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
