I want to build a device that will work like a voltage dependent on-off switch.
Specifically, the device will not allow electricity to be let through until the voltage reaches approximately 120v AC. That is 100v AC will not be allowed to pass. If voltage drops after reaching 120v then the device will also not the voltage pass.

I am unfamiliar with most components, so does anybody have any suggestions?

 

Something along the lines of a A DIAC driving a TRIAC could work, might need two DIACs in series, or a special voltage Diac to get that exact range, but the simplest solution I can come up with in a snap. Other support components would be also required, those are just the meat of it.

Are you certified to work with line powered items? If not, I'm unable to assist beyond the suggestion above, as it is potentially LETHAL if done incorrectly.

 

Alsol, a voltage divider followed by a peak detector and a comparator will work. The comparator will compare the peak voltage of the mains supply, compare it with a threshold value and drive a TRIAC accordingly.

 

Possibly better to use a transformer to step the AC down to 12 volts, make a power supply based on that, and use a window comparator to monitor the transformer output (rectified). The comparator can control a SSR to turn the AC line on and off according to the setpoints.

 

Or, maybe one of these will help you:

http://www.sola-hevi-duty.com/products/powerconditioning/cvs.htm

You can get one on ebay fairly cheaply.

John

 

Possibly better to use a transformer to step the AC down to 12 volts, make a power supply based on that, and use a window comparator to monitor the transformer output (rectified). The comparator can control a SSR to turn the AC line on and off according to the setpoints.

Why not 100V worth of DIACs aka "AC Zeners" on a Triac Gate? Sloppy, but quick. Too sloppy?

 

Thank you for your suggestions,
To give a little insight...

I have a voltage sensitive device- basically a fluorescent lamp- but the wiring to my work area goes through a dimmer. This causes my device to die because the commercial balast can only handel a 120v.

My hope is to ensure only 120v go to the destination- I am uninterested in any dimming effects...

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I am considering DIACs, but do they act as thyristors or zener diods?

That is if the voltage reaches 120v and then falls to 100v -will the current stop?

 

Diacs let voltage through once there is a potential of around 30-40V across them.

Now that I think of it, a TRIAC wouldn't work in your situation, as the Diacs would turn on for only half of each cycle.

Simpler solution: Remove the dimmers.