I have a Hometics Zen table-top water fountain consisting of a small AC water pump that circulates water through a display. I want to add some LEDs to brighten up the unit. Before I construct an LED circuit I'm looking for some expert technical advice.
The display right now has a wall-mounted 12 Vac 200ma transformer. At no-load voltage it puts out 16.1 Vac. The 12 Vac 2W motor is connected through a switch to the transformer. I want to add two opposite polarity series legs of two blue and one white LED in parallel with the motor. A capacitor in each LED leg drops any excess voltage. I realize that I won't get full brightness due to the a lesser RMS voltage compared to a plain DC circuit.
The blue LEDs have a 3.5 volt drop at 20 ma. The white LEDs have a 3.5 volt drop at 30 ma. Assuming 20 ma current through each leg and no-load output, I calculate a 5.6 Vac drop across the capacitor. At 60 Hz, that's 280 O impedance; a 9.5 microfarad capacitor should drop the correct voltage.
My questions:
1. Am I out to lunch with my circuit? I'm reaching back to my Navy days with AC circuit theory, so I'd like a backup here.
2. Is my capacitor value close?
3. With a 10 ma operating current difference, will the white LEDs even light? Or will the white LED overdrive the blue LEDs?
4. Will there be any issue with each LED leg during the reverse AC cycle?
Thanks for everyone's help with my first circuit engineering in years.
The display right now has a wall-mounted 12 Vac 200ma transformer. At no-load voltage it puts out 16.1 Vac. The 12 Vac 2W motor is connected through a switch to the transformer. I want to add two opposite polarity series legs of two blue and one white LED in parallel with the motor. A capacitor in each LED leg drops any excess voltage. I realize that I won't get full brightness due to the a lesser RMS voltage compared to a plain DC circuit.
The blue LEDs have a 3.5 volt drop at 20 ma. The white LEDs have a 3.5 volt drop at 30 ma. Assuming 20 ma current through each leg and no-load output, I calculate a 5.6 Vac drop across the capacitor. At 60 Hz, that's 280 O impedance; a 9.5 microfarad capacitor should drop the correct voltage.
My questions:
1. Am I out to lunch with my circuit? I'm reaching back to my Navy days with AC circuit theory, so I'd like a backup here.
2. Is my capacitor value close?
3. With a 10 ma operating current difference, will the white LEDs even light? Or will the white LED overdrive the blue LEDs?
4. Will there be any issue with each LED leg during the reverse AC cycle?
Thanks for everyone's help with my first circuit engineering in years.