How can I determine the value of a pot in circuit? - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: October 09, 2021, 03:31:41 pm
content: I've built an audio preamp with a Baxandall 2-band active tone circuit that works quite well. I'm preparing a report on the project and I have some Spice simulations of the response of the tone circuit. The circuit has some interesting characteristics that arise from the bass and treble circuits interacting (see schematic below). The Bode plotter on my Siglent 1104X-E is working as expected, but I'm not sure how to figure out what the pots are set at.The bass and treble pots are 100k, linear taper. In circuit with the power off they measure about 60k ohms. I tried using my ohmmeter to set the pots at 25% and 75% of the measured value but the Bode plot I got was far from the simulated one for the same values. I considered (for a second) measuring the pots with the circuit energized but even if that didn't blow up my meter I expect the presence of any voltage at the pot leads from the rest of the circuit would skew the resistance measurement anyway.So my question is, is there a way to determine an equivalent 25%/75% setting for the purpose of making a Bode plot that is useful for comparing with the simulated response? It's easy enough to tweak the pots to get the Bode plot to show what I expect, but then what do I know about the value of the pots? I guess I could do that and then power down the circuit (wait a few minutes for all the caps to bleed off) and then measure the pots again. That should at least tell me what resistance values create the same response as the simulation. I feel like I'm overlooking some basic thing that will unlock this puzzle.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 09, 2021, 03:32:24 pm
content: Here are some pics to support my question.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 09, 2021, 03:33:53 pm
content: Here's the schematic of the circuit.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 09, 2021, 03:38:09 pm
content: For some reason the forum is flagging my scope shot showing the Bode plot. It's a 750kB .png file so not sure why but I'm unable to post that one.

author: EPAIII
author_rank: Super Contributor
timestamp: October 11, 2021, 02:40:20 am
content: Measuring the resistance (or impedance) of any component "in circuit" is always an iffy proposition. The circuit that surrounds the component will almost always have measurable values for the measurements and these values will be in parallel with those of the component of interest. In troubleshooting, I have tried many times to measure across "in circuit" components with an Ohm meter but most of the time the values found that way are far from the values taken with the component taken out of the circuit. I have never found a reliable way of getting around this in over 45 years of my professional career plus another 15 years or so before and after that.In short, if you want an accurate and practical way to set a pot to a 25% or 75% position, you can either rely on the mechanical position of the shaft (not very accurate), or simply REMOVE it from the circuit.I said "practical" way above. You could always construct the circuit in a circuit simulation program (SPICE) and use fixed resistors to simulate those two settings. Then use the functionality of the program to take readings. TI's TINA circuit simulation program has an Ohm meter that can be used for this. But I do not consider this to be very "practical" because it would be far easier and faster to just un-solder two of the three connections to a pot instead of constructing the entire circuit using a circuit simulation program and making sure that ALL the other possible settings in that circuit did match those in your physical circuit. Simply un-soldering is how I did address this problem many, many times over my career. So I know it is far more practical.Please notice that you do not need to disconnect (un-solder) all the leads of a component in order to isolate it from the circuit. You can always leave ONE lead connected.Another shortcut that I have used is to cut either the leads of the component or cut the circuit traces leading to it. Those cuts can be easily re-connected with a drop of solder bridging the gap. This can be super fast and, while it does leave the circuit a bit unsightly, it does no permanent damage.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 11, 2021, 03:25:27 am
content: That makes sense. So is it the case that if I did that and set the pots to 25/75 "out of circuit", and then resoldered to test the response of the filters, that that would be a reliable representation of the performance? If the presence of other components in parallel affect the value of the resistance at the wiper when the pot is in the circuit, then does it even mean anything to say the pot is at 25% of 100k if it actually isn't 25k ohms in circuit? I have done the simulation , fwiw.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2021, 05:55:03 pm
content: Besides measuring the resistance of the poteniometers out of circuit which is easy enough, they could be mechanically calibrated to show the resistance directly from the angle of their shaft. It seems crude but 5% accuracy is only one part in 20 and you can easily do better than that.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 11, 2021, 07:24:29 pm
content: That's an excellent idea.Do you think the setting has any meaning if the value changes in circuit? I'm trying to wrap my mind around the legitimacy of the result if I set it at 25% and then take the measurement in circuit knowing that the actual resistive value is not the same.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 07:42:03 pm
content: The value in circuit isn't of consequence. The value of the resistance OF THE POT is the important number.

author: hummusdude
author_rank: Regular Contributor
timestamp: October 11, 2021, 07:52:48 pm
content: That's what I thought. Thanks for spelling that out for me!