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)????? > > > >
