07-08-2015, 08:28 PM
It doesn't seem to be commonly known but there is a solution to the log/antilog/special law potentiometer problem and it surprisingly simple!
If you put a resistor between the slider and one end of the pot, you will alter the law of the pot. For a log law, the resistor should be between the slider and the earthy end. For an anti-log law simply move the resistor so that it is between the slider and the 'hot' side of the pot.
When calculating the resultant characteristic it is important to consider the external circuitry.
For example, I once calculated that putting a 15k resistor from slider to earth on a 47k pot would modify the law to match a varicap diode's characteristic.* In this case, however, the top end of the pot was connected to the 30V varicap PSU.
If there is external resistance at either end of the pot - usually at the top end - this will need to be taken into account and, if the total resistance is important, a higher value of pot may need to be used as the basis of the exercise.
I would suggest temporarily tacking a resistor across an existing linear pot of the same or higher value as the original, then using a 'suck it and see' approach to determine how close to the desired law the result is.
If you think this approach is somewhat crude, you might like to ponder on the fact that many log law pots are nothing of the sort! Essentially, the track is laid down in two overlapping linear sections of different values so that the selected portion of the track increases slowly in value until about the halfway stage, then increases rapidly to maximum!
Your ears will tell you when you've got it right!
* The varicap diode law is much sharper than the normal log characteristic so a starting value of ~50% of the linear pot's value might be a useful starting point.
If you put a resistor between the slider and one end of the pot, you will alter the law of the pot. For a log law, the resistor should be between the slider and the earthy end. For an anti-log law simply move the resistor so that it is between the slider and the 'hot' side of the pot.
When calculating the resultant characteristic it is important to consider the external circuitry.
For example, I once calculated that putting a 15k resistor from slider to earth on a 47k pot would modify the law to match a varicap diode's characteristic.* In this case, however, the top end of the pot was connected to the 30V varicap PSU.
If there is external resistance at either end of the pot - usually at the top end - this will need to be taken into account and, if the total resistance is important, a higher value of pot may need to be used as the basis of the exercise.
I would suggest temporarily tacking a resistor across an existing linear pot of the same or higher value as the original, then using a 'suck it and see' approach to determine how close to the desired law the result is.
If you think this approach is somewhat crude, you might like to ponder on the fact that many log law pots are nothing of the sort! Essentially, the track is laid down in two overlapping linear sections of different values so that the selected portion of the track increases slowly in value until about the halfway stage, then increases rapidly to maximum!
Your ears will tell you when you've got it right!
* The varicap diode law is much sharper than the normal log characteristic so a starting value of ~50% of the linear pot's value might be a useful starting point.
