren't there dirt cheap programmable resistor substitution boxes? Forget the requirement of being 0.1% or even 1% precision for the moment; surely there is a technique (as just mentioned above) where you could cheaply generate the target resistences in a consistent way without resorting to mechanical substitution (e.g. rotary switches, DIP switches, jumper tabs).

The cheapest resistor substitution device (there's also a capacitor version) is the one Gerry Sweeney made. I think the other ones with rotary dials are probably nicer in the hand, and look better, and perhaps are easier to use.

However, you can simply make the argument that a programmable one would be orders of magnitude easier to use (no removing jumpers and putting them back). You could also realistically do resistance sweeps to search for the optimal point you are presumably trying to find...

Anyone know about this kind of thing?

[ManateeMafia]

I think IET Labs PRS series are ok. GPIB interface or local control.
http://www.ebay.com/itm/iET-Labs-PRS-201-1EEE-RM-Programmable-Resistance-Substituter-Used-Working-/382061957304?hash=item58f4aad0b8:g:0kYAAOSwuMZZAmJw

[Fungus]

You could also realistically do resistance sweeps to search for the optimal point you are presumably trying to find...

Search for a given current. That would be fun.

Or manually dial a voltage or current on the LCD display then see a readout of the resistance needed to obtain that.

[technogeeky]

You could also realistically do resistance sweeps to search for the optimal point you are presumably trying to find...

Search for a given current. That would be fun.

Or manually dial a voltage or current on the LCD display then see a readout of the resistance needed to obtain that.

I am not very creative with circuit design yet, but I sort of see two options that both suck:

In one, you would manage the mess of using banks of solid state relays to connect and disconnect various SMD resistors.

In another, you would use some active device. But what? (digital potentiometers seem to be out; you need to be able to handle maybe a few hundred volts; you need better precision then 20%)

I'm not sure studying e.g. the aforementioned IET PRS will yield insight into the kind of small cheap thing one would want to build.

[Someone]

surely there is a technique (as just mentioned above) where you could cheaply generate the target resistences in a consistent way without resorting to mechanical substitution (e.g. rotary switches, DIP switches, jumper tabs).

Sure is, but it needs calibration for the range you want to use and its not particularly stable over temperature (both are solvable):
https://en.wikipedia.org/wiki/Resistive_opto-isolator
Otherwise called a vactrol.

[1audio]

Even the optoisolators have significant drawbacks. They change with light and temperature and they are not all that predictable with some hysteresis changing depending on going up or down.

A scheme using FET's would not work predictably for higher power and would most likely need an opamp and some other tricks to get a known resistance. A resistor array would need mechanical relays for the lowest values and FET's could work for higher values. The FET's could be controlled from PV optocouplers. It would be a lot of stuff to solve a problem.

Another option would be a 2 gang 10 turn pot + motor to drive using the second pot to servo to the desired value. Still a lot of stuff and only 3 digits.   I have used pots to tweak analog circuits in design. Turn the knob until you get the desired result and then measure.

[forrestc]

1, 1.8, 3.3, 6.2, 12, 22, 43, 82, 160, 300, 560, 1K1, 2K0, 3K9, 7K5, 13K, 24K, 47K, 91K, 180K, 330K, 620K,  1.2M, 2.2M, 4.3M, 8.2M.

Ok, so how do you plan to set 1R4 to 1% with those values?

I don't.   Let me see if I can clarify:

With the arrangement above (with a switchable relay 'shorting' each resistor when contacts are closed), you can set the box to any ohm value +- appx 1ohm.  All the resistors are in series, so with all of the relays open, you get the sum of all of the resistor values.  With all of the resistors closed, you get whatever the contact resistance of all of the relays are.   So, you effectively you can set the box to a nominal value from something around 1ohm, to something over 10Mohm.

This is not the accuracy of the box - it's the resolution, or in ADC terms, step size.  You can't get 1R4, but you can definitely set to a value you believe is something like 23.837K+-1Ohm.   To do this, you need the actual measured resistance of the box when all relays are closed, and then the resistance delta for each relay when opened individually - which will be close to the value of the resistor in parallel with the contact.   Once you have those values, writing a bit of code to adjust the relays appropriately to make any value by opening the correct relays is rather trivial.

Which gets us back to the accuracy question.

Below about 100 ohms you're limited by the appx 1 ohm step size.

Above 100 ohms you're limited by drift effects of the resistors+relay+copper+everything else.   Trying to set a box made from 50ppm resistors to something like 10,000,000 ohms and expecting it to be +- 1 ohm isn't likely, since 1 degree of temperature drift will net you 500 ohms of change.   But expecting 1% at the top end of the scale is more than reasonable.   Probably more like 0.1% (but don't quote me on that as it would need to be characterized/simulated).

In relation to the goal, it's mostly these two:

a) cover whole range, with any value from the range, to 1%,
 c) cover unspecified count of randomly distributed values but not using more than 26 relays and 34 m of wire,

With the caveat of the low end of the scale is going to be swamped by the step size, and the number of relays+resistors is still to be determined.

or an additional option:  Buy something off the shelf which simulates a real resistor.

[forrestc]

I think IET Labs PRS series are ok. GPIB interface or local control.
http://www.ebay.com/itm/iET-Labs-PRS-201-1EEE-RM-Programmable-Resistance-Substituter-Used-Working-/382061957304?hash=item58f4aad0b8:g:0kYAAOSwuMZZAmJw

Wow!  THANK YOU!!!!!

How the heck did I not try "programmable resistance" in my search?  I know I searched for "programmable decade" and a dozen other options such as "resistance source".   But a ebay search for 'programmable resistance' yields very interesting results. 

This appears to be exactly what I'm looking for....

[PointyOintment]

I think IET Labs PRS series are ok. GPIB interface or local control.
http://www.ebay.com/itm/iET-Labs-PRS-201-1EEE-RM-Programmable-Resistance-Substituter-Used-Working-/382061957304?hash=item58f4aad0b8:g:0kYAAOSwuMZZAmJw

Wow!  THANK YOU!!!!!

How the heck did I not try "programmable resistance" in my search?  I know I searched for "programmable decade" and a dozen other options such as "resistance source".   But a ebay search for 'programmable resistance' yields very interesting results. 

This appears to be exactly what I'm looking for....

I don't remember you clarifying the reversibility requirement, but that thing doesn't look like it would work for that, having designated high and low jacks.

…regular decade box and a handful of motors?

[ManateeMafia]

It should work just fine. I happen to have a PRS-201 model and took a few photos. The room was a little dark but I think you can get a good idea of its capabilities.

I slowly rocked each of the decades and could hear relays chatter. Some did it more than others so perhaps mine spent a good part of its life in remote mode? The Coto relays are a nice touch and IET could have cheaped out. These are hermetically sealed and low thermal.
There are two different types of boards in there.