Power On Delay/Loss of AC Detector Relay - diyAudio
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Old 25th January 2013, 12:33 PM   #1
weslito is offline weslito  United Kingdom
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Default Power On Delay/Loss of AC Detector Relay

I'm working on a power on/off relay circuit for any line level equipment with AC power in and onboard DC supply.

It has a power on delay of ~1s (set by R6/C2), as well as AC detection to turn the relay off before the DC supply collapses, eliminating nasty noises/DC on power down.

During normal operation, AC enters through D1, keeping C1 discharged through Q1. As soon as the AC power is lost, Q1 stops conducting, C1 charges through R4 and turns on Q2. This rapidly discharges C2 and steals the base current of Q3 which subsequently opens the relay. A zener diode D4 is used to reduce the turn off time of the relay.

Figure 2. shows the turn on delay. Red = C2 voltage, Green = relay coil current.
Figure 3. shows the AC (blue) cutting out at 6s. Time until the relay (green) turns off is ~20ms on the simulation.

I suspect R4/C1 may need a bit of tuning to ensure Q2 stays off during normal conditions, 47K might be better.

Think it'll work?
Attached Images
File Type: jpg Figure1.jpg (112.0 KB, 165 views)
File Type: jpg Figure2.jpg (91.7 KB, 143 views)
File Type: jpg Figure3.jpg (100.8 KB, 136 views)
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Old 26th January 2013, 02:49 PM   #2
rsavas is offline rsavas  Canada
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Looks very similar to what what Doug Self has used in his books. He has provided improvements to the same basic ckt that you have posted. I have also referenced Pioneer stereos as another methods of doing a discrete protection/turn-on delay ckt. Look at SX-1250, SPEC-1 pre-amp and others of the same error, schematic/service manuals can be obtained at
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