PDA

View Full Version : A general question about the potentiometer (pot) and itís 3 pins



Silverwater
01-14-2011, 03:20 PM
A general question about the potentiometer (pot) and itís 3 pins:

In the educational projects the ďPotentiometerĒ is described as follow:
{Reference: Basic Analog and Digital, Student Guide, VERSION 1.4, page 5}


ďAs the wiper gets closer to Vdd, the voltage measured at the wiper terminal will approach
the value of Vdd, which is 5 volts. Likewise, when the wiper is closer to Vss, the voltage
at the wiper terminal will be closer to Vss, which is 0 volts. As the wiper terminal travels
between Vdd and Vss, the output measured at the wiper terminal varies between these
two values in a manner analogous to a door as it opens and closes.Ē
I get quite the opposite! As the wiper gets closer to Vdd the resistance goes up (gets to maximum) and the voltage approaches zero. Why is this?

Mike Green
01-14-2011, 03:50 PM
You get the opposite because something (the pot or Vdd/Vss or the meter you're using for measurement) is connected backwards.

Voltage and resistance are always measured between two points in a circuit. If you're measuring resistance between the wiper and one end of the pot, as the wiper gets closer to that end, the resistance will approach zero. If you measure resistance that's going up, you're using the opposite end of the pot from the one you think you have. If the voltage is approaching zero, the wiper is getting closer to Vss (the zero voltage point in the circuit), not Vdd.

Leon
01-14-2011, 05:24 PM
When connecting a pot I always visualise how it operates, and wire it so that the voltage or signal is at maximum when it is turned clockwise. My PCB symbols for pots have a little arrow on them to remind me how they should be connected.

Silverwater
01-14-2011, 06:12 PM
Thanks, Mike I now know my mistake.
I had my voltmeter measuring between the wiper and end connected to Vss.
Summarizing:
Using a setup, that opposite the wiper are two ends; one connected to Vdd and one connected to Vss; the following is true:
If the wiper is closer to the Vdd end, that end has higher resistance and therefore higher voltage, if the voltage is measured between the Vdd end and the wiper.(V=IR). So it goes to say, that the closer the wiper is to an end, the measured resistance at that end increases and the wiper voltage increases.
Please confirm.
The reason I’m asking is that you wrote in your response, “…If you're measuring resistance between the wiper and one end of the pot, as the wiper gets closer to that end, the resistance will approach zero…”. I performed this measurement with just the pot and an ohmmeter and measured that as the wiper gets closer to that end the resistance approached maximum; the opposite!?
Please excuse me if I’m wrong again.
Thank you for your time!

Leon
01-14-2011, 06:55 PM
You just need to visualise what is going on, as I suggested. If the wiper is close to the end connected to ground, the output will be close to ground. With the wiper at the other end of the track, you will get a high voltage.

Mike Green
01-14-2011, 06:56 PM
I don't know what you are measuring. Physically, you must measure a lower resistance between the wiper and one end of the pot as the wiper approaches that end. There is no way you can measure a higher resistance unless you have things connected wrong (like using the wrong end of the pot's resistance element). That said, you can't measure resistance accurately if there's power applied to the circuit. A meter measures resistance by applying a voltage to the resistance and measuring the amount of current that flows. If there's another source of power, it throws the measurement off.

Inside the pot, there's a strip of carbon-based resistive material with connections at each end (the fixed ends of the resistance). There's a wiper that moves from one end of the strip to the other and a low resistance (essentially zero resistance) contact to that wiper. As the wiper moves closer to one end of the strip, there's less and less resistive material between the wiper and the fixed contact at the end of the strip, therefore less resistance. When the wiper reaches the end and touches the fixed contact at that end, there's essentially zero resistance between the wiper and the fixed contact. If you measure the resistance between the wiper and the other fixed contact (at the other end of the strip), you'll essentially measure the resistance of the entire length of the strip.

Mike G
01-14-2011, 06:57 PM
If you have an ideal 100k POT then the resistance across the whole POT is 100k. If the wiper is in the middle then the resistance measured from wiper to either end of the POT is 50k. As the wiper is turned, one side (measured wiper to end) will increase in resistance while the other side decreases. The total resistance (adding both sides) is always 100k. From there it's just a matter of Ohms law.




5 5
(-) ---------- (+)
^

2.5 7.5
(-) ---------- (+)
^


If you have 10V across the 100k POTs above. The first one, as measured from wiper to ground, is 5V on both sides. In the second you have 2.5V in one side and 7.5V on the other.

Just get a POT and a multimeter in resistance mode. Connect one lead of the meter to the wiper and the other lead to one of the remaining POT nodes. Turn the POT and watch the meter.