If anyone wants to take this entire comment thread and boil it down to an article for the Tech Info page at repeater-builder let me know.
Mike At 06:26 PM 03/26/08, you wrote: >The protection diode is typically across the power leads to the radio >being protected (in parallel) , and not in series wit the radio because >of both the forward drop of a silicon diode (around 0.7 V depending on >the characteristics of the diode) , and the current carrying capability >of the diode. > >The Cathode of the protection diode connects to A+. The Anode of the >protection diode connects to - which when you use the bottom battery in >a string of 2, 12 volt batteries is chassis / frame Ground. Much like a >Zener that clamps -this clamping voltage is what a distinguishes a >transorb from an ordinary protection diode. > >When both batteries are alive and well, the 2, 12 volt batteries in >series form 24 volts which when applied through the solenoid, to the >starter motor, and back to chassis form a complete circuit or loop. > >When someone forgets and leaves the radio turned on, connected across >the bottom 12 volt battery and discharges the bottom battery, the top >battery remains charged. > >The bottom or discharged battery is depleted and cannot power the radio >or aid or oppose current through the starter motor. It is as though the >bottom battery were removed. > >Now when the operator engages the starter motor, only the top battery >has the potential to cause current to flow. > >This current flows from the + post of the top battery, thru the >solenoid, thru the starter motor, thru the radio (the battery that the >radio is connected across is dead and for all practical purposes does >not exit) and completes the circuit at the - post of the top battery. >(I don't want to start an electron current vs conventional current >discussion). > >The starter will try and draw several hundred amps. The radio and >protection diode can only handle maybe 20 amps, and then only for a >brief period of time. > >If the radio is fused, the fuse should blow. If the radio is not fused, >the radio will blow, because the current carrying capability of the >protection diode is no match for the current carrying capability of a 24 >volt starter motor and 500 mcm cable... > >I hope this makes sense, Dave, 73, Steve NU5D > > >Dave Gomberg wrote: > > At 09:56 3/25/2008, Steve S. Bosshard (NU5D) wrote: > > > >> Anyhow, without a fuse, the reverse protection diode or transorb in the > >> radio tries to short and shunt the reverse current from the radio. > >> > > > > So you are saying it is in parallel (and reverse direction), not > > series (in forward direction)????? > > > > > > > > > > >------------------------------------ > > > >Yahoo! Groups Links > > >
