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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)

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  • \$\begingroup\$ Not much of a "string" if it has the Vf of a single LED is it? \$\endgroup\$ – Spehro Pefhany Dec 9 '20 at 22:31
  • \$\begingroup\$ You can use variac as variable resistor in some timing circuit. PWM is the best choose to control LED brightness. \$\endgroup\$ – user263983 Dec 10 '20 at 1:46
  • \$\begingroup\$ @SpehroPefhany The string is a parallel string, so it actually is a led string of 100 pcs, and it can handle more current, but the nice output at 250mA is sufficient, so limiting at roundabout that would be fine. I'll add this to the question, thanks \$\endgroup\$ – ExploWare Dec 10 '20 at 7:46
  • \$\begingroup\$ Maybe you can hack something together with a few volts A/C into a full wave bridge rectifier feeding a resistive voltage divider to feed the input of a comparator driving a FET and get line frequency PWM out of it based on the fraction of the time the waveform is above the threshold. Or just see if you can excite and measure the variac with an MCU and then turn around and PWM a FET. In either case I'm saying run the LEDs off a DC supply just enough higher that you can use a current limit resistor. \$\endgroup\$ – Chris Stratton Dec 10 '20 at 8:13
  • \$\begingroup\$ an MCU would be an option, but I was struggling how to use the variac's AC characteristic on a digital input. But maybe the ADC for example of an ESP8266. which is commonly limited at 1V, could be used with a 27K and 1k2 voltage divider, and just multiple measurements. Keeping the 24VAC transformer as a supply, also supplying a rectifier and voltage regulator to the 5V MCU supply. Or should I even consider supplying an high-frequency output-signal from the MCU onto the Variac? I have no idea how to read that out :) \$\endgroup\$ – ExploWare Dec 10 '20 at 8:46

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