Resistance source?

[forrestc]

Before I go and build one I figured I'd ask if there is something I should be searching for which does this.

I'm looking for the equivalent of a digital potentiometer.  I have some automatic test procedures where I need to vary the resistance across a pair of terminals.     

Off the shelf digital pot ic's (that I've found) are relatively low voltage devices across the pot - I may end up with around 50V across the pot, and so a 5V one won't work.

I considered (and experimented with) a digital load set to constant resistance mode but there are some challenges there when there is very little current thorough the resistor - these are really designed for sinking current.  And there seems to be various stability issues since the load is adjusting the effective resistance of the FET's based on current through and voltage across them.

I looked at various process calibrators (such as the fluke 725), but they all seem to top out at around 3-5K and aren't cheap.

I did find the fluke (and similar) multifunction calibrators.   The older ones like a 5100B *may* work, but they're big and heavy and old enough that I would worry about their reliability without a refurbishment.   The more modern ones are very expensive.

I'm almost at the point of building a decade box with a pile of resistors and relays instead of switches.   There are a lot of things going for this option, with the main disadvantage being the fact I have to build it (I have lots of other projects I would rather build).   I'm hoping that someone knows of something else I haven't looked at just because I don't know what to search for.

In an ideal world, I'd love to see something like 1% over the range of 1 ohm to 1Mohm.  But I realize I don't live in an ideal world, so I'd accept something less than that - I definitely need this to go up into the 1K-100K range though since that is the area of interest for the current project.    At the higher ranges (10K-100K) I may have up to 50V across this, so I need something which would survive that (1/2 or 1W).

Any ideas of something else I should look at before I go build me a suitable equivalent?

P.S. I just realized I hadn't searched for 'programmable decade box'.  Seems like there's some options out there, but still fairly pricey for a couple dozen relays + resistors ($1.5K is the cheapest I found).   So still looking...

[coromonadalix]

you could build such an item ?   1% resistors in series ...  and create ranges with small relays, and code something with an arduino

[ruairi]

You might be able to mod an audio relay attenuator, there are quite a few designs available, usually controlled by a pic.  I've seen an arduino design too that may be more easily hackable to what you need.

I've used several designs that give 64 x 1dB steps.

[Seekonk]

I took apart a relatively cheap stereo.  It had a motorized pot for remote control.  With a stereo pot, one would give position, the other resistance.  These must be located on ebay.  Recording studios used to have these motorized pots

[coromonadalix]

oohh  good one Seekonk   never tough of this one,  but the op want an 1% precision,  maybe hacking one with an 1% pot

[pigrew]

What frequency response is needed? If it isn't too high, perhaps a FET could be used an "electronic load".

[Brutte]

A resistor is a device that meets the equation: R=U/I

If you want to have a variable R that can be controlled by an external signal (a voltage perhaps) then you can use a multiplying op-amp. Of course this is for low-ish powers if you want to drive that directly. With I²R above 100mW I'd recommend a power stage.

Anway, a multiplying opamp works more or less like an opamp with ln() converter on all inputs and ln^-1() on output.

So:
     ln(U) - ln(I) - ln(R) = 0
This is a summing amplifier that sums three signals.
If some device can make that happen at all times then it is indistinguishable from a resistor (at least for the circuit that is wired into it).

[Fungus]

oohh  good one Seekonk   never tough of this one,  but the op want an 1% precision,  maybe hacking one with an 1% pot

You can't put 50V across a regular potentiometer.

If there's a limited number of fixed values then relays/MOSFETs + fixed resistors is easy.

If you need a fully programmable capability then how about a "Binary Resistance Box" with resistors that add up in powers of two and a MOSFET to bypass each one. Switch on the right combination of MOSFETs and you can get any value up to twice your maximum resistor -1.

(Has this ever been done? Should I patent my idea?)

[babysitter]

You do automated test equipment ? Chances are you have some switching boxes, like keithley 7000s or hp 3488. (If not - get some!)
Just add a string of resistors and use a relay card to short out some of them if demanded, do measurements if it qualifies for your task... tadaa, programmable resistor without adding too many boxes or program lines.

[bson]

You could use a BJT like a 2N3904 controlling the LED current of an LDR like an NSL-32SL2.  The base voltage then comes from a function gen which of course can be remote controlled. You'll need to calibrate for R(Vb).

Or, should you happen to have one, a lateral power MOSFET, but these are hard to find and expensive when you do, so practical only if you happen to have some on hand.  (If you buy them on ebay they're guaranteed to be fake.)  Verticals will have too small of an ohmic region in terms of Vgs to be practical, maybe only a volt or two.

[NorthGuy]

It is very difficult to get 1 Ohm with 1% precision. It is even more difficult to put 50V across it, which would require 2500W.

If you remove the low end from your specs and settle for 500 Ohm to 1 MOhm range, then it's only 12 resistors and 11 relays to get all the possible values in the range with 500 Ohm step.

[Cerebus]

It is very difficult to get 1 Ohm with 1% precision.

Depends what you mean by "get". If you mean "obtain" then no, it's not, you just have to know where to look. Just the other day I bought some 2 ohm 0.1% 0.5W 5 ppm/C resistors from Vishay from a range that includes a 1 ohm 0.1% part.

If on the other hand by "get" you mean "engineer", then yes. Just to begin with, at that level you have to specify where along the lead you are measuring the resistance. Most PCB traces have enough resistance that you have to take them into account at that level.

Case in point, those 2 ohm resistors mentioned above were to replace the lowest 2 values in a 0-99999 ohm resistance decade box that's a little bit special. It was made by Vishay in 1969 and originally contained 25 hermetically sealed Vishay HP202 resistors. The two lowest values had been blown by the previous owner and needed replacing. I lucked on someone with some stock of suitable, albeit inferior to the original, 2 ohm resistors in stock and I ended up using one 2 ohm and a pair of 2 ohm resistors in parallel as a 1 ohm resistor. The values of the original resistors clearly take the boxes switches and wiring into account, they have specified values of 1.9860 ohms and 0.9824 ohms. I now have to remember that if I use the lowest dial on the box I could be out by a few milliohms.

[forrestc]

It is very difficult to get 1 Ohm with 1% precision. It is even more difficult to put 50V across it, which would require 2500W.

If you remove the low end from your specs and settle for 500 Ohm to 1 MOhm range, then it's only 12 resistors and 11 relays to get all the possible values in the range with 500 Ohm step.

Figure this is a good one for me to reply to and clarify my needs.

Just to dispel a couple of things I think I may have not clarified well:  I don't need 50V survivability at the bottom end.    What I do need is a box which can handle 50V at appropriate portions of the curve, say with a 100K resistor across the load.   

The problem I am having with all of the active solutions (fet, etc) is generally that they are unidirectional (they don't work if you switch the leads), or don't support a wide enough range of voltages.   Plus, the stability of some of these solutions are less than adequate.   I'll take some time to review the suggestions in this thread to see if any of them lead me down a path which I haven't already considered, as often I find that I just simply missed a logical solution that somehow I discarded.

Generally, what I need would be met perfectly by a typical 1W 1% 'decade box'.  You know, the type with lots of resistors, each being able to be shorted with a switch.   With all the switches closed, you actually get close to zero ohms (effectively the aggregate resistance of all of the switches when closed, plus the internal wiring resistance).   With all of them open, often you'll get on the order of 10M or so.   The only problem with this is the lack of automatic control.

Which is what I am headed toward - load up a whole bunch of 1% 1W resistors across a whole bunch of relays, control the whole thing with a microcontroller.    Where a whole bunch means something on the order of 28.  (probably a few less, as I suspect I can actually just use powers of two all the way through, which means somewhere around 20.. for 1ohm to 1Mohm. ).   Probably can calibrate the whole thing against my reference 7 1/2 digit DMM to help improve accuracy (i.e. use the lower value resistor(s) to trim the higher value resistors if needed)

My problem is that I just don't need another R&D project right now as I'm extremely busy with the projects I'm currently working on, so I was hoping something off the shelf and relatively inexpensive (definitely under $1K, although that is probably still too high) existed.   At this point, I'll probably go ahead and spend the day or two it will take to get this put together, unless I find something else.

[Brutte]

Generally, what I need would be met perfectly by a typical 1W 1% 'decade box'.

Which is what I am headed toward - load up a whole bunch of 1% 1W resistors across a whole bunch of relays, control the whole thing with a microcontroller.    Where a whole bunch means something on the order of 28.  (probably a few less, as I suspect I can actually just use powers of two all the way through, which means somewhere around 20.. for 1ohm to 1Mohm. ).   

You won't be able to buy resistors with nominal values in the power of two. That is because these are sold as the N-th root of 10. Go, figure.
To get 1% match you need 96-th root of 10. At least E96. Otherwise there would be voids in resistance values.
Concluding, you need 6 decades of E96.
Or some wild switching board that adjusts that in parallel/series.

Even if you find some custom power-of-two resistor series (if you expect 20 of them to span 6 decades), how do you imagine rearranging them with relays?

Now it ispossible but if you consider the series of:
- 1R,2R,4R,8R,.....,524288R  (20 powers of 2)
even when these are 1% nominal, there are huge voids within that series near bottom end. You want 2.5R? Get 2R and accept 20% error..

Anyway, pictures are welcome!

[john_c]

Why does it need to be bidirectional? Are we talking AC, or just that the current direction may change from time to time?

[john_c]

Another thought: is it okay if it takes a few seconds to set the resistance? Because you could take something like this:

http://www.tedss.com/2021013231

and attach a worm-gear drive. Even if you hired somebody to custom build that... could probably be < $1k, right?

[Cerebus]

Generally, what I need would be met perfectly by a typical 1W 1% 'decade box'.

Which is what I am headed toward - load up a whole bunch of 1% 1W resistors across a whole bunch of relays, control the whole thing with a microcontroller.    Where a whole bunch means something on the order of 28.  (probably a few less, as I suspect I can actually just use powers of two all the way through, which means somewhere around 20.. for 1ohm to 1Mohm. ).   

You won't be able to buy resistors with nominal values in the power of two. That is because these are sold as the N-th root of 10. Go, figure.
To get 1% match you need 96-th root of 10. At least E96. Otherwise there would be voids in resistance values.
Concluding, you need 6 decades of E96.
Or some wild switching board that adjusts that in parallel/series.

Even if you find some custom power-of-two resistor series (if you expect 20 of them to span 6 decades), how do you imagine rearranging them with relays?

Now it ispossible but if you consider the series of:
- 1R,2R,4R,8R,.....,524288R  (20 powers of 2)
even when these are 1% nominal, there are huge voids within that series near bottom end. You want 2.5R? Get 2R and accept 20% error..

Anyway, pictures are welcome!

You can do this with E24 series resistors, 4 per decade with 4 SPST relays per decade. Simply connect 1, 2, 3 and 3 ohm resistors in series (or 10, 20 etc. etc) and then connect a SPST relay across each resistor to short it out if it's not in use. It should be obvious that any value in the range 0-9 ohms can be made with these combinations. Simply connect as many decades in series as you need, obviously increasing the resistor values by a factor of 10 each time. I'd suggest latching relays for all the obvious reasons (power consumption, induced currents etc.).

There is an electronic solution that could use a digital potentiometer chip that is very simple in principle - just use a variable gain amplifier, gain set by the digital pot, to multiply the voltage from a reference resistor to synthesize a resistor that is (1+gain) times the reference resistor. In practice to make that work it requires a floating supply relative to the circuitry that needs to 'see' the synthesized resistance, also needs an op amp with an output capability that meets your voltage and current requirements (which could both be high compared to typical op amp specifications) and needs an isolated SPI (or similar) interface to control the digital potentiometer. Those alone make it sound more impractical than good old relays. Also an 'electronics' solution would struggle to work well over, say, a 10,000:1 range (think "gain bandwidth product") whereas the relay based solution scales better.

[Brutte]

You can do this with E24 series resistors, 4 per decade with 4 SPST relays per decade. Simply connect 1, 2, 3 and 3 ohm resistors in series (or 10, 20 etc. etc) and then connect a SPST relay across each resistor

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%??

Also E24 series resistors are about 24-th root of 10 apart. That is 10% of one value with respect to the next one (1R1/1R = 1.1). Not sure how do you expect to fill those voids to get 1% coverage.

Otoh, if the coverage can have voids of arbitrary size, why not two resistors: 1% 1R and 1% 1M, one relay and we have a variable resistor that covers 1R-1M range 

It should be obvious that any value in the range 0-9 ohms can be made with these combinations. Simply connect as many decades in series as you need, obviously increasing the resistor values by a factor of 10 each time.

This means 4 binary steps over 6 decades = 24 relays at least. Sweet.
Mind in this idea there are still huge voids at lower end. For example between 1R and 2R there is a void like hell. 100% jump between adjacent values. Nowhere near 1%.

I would love to see that contraption. Pictures welcome. I seriously doubt this route is within 1k$ range but I'd love to prove wrong.

[Fungus]

It is very difficult to get 1 Ohm with 1% precision.

Really?

http://www.ebay.com/itm/262880450911

Very interesting. 1,2,4 and there you stopped. Lets proceed: how do you plan to buy 2^19? That is 524288R.

You don't buy them, you start with 680k and use a file to 'adjust' them. 

[Brutte]

You don't buy them, you start with 680k and use a file to 'adjust' them. 

Here on northern hemisphere filing down 680k resistor won't get you nowhere near 524288R. Doh, those Australians.. Is everything down there upside-down?
But I get your point - any resistance is obtainable, with any tolerance, with adequate funds and adequate patience.

[NorthGuy]

Depends what you mean by "get".

By "get" I mean making sure that it is still within 1% of 1 Ohm after you account for all the wire resistances, relay resistances, fuses, lead wires to the DUT etc.

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.

If you want the result to be within 1%, you only need the resistors to 1%, so anything having real resistance from 520k to 530k will do.

Even if you use several resistors to produce desired values by combining them, binary is still better than decades because it minimizes the number of relays.

[Cerebus]

You can do this with E24 series resistors, 4 per decade with 4 SPST relays per decade. Simply connect 1, 2, 3 and 3 ohm resistors in series (or 10, 20 etc. etc) and then connect a SPST relay across each resistor

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%??

Read that again: "Simply connect 1, 2, 3 and 3 ohm resistors in series" and I kept using the word "decades", at no time did I mention powers of 2. Note that 10, 20 and 30 are all standard E24 values and that E24 values are also E96 values so available in at least 1% precision and in practice in much, much greater precision.

1 = 1
2 = 2
1+2 = 3
1+3 = 4
2+3 = 5
3+3 = 6
1+3+3 = 7
2+3+3 = 8
1+2+3+3 = 9

Now multiply all those by 10 and do it again, and again...

Also E24 series resistors are about 24-th root of 10 apart. That is 10% of one value with respect to the next one (1R1/1R = 1.1). Not sure how do you expect to fill those voids to get 1% coverage.

Otoh, if the coverage can have voids of arbitrary size, why not two resistors: 1% 1R and 1% 1M, one relay and we have a variable resistor that covers 1R-1M range 

It should be obvious that any value in the range 0-9 ohms can be made with these combinations. Simply connect as many decades in series as you need, obviously increasing the resistor values by a factor of 10 each time.

This means 4 binary steps over 6 decades = 24 relays at least. Sweet.
Mind in this idea there are still huge voids at lower end. For example between 1R and 2R there is a void like hell. 100% jump between adjacent values. Nowhere near 1%.

I would love to see that contraption. Pictures welcome. I seriously doubt this route is within 1k$ range but I'd love to prove wrong.

It uses rotary switches rather than relays but operates on exactly the same principle and it's sitting not three feet away from me. Original factory spec was 0.005% across 5 decades; how close it is now I can't say as it's still more accurate that the most accurate meter I own (a 200,000 count Keithley 197) but it hasn't drifted much since it was made in 1969.

Approximate cost to reproduce with 6 decades with 0.1% through hole 15ppm resistors (TE connectivity R series 1/4W @£0.40 each) 4x6x0.4 = £9.60, call it a tenner. Twenty four dual coil latching relays at ~£4.00 each, call it £100. A board, call it a tenner. Six x TPIC6595 8 bit open drain shift registers to drive the relays @£2.50 each, £15. A handful of catch diodes and the total comes to around £140. Say £200 once it's in a box with some connectors and your favourite cheap micro dev. board to drive the whole thing.

If one wants total overkill, go to Vishay for some S102 0.005% 2ppm/C resistors at ~£20 each and the total would be closer to £600-700.

[Cerebus]

Even if you use several resistors to produce desired values by combining them, binary is still better than decades because it minimizes the number of relays.

A 0-999999 ohm range in binary will take you 20 relays and 20 hard to source resistors and 2 1/2 octal relay drivers (or 5 if you use dual coil relays). In decades it will take another 4 relays and 24 easy to source resistors. Sometimes counting on your fingers or toes is the easier engineering option.

[NorthGuy]

A 0-999999 ohm range in binary will take you 20 relays and 20 hard to source resistors and 2 1/2 octal relay drivers (or 5 if you use dual coil relays). In decades it will take another 4 relays and 24 easy to source resistors. Sometimes counting on your fingers or toes is the easier engineering option.

Not really that hard to source. For example, DigiKey has 73 different part numbers in stock, any of which would be suitable for the 524k resistor being discussed.

https://www.digikey.com/products/en/resistors/chip-resistor-surface-mount/52?k=&pkeyword=&pv1=586&FV=ffe00034&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

You can even get picky about what to choose.

[Cerebus]

A 0-999999 ohm range in binary will take you 20 relays and 20 hard to source resistors and 2 1/2 octal relay drivers (or 5 if you use dual coil relays). In decades it will take another 4 relays and 24 easy to source resistors. Sometimes counting on your fingers or toes is the easier engineering option.

Not really that hard to source. For example, DigiKey has 73 different part numbers in stock, any of which would be suitable for the 524k resistor being discussed.

https://www.digikey.com/products/en/resistors/chip-resistor-surface-mount/52?k=&pkeyword=&pv1=586&FV=ffe00034&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

You can even get picky about what to choose.

That's one value, you need the whole 2ⁿ series and have to put up with roughly tolerancing to hit values like 4096 (nearest E96 402 or 412), 8192 (nearest E96 806 or 825) to some nominal accuracy, just as you've done with 523k for  524.288k. That's going to throw any idea of linearity out of the window. Whereas if you stick with decades you don't have to do any fudging, you keep linearity and you get to choose a straightforward tolerance from the standard ranges available, 1% 0.1%, 0.05% and so on to as arbitrary a level of accuracy as your wallet will allow.