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Subject: Re: How can I use 15vAC to trigger a relay?

Date: 10/23/00 at 6:40 AM
Posted by: Alan Dorman
E-mail: aldor@coam.net
Message Posted:

In Reply to: How can I use 15vAC to trigger a relay? posted by KDC on 10/01/00 at 10:24 PM:

Well the easiest way is to connect a 24 VAC relay to the circuit. The 15 VAC (18 VAC?) voltage should be enough to turn on the relay.
I did the DC route when I changed mine over. I needed to make a louder sound in order to hear it in my shop so I connected the two wires to the AC input of a Full Wave Bridge and across the + and - output of the bridge I connected a 470 Mf 50VDC electrolytic capacitor, polarity matching the FWB. When the doorbell switch is pressed the capacitor charges almost immediately. When it is released the capacitor discharges through the load, in this case a pulsing (beep, beep, beep) Sonalert and the beeps would fade off as the capacitor discharged. If you decide to use a DC relay as the load a 24 VDC relay should work well. Take the ((15 VAC times 1.414) minus 1.4) gives you the voltage that the capacitor will charge to. It is more than the RMS AC rating given for the transformer. If the relay appears to stay stuck on long after the doorbell switch is released then you need to connect a resistor across the + and - of the capacitor in order to discharge it fast enough. A 1 Watt, 1000 ohm resistor should work. Use the following formula to determine the discharge rate, T=RC. In this case 1000 (in ohms) x .00047 (in farads) = .47 seconds to totally discharge. Calculate the current flowing through the resistor using I=E/R or current = Voltage divided by resistance. Then, by knowing the current calculate the wattage of resistor you will need using P = I x E

Alan Dorman
aldor@coam.net

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