I have a beautiful old variac, with a sliding (rotating) center contact. I would love to use this on a nice LED string inside a glass bulb on top of the device. But also, use the variac as the dimmer, by rotating that glass bulb.
The contact points are bare metal, so mains is not an option. I have a nice AC/AC transformer for 240V -> 24V AC, That would be nice, but I have to limit the current. This LED string gives a nice light-output at 250mA, which flows at about 3.1 volts (it's 100pcs parallel, no datasheet available. Used to be battery operated, 3x AA with a 10ohm resistor).
What would be a good way to drop the voltage, keeping the Variac in circuit? I have been thinking of an capacitor after the coil, but I wouldn't know if the current-output would be constant at the frequency, or also depending on the variac-state?
Or would a decent resistor be the only option? (5.5 watt minimum, 20.9V at .250A)