In a recent project I used relays to electronically control a multi-room speaker selector. I wired these relays in parallel with the physical switches in the box.
When I was designing it, I wasn't sure how to analyze how much current could be conducted. The amplifier is rated for 2x 25W RMS, speaker impedance 8 ohms; thus the theoretical maximum current given by P = I^2 / R is sqrt(25 W / 8 ohm) = 1.77 A. In practice, when I attempted to measure AC current at high volume with a multimeter, I saw <100mA. I didn't have a scope at the time, but I imagine that because it's not a sine wave, it'd be better to use an oscilloscope and measure the voltage across a shunt.
I ended up using relays rated for 2A contact current and random ~24 AWG wire I had on hand. The project works very well, but I'd like to better understand what is actually going on.
- Is the theoretical current above correct? Why is the measured current so much lower?
- Each room has an "impedance matching volume control." Does this mean that they simply put a resistor in series with the speakers, so that when multiple channels are on, the amplifier doesn't see <8 ohm impedance? (I can't find much information about how these work.) That could explain the low current.
- I'm frankly a bit surprised that a small amplifier like this can drive 5 rooms/10 speakers at a pretty high volume. The speakers and in-wall volume controls were professionally installed before we moved in. Is it likely that there is actually some powered amplification hidden in each room?