In darkroom fix
I am in a good power state in my house, but am not in control of when the washing machine is running, the fridge, freezer, dehumidifier or furnace fan start. Then there is the roller processor wash dry unit dryer, which cuts in from time to time. All of these can cause voltage fluctuations.
I run the darkroom on its own circuit, and the roller processor on a sepearate circuit yet again.
The best fix was a surplus find. A .25kVA ferroresonant transformer. It lives in the darkrrom now, and powers the enlarger and nothing else. For $60 it seemed a small price to pay for peace of mind, particlarly when I was learning to print RA-4 using roller tubes for the chemistry. The last thing I wanted was variations in bulb brightness from print ot print, when trying to figure in what direction to go with filtration for the next print iteration.
No exposition on transformer regulation could be called complete without mention of an unusual device called a ferroresonant transformer. Ferroresonance is a phenomenon associated with the behavior of iron cores while operating near a point of magnetic saturation (where the core is so strongly magnetized that further increases in winding current results in little or no increase in magnetic flux).
the tex that follows has been snipped from the "all about circuit" web site:
While being somewhat difficult to describe without going deep into electromagnetic theory, the ferroresonant transformer is a power transformer engineered to operate in a condition of persistent core saturation. That is, its iron core is stuffed full of magnetic lines of flux for a large portion of the AC cycle so that variations in supply voltage (primary winding current) have little effect on the core's magnetic flux density, which means the secondary winding outputs a nearly constant voltage despite significant variations in supply (primary winding) voltage. Normally, core saturation in a transformer results in distortion of the sinewave shape, and the ferroresonant transformer is no exception. To combat this side effect, ferroresonant transformers have an auxiliary secondary winding paralleled with one or more capacitors, forming a resonant circuit tuned to the power supply frequency. This tank circuit serves as a filter to reject harmonics created by the core saturation, and provides the added benefit of storing energy in the form of AC oscillations, which is available for sustaining output winding voltage for brief periods of input voltage loss (milliseconds' worth of time, but certainly better than nothing).
Ferroresonant transformer provides voltage regulation of the output.
In addition to blocking harmonics created by the saturated core, this resonant circuit also filters out harmonic frequencies generated by nonlinear (switching) loads in the secondary winding circuit and any harmonics present in the source voltage, providing clean power to the load.
Ferroresonant transformers offer several features useful in AC power conditioning: constant output voltage given substantial variations in input voltage, harmonic filtering between the power source and the load, and the ability to ride through brief losses in power by keeping a reserve of energy in its resonant tank circuit. These transformers are also highly tolerant of excessive loading and transient (momentary) voltage surges. They are so tolerant, in fact, that some may be briefly paralleled with unsynchronized AC power sources, allowing a load to be switched from one source of power to another in a make-before-break fashion with no interruption of power on the secondary side!
Unfortunately, these devices have equally noteworthy disadvantages: they waste a lot of energy (due to hysteresis losses in the saturated core), generating significant heat in the process, and are intolerant of frequency variations, which means they don't work very well when powered by small engine-driven generators having poor speed regulation. Voltages produced in the resonant winding/capacitor circuit tend to be very high, necessitating expensive capacitors and presenting the service technician with very dangerous working voltages. Some applications, though, may prioritize the ferroresonant transformer's advantages over its disadvantages. Semiconductor circuits exist to condition AC power as an alternative to ferroresonant devices, but none can compete with this transformer in terms of sheer simplicity.
