I need a programmamable load (pure resistive) between 10ohm and 10Mohm, I know there´s digital potentiometers, but the maximum range I´ve seen is 200k. Does anyone know how to solve this problem, I really dont want to put 50 of those in series.

 

I'm just speaking conceptually here, but is there some way that you can play with the scale of the digital potentiometer?

 

Unfortunately not. At lest not that I know of, and I really doubt there is.

 

What resolution do you need? - it's rather crucial to any solution, the higher the resolution the greater the cost!.

As an absolute minimum you would only require two resistors switched into circuit, one of 10 ohms and one of 10Mohms.

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Why does it have to be resistive? How much current at 10 ohms and how much voltage at 10 megohms?

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Is it a DC load or an AC load? If AC, what frequency range? What kind of voltage/current are you planning across the load? And, as Nigel mentioned, what kind of resolution are you looking at? The range is quite large!

 

I´m building an automatic measurment system for Grätzel solarcells, it´s supposed to measure voltages between 10mV and 1V with a resolution of 10mV, currents from 1uA to 10mA with 1uA resolution. The range of the load should be approx 1ohm to 10Mohm in say 100 steps (log). All DC. The cost is not a big issue.

I have everything under control except this load issue, the big range is the problem.

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You could switch resistors in a binary sequence - so you would have 1,2,4 and 8 ohm resistors, and the same in each decade - this would take a lot of resistors and I/O, but would give you one ohm resolution from 1 ohm to 10 MOhm.

To do only 100 steps you could use just 7 resistors (giving 128 possible values) - you would need to carefully workout the resistors values, as they would be added in parallel for the lower currents - as you want a log variation, it may be possible to work out reasonable values. Increasing to 8 resistors would give 256 possibilities, and it could be easier to select 100 out of 256, rather than 100 out of 128.

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To cover 7 decades in 100 steps takes approximately 14 steps/decade. In the first decade, you resistor values would be

1.00
1.18
1.39
1.64
1.93
2.28
2.68
3.16
3.73
4.39
5.18
6.11
7.20
8.48

The next decade's resistors are 10 times the above values, and so on. Do you really need that much resolution?

 

WOuld you consider a programmable current source? 1 / (Gm) of your source looks purely resistive at DC and with a cheap D/A for programming it, you could get sub microamp to Amps (which would give you 10MOhms to sub 1Ohms) You can get very good precision out of this as well.

I think a power FET (or Bipolar), opamp, 12 bit D/A and some passives would do it.

You do not even really need the D/A if precision is not of concern. A voltage divider and pot would work just as well.
I've made crude programmable current sources like with 100 nA to Amps range like this for troubleshooting other circuits.

The circuit will behave at DC as if you were placing a variable sub 1 Ohm to 10 MOhm resistor at the load.

 

I´m not sure what you mean, do mean by hand changing resistors? It´s supposed to be automated.

Figuring out the different values is not a big thing nor very difficult.

It sounds like a good idea, but are you sure that it would behave exactly is if it was an ordinary resistor?

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\"I don¹t know what kind of weapons will be used in the third world war, assuming there will be a third world war. But I can tell you that the fourth world war will be fought with stone clubs.\" - Albert Einstein

 

I didn't mean to imply that it was difficult. I was only trying to illustrate how close together the resistor values are, bringing up the question (in my mind) of whether you need such fine resolution.






.

 

The current source is not like a resistor because if the voltage increases, the current stays the same. What are you trying to accomplish? Are you trying to determing the source resistance of the solar cell? Why change the load over such a wide range?

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No, switch them with either transistors, FET's or even relays. By putting 7 resistors in various parallel combinations (and also individually) you could get 128 different values. You would need to work out what values you would need to give the required results - you may need to use more than 7 to get the results you require, and pick the particular combinations which give the values you want.

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I´m sorry if I was unclear of what i want to do. The project is to get the voltage-current characteristics for solarcells with different loads.

Unfortunately I don´t decide the resistor resolution.

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\"I don¹t know what kind of weapons will be used in the third world war, assuming there will be a third world war. But I can tell you that the fourth world war will be fought with stone clubs.\" - Albert Einstein