Battery Master Solenoid (Contactor) Diode
Posted: Wed May 01, 2002 12:31 am
Later Cessna's came out with solid-state avionics and began to suffer excessive radio failures. This was ultimately traced to voltage spikes that occur when the battery Master switch was disengaged with radios still online. Early airplanes had their radios wired to be switched on and off individually rather than through a radio-master. Many Cessna 170's are still wired that way. If you have a radio/avionics master-switch then doubtlessly you turn it off before shutting down (except when you suffer senior moments and forget.)
At any rate, Cessna came up with a Service Letter that addressed the issue (the exact SL # escapes me just now......what was it I was talking about??) Oh, yeah,...anyway the fix was to put a diode at the Battery relay/solenoid/contactor. This diode is attached to the small terminal that leads to the actual master switch and also to the large terminal that comes to the relay from the positive battery terminal. The diode points toward the positive battery cable terminal.
What was happening was that the electro-coil within the relay (solenoid) gets it's power directly from the positive battery cable, and it's circuit is completed to ground through the actual aircraft Master switch in the cockpit. This grounding of the coil activates the electromagnet formed by the coil and pulls the large internal contactor closed to complete the circuit across the relay, thereby sending battery power onto the main aircraft electrical buss.
When at the end of a flight, the pilot turns the Master switch back to the off position, the ground circuit of that coil is broken and a large voltage "spike" occurs as that coil's magnetic field collapses. This happens almost as fast as most central-Texas "B"-models. The 600+ volts formed by that spike gets on across the relay and into your aircraft's electrical buss faster than the relay contacts can physically open, which gives any appliances that are still online a 600+ volt shock! So, your avionics that are still online, as well as your strobe power supply, and your digital clock, etc., etc., are taking a beating, ...unless that diode is in place. That diode allows that 600 volts to find a place to be absorbed in the ship's battery (which is slower than a Grand Prairie "A"-model to re spond to large voltage spikes, and the actual current is small, so it isn't harmed.)
In other words, if you don't have such a device wired across from your battery master relay's small terminal to it's large terminal that the battery positive cable is attached to,...then your radios, etc., are at risk.
These diodes can be bought expensively I'm sure, from Cessna, or you could get one cheaply from Radio Shack (Cat. No. RSU 11928397) for about 89-cents.
Or you could do what I did. I mentioned it to Wendell Wyborny who promptly produced a variation on the theme for me. He used a small 18v/100Amp MOV (Metal Oxide Varistor) to accomplish the same thing. It was only slightly larger than a pin-head. (MOV's will equalize large voltage differences across their leads, but will not bother with voltages that are essentially the same. In other words, while the battery relay is ON, the MOV is like an open circuit. But when the voltage spikes high when you switch the relay off, then the MOV shorts the excess voltage back to the battery positive terminal. There's no advantage to using a MOV other than they are not polarity sensitive, so if you made certain that the diode is not installed backwards, then you'll be fine.)
Remember, you want the current to flow from the small terminal toward the large postive battery cable terminal. (The diode will be marked to indicate direction of flow.... >| means the current will flow to the right.)
At any rate, Cessna came up with a Service Letter that addressed the issue (the exact SL # escapes me just now......what was it I was talking about??) Oh, yeah,...anyway the fix was to put a diode at the Battery relay/solenoid/contactor. This diode is attached to the small terminal that leads to the actual master switch and also to the large terminal that comes to the relay from the positive battery terminal. The diode points toward the positive battery cable terminal.
What was happening was that the electro-coil within the relay (solenoid) gets it's power directly from the positive battery cable, and it's circuit is completed to ground through the actual aircraft Master switch in the cockpit. This grounding of the coil activates the electromagnet formed by the coil and pulls the large internal contactor closed to complete the circuit across the relay, thereby sending battery power onto the main aircraft electrical buss.
When at the end of a flight, the pilot turns the Master switch back to the off position, the ground circuit of that coil is broken and a large voltage "spike" occurs as that coil's magnetic field collapses. This happens almost as fast as most central-Texas "B"-models. The 600+ volts formed by that spike gets on across the relay and into your aircraft's electrical buss faster than the relay contacts can physically open, which gives any appliances that are still online a 600+ volt shock! So, your avionics that are still online, as well as your strobe power supply, and your digital clock, etc., etc., are taking a beating, ...unless that diode is in place. That diode allows that 600 volts to find a place to be absorbed in the ship's battery (which is slower than a Grand Prairie "A"-model to re spond to large voltage spikes, and the actual current is small, so it isn't harmed.)
In other words, if you don't have such a device wired across from your battery master relay's small terminal to it's large terminal that the battery positive cable is attached to,...then your radios, etc., are at risk.
These diodes can be bought expensively I'm sure, from Cessna, or you could get one cheaply from Radio Shack (Cat. No. RSU 11928397) for about 89-cents.
Or you could do what I did. I mentioned it to Wendell Wyborny who promptly produced a variation on the theme for me. He used a small 18v/100Amp MOV (Metal Oxide Varistor) to accomplish the same thing. It was only slightly larger than a pin-head. (MOV's will equalize large voltage differences across their leads, but will not bother with voltages that are essentially the same. In other words, while the battery relay is ON, the MOV is like an open circuit. But when the voltage spikes high when you switch the relay off, then the MOV shorts the excess voltage back to the battery positive terminal. There's no advantage to using a MOV other than they are not polarity sensitive, so if you made certain that the diode is not installed backwards, then you'll be fine.)
Remember, you want the current to flow from the small terminal toward the large postive battery cable terminal. (The diode will be marked to indicate direction of flow.... >| means the current will flow to the right.)