File:UPC1237 speaker protection 3. Add delayed muting relay.png

English: Use of uPC1237 speaker protection IC. 3. Adding a delayed muting relay. The idea is to keep the input signal exactly at zero when the main relay turns on and off. This (hopefully) eliminates arcing and extends the life of the relay. With +30V rail voltage, the as-built delay is about 0.15 s at power-on and under 0.1 s at power-off. This, of course, depends on the relays used - in my case, Songle SMIH-24VDC-SL-C for speakers and Takamisawa RY24W-K for muting. The power-on delay is easy to set (RC to the base of transistor), but the power-off isn't. Quite counterintuitively, in this circuit at +30V power voltage the muting relay turns off faster than the main speaker relay. At +45V, however, the delay is almost imperceptible (tens of milliseconds, but same sequence). I tested this only at zero DC current through the relays' contacts; presence of substantial currents will almost certainly increase the turn-off delay. Breaking currents always takes time and effort.
Tip: if the muting relay had two sets of contacts (as is the case of Takamisawa RY series), only one set is necessary for muting. The other set may conveniently switch the on/off LEDs which may be of a common-anode combination type.
Update: The turn-on delay network R5C4 (same as in datasheet) considerably slows down the turn-on transient of the uPC1237's output transistor. This requires a fairly hefty C5 of at least 470uF. With 220uF the circuit may not work correctly, to the point where the relays won't turn on at all. The only way to ensure relay turn-on is to increase C5 to 470 uF; this certainly works with 100 uF timing cap. Likewise, if you feel like increasing turn-off delay, increase C5 accordingly.