loudspeaker protection and muting circuit trouble :( - diyAudio
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Old 21st March 2004, 01:52 AM   #1
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Question loudspeaker protection and muting circuit trouble :(

Ok, I built Rod Elliots Loudspeaker protection and muting circuitry and its not working properly.

http://www.sound.westhost.com/project33.htm

i used a BD140, 2 mpsa42's and 1mpsa92 for the transistors, and the mpsa are wired backwards. ...and hence they are inserted backwards. The reason I used them is because my PS is +-70vdc...which is a little higher than Rod's recommended voltage. Ok, here's the problem. I have an LED helping me indicate power output to the relays. When I turn AC power on, nothing happens, relays don't flip. If I attach 70vdc to the AC input terminal, relays flip and STAY flipped (as opposed to only flipping for 2 seconds and flipping back). I haven't even tried the DC dectection yet, since AC doesn't seem to be working properly. since i have gotten the relays to flip somehow, i'm pretty sure the relay resistor is correct, but somewhere i put the wrong component in....though I kinda doubt it. Its a simple circuit. Can anyone have a look at it and let me know where my error might be? Thanks!

-Matthew K. Olson
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Old 21st March 2004, 02:18 AM   #2
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Ok, never mind. My trouble was that I was taking the AC from the line ac....instead of after the transformer at the bridge rectifier. Also, the circutry is a little backwards from how I imagined it, I thought that if DC was detected...the relays would flip from rest to switched. Instead, the relays flip from switched to rest position. so instead of me having my LED come on only when DC is detected....its on all the time and when DC is detected it goes off. Hmm. have to figure out something there. later!

-Matthew K. Olson
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