An alternative approach to adjust something very accurate in electronic circuits by assembling fix value resistors instead of using trimmers or potentiometers.
A resistor can be precisely tuned by placing additional resistors in parallel. We present a technique using 4 parallel resistors, of which two are in series (A and B in schematic below). We use a potentiometer parallel to the existing resistor to get the target resistance, which will be placed next. We use this python script to successively determine the resistors A and B, then C, and later D. Depending on the initial error of resistor E, we reduce the error by two to three orders of magnitude by placing the resistors A and B, and one to two orders of magnitude each for C and D.
Suggested fix value resistors |
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Trimmers and Potentiometers are bulky, expensive and have a limited resolution. In addition cermet trimmers show a lot of flicker noise. If you need something really stable, this approach could be something for you.
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Resistor E is assembled (optional). Offset has to be adjusted by reducing the total resistance. The resistor is a part of a circuit where the optimal point can be measured. Let's assume it is an offset of an amplifier. |
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Trimmer P is connected temporary and adjusted until offset is zero.
Trimmer is removed and resistance P is measured. Target resistance: 11203 Result: A: 11000 B: 220 C: 12000 D: 11000 |
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Assembling of resistors A: 11000 B: 220 |
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Trimmer P is adjusted until offset is zero. Trimmer is removed and resistance P is measured. Target resistance: 5700 Result: A: 5600 B: 110 C: 6200 D: 5600 |
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Assembling of resistor C: 6200 |
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Trimmer P is adjusted until offset is zero. Trimmer is removed and resistance P is measured. Target resistance: 830000 Result: A: 820000 B: 11000 C: 910000 D: 820000 |
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Assembling of resistor D: 820000 |
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Example: set of fix value resistors. |
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A device to speed up the trimming process. Switch to Rx, test-pins pressed on the circuit board, adjusting a trimmer. The range can be selected. Switch to Multimeter. Reading the resistance of the trimmer from the multimeter. |
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An example of a circuit board. R11 and R111 are assembled initialy to make the circuit working. According to the initial offset the left side R11 or the right side R111 has to be reduced. The yellow letters show an adjustment of the left side. A good trimmer would need more space on the pcb. |
If you do this you end up at very high resistance values. Thinfilm resistors ar avaiable only up to 1 MOhm. By assembling two in the first step, you get a more accurate first step and less sensibility in the following steps.
In the simulation you can see the relative error to the ramdomly choosen target value. (Target value for A and B)
It is important to have good thinfilm resistors. I measured the typical error of 55 piece 10k resistors (ERA6A) and got a maximum error of 0.015% from 10kOhm. This tolerance is used in the python script.
Consider the tolerance of the multimeter used to measure the trimmer P and the limited accuracy of the fix value resistors and the possible change of the resistance value due to soldering. By doing it iterative, errors in the former steps will be corrected in the following steps.
I developed this concept because i needed a solution. It fulfilled my requirement perfectly. It is possible there is an more efficient way to do it. Suggestions are welcome!