Doug, The circuit you're looking for actually comprises two separate functions. The first part is to separate the charge/power supply from the battery and radio, and that can be done with a Schottky diode. Why a Schottky diode, instead of an ordinary silicon power diode? Because the technology behind the Schottky diode means very low forward voltage drop, 1/2 to 1/3 that of an ordinary silicon diode, and that means that little voltage is lost in the diode- and less power loss.
The second part is a bit more difficult, that of disconnecting the battery when the voltage falls below a specified point. Some stations, such as the Motorola MTR2000, have an internal low-voltage disconnect and the point is moot. For the other, less intelligent stations, there is the Motorola HLN9455 Battery Revert Accessory. This module installs inside of GR1225, RKR1225, and GR400 X-Pand stations. Besides handling the low-voltage disconnect chore, it also provides a signal to the R1225 radio that the station is on battery power, and that causes a periodic tone to be broadcast. (The MTR2000 station has that tone capability built-in.) The HLN9455 is currently priced about $110, and it includes the Instruction Sheet 6880905Z74, which is not available separately. If you do not wish to buy the HLN9455 Battery Revert Accessory, I will provide just the circuit description and schematic upon request to my call at verizon dot net. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of dadavies3 Sent: Monday, July 30, 2007 3:54 PM To: [email protected] Subject: [Repeater-Builder] Low Voltage Disconnect Circuit Looking for a fairly simple low voltage disconnect circuit that will allow battery operation when the power supply/charger shuts down and will disconnect the battery when its voltage drops below a pre- determined level. I checked the files section but couldn't find anything. If anyone has such a circuit, please let me know. Thanks for any and all help. Doug VE7DRF
