I have a "controller" that energizes one relay if line voltage drops below 118v, and energizes another if it goes above 128v. Normal line voltage is around 123v AC.
Controller plugs into a base; I have spare bases that I can adapt to go "between" the field base and the controller.

I hoped to put some kind of variable voltage divider-type circuit into the spare base, to provide field personnel with a semi-compact device that will allow field testing of the OV/UV trip function; otherwise they have to send the controllers in to central test lab (from all over the state).

Size is a consideration; I'd like to keep it small, hopefully 2 or 3 inches square max...? The base adapter I'll be using is round, about 6 inches in diameter, and about 2 inches deep (very similar to electric meter on your house).

I'm guessing I won't be able to fit a solution inside this base; any suggestions on what to use/ how to configure a circuit like this? Thank you!

 

We have used a variable transformer such as a Staco brand to obtain a adjustable AC output. Their L-series are cord and plug 120v in/0-140v out. Also good for supercharging you soldering iron.

 

What is the power for the "divider"? Because the size of transformer maybe too large for your 2-3 square inch.

 

This depends on just how precise you need the output of this tester. If you series (in phase)the output of a 1:1 transformer (123V in 123V out) and a 5 V transformer (123Vin 5V out) you would get your 128V and if you series (out of phase) you will get 118. This is the buck and boost principle. I think you can use a DPDT switch to connect your transformers.

If you want to control the output more precisely, you can just connect a 1:1 and a 12 volt in series to give you your 135 VAC. Then use a variable and fixed resistor in series across the output of the transformers. Take the output from the bottom of the fixed resistor, and the wiper of the variable to your controller, and using a DMM adjust for the exact voltage you need. I assume the controller has a high input impedance, and would use very little current, so scale your components to match.