If you are concerned about your batteries lasting as long as possible when running on backup, I would NOT use a relay. Even at 100ma of current draw, that's a LOT of current in a backup / solar situation. Since the RF PA only draws current when in transmit, (Class 'C') you can hook it directly to the battery all the time when line voltage is not present. This can be done easily with normally closed contacts on a relay that is fed from the AC line.
Have a look at the cutout circuitry of this solar charge controller: http://www.solorb.com/elect/solarcirc/spc3/ They use a voltage comparator to do the switching. I would think a circuit could be designed that simply used a 10V zener diode and a transistor to feed the power FET. Probably until you would go through all of that, you could have the comparator circuit built and ready to go. The moral of the story is that a comparator and power FET switch would draw FAR less current than using a relay coil. You could also set the comparator EXACTLY where you want the circuit to turn off. Be sure that the hysteresis resistor is a small enough value so that when the circuit cuts out with the transmitter active, it won't turn back on when the battery voltage recovers. Of course, this could be tailored to taste. Scott Scott Zimmerman Amateur Radio Call N3XCC 474 Barnett Rd Boswell, PA 15531 ----- Original Message ----- From: "Al Wolfe" <[EMAIL PROTECTED]> To: <Repeater-Builder@yahoogroups.com> Sent: Tuesday, October 21, 2008 9:15 AM Subject: [Repeater-Builder] Re: Shutting Down Battery Back up > Don, > Low battery voltage shut-down can be done quite simply using three or > four parts: a normally open contact relay, a resistor, and a cap. The > battery goes to one side of the N. O. contact and the load (repeater?), > power supply, and relay coil to the other N. O. contact. The other side of > the coil goes to ground via a resistor. The resistor value is determined > empirically and should be selected so that the relay will drop out at > about > ten volts or whatever you decide is the minimum useful voltage. Use a > variable DC supply to determine the proper resistance value. > > You should be able to find a 12 volt relay that needs less than 100 ma. > through the coil. An appropriate resistance might be 22 or 27 ohms for > this > relay. > > A 1000 MFD cap should be in parallel with the resistor so that when > power is restored from the power supply, full voltage is applied to the > relay coil for a time to energize it. The resistor also lowers the power > used by the relay to a degree. The ubiquitous chatter diode across the > relay > coil is a good idea as well. > > I have used this method for several applications at work and it is > virtually fool proof. > > Good luck, > Al, K9SI > > > >> Now the question and I have not seen this talked about I would >> assume all I would need is a Normally closed Relay and as the Voltage >> dropped below >a Certain Level it would open and just break the >> connection to the Battery back up , Is this the way to do it > > >> Thanks > > >> Don KA9QJG > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.173 / Virus Database: 270.8.1/1731 - Release Date: 10/17/2008 7:01 PM
