# Explain trim pot for VU meter

I have the following schematic that is supposed to rectify the signal going to a VU meter and also trim it, so the user can adjust the position of "0dB".

What I don't understand is why and how the resistor is connected in that way, shorting legs 2 and 3. Is this working as a simple voltage divider? Or just a rheostat? How does it work and how does this trim the actual display?

The schematic is preceeded with a buffering op-amp.

The pot is connected as a rheostat. It will vary from about zero ohms when the wiper is at the top to about 20K when it is at the bottom.

The diodes are connected as a bridge rectifier so that the current through RV5 passes through the meter in the same direction (pin 2 is positive, pin 1 is negative).

The meter has some current sensitivity and resistance so it adjusts the input voltage (in this case average AC voltage) that will result in full scale needle deflection.

• Ok, thanks. But that means I could leave pin 3 of the pot unconnected, right? Also, does this mean that the meter deflection is set by series resistance? What happened, if pin 3 of the pot was connected to ground, making it a voltage divider? – jilski Oct 30 '15 at 19:31
• In most applications pin 3 would be left connected as shown so that in the event of high resistance wiper contact at least some current would get through. (Calibration would be off, of course.) From what I remember (30 years ago) the VU circuit uses the non-linearity of the diodes to give a non-linear motion to a linear meter - more like a logarithmic deciBel scale. The in-series resistor is part of the deal! It and the coil are well behaved linear resistance elements. The diodes - as well as rectifying - give the non-linear "correction". I'm sure a professional VU meter is more complex. – Transistor Oct 30 '15 at 19:51
• It would behave quite differently with a voltage divider because of the nonlinearity of the two diode drops in series with the meter coil. Leaving pin 3 open would have little noticeable effect- it slightly reduces the contact resistance variation (like noise) during adjustment. – Spehro Pefhany Oct 30 '15 at 19:52