jamesm Posted January 12, 2022 Report Posted January 12, 2022 (edited) Hi, Since my '67c was manufactured before radio master switch was OEM installation. I have started looking into design radio /avionics master switch in a '67c model. I have noticed that the later year (mid 70's) C model's have a switch radio master switch I think 21P0B6 (could not read the prints) From my calculations max current draw is about 23 amps @14Vdc. Which if I add a switch de-rate factor of 1.3 pushes near 30 amp rating dc switch contact rating. Short of installing a radio master relay and switch (more complexity) or a circuit breaker type switch. Which from what I have seen so far other installations or manufacturers appear to be switch type circuit breaker as avionics radio master switch. Is there any other installations out there were the avionics radio master switch is actual switch not circuit breaker type switch or relay and switch combination? What is the common practice for avionics radio master switches? I am looking for toggle switch. Edited January 12, 2022 by jamesm Quote
Marauder Posted January 12, 2022 Report Posted January 12, 2022 Hi, Since my '67c was manufactured before radio master switch was OEM installation. I have started looking into design radio /avionics master switch in a '67c model. I have noticed that the later year (mid 70's) C model's have a switch radio master switch I think 21P0B6 (could not read the prints) From my calculations max current draw is about 23 amps @14Vdc. Which if I add a switch de-rate factor of 1.3 pushes near 30 amp rating dc switch contact rating. Short of installing a radio master relay and switch (more complexity) or a circuit breaker type switch. Which from what I have seen so far other installations or manufacturers appear to be switch type circuit breaker as avionics radio master switch. Is there any other installations out there were the avionics radio master switch is actual switch not circuit breaker type switch or relay and switch combination? What is the common practice for avionics radio master switches? I am looking for toggle switch. I’ve never seen one just as a switch. My 1975 F didn’t have a radio master as well. The mechanic went with the relay - switch combination. Sent from my iPhone using Tapatalk Pro Quote
Skates97 Posted January 12, 2022 Report Posted January 12, 2022 Somewhere over the years someone installed a circuit breaker type switch as an avionics master in my 64D. I would have to dig back through the log books or stick my head up under the panel to see what the amperage is but it is one of these. https://www.aircraftspruce.com/catalog/elpages/pbcircuitbrkr2.php Note that they do not have FAA approval but were OEM in certain planes, not sure that the Mooney is one of them, a conversation to have with your AP/IA. 1 Quote
EricJ Posted January 13, 2022 Report Posted January 13, 2022 A switch and relay is nearly always done as best practice for high-current applications like you are describing. This moves high current faults away from the user contact and usually reduces cost. 1 Quote
carusoam Posted January 13, 2022 Report Posted January 13, 2022 Read up on the failure resistant methodology of the avionics bus… Not only is there a switch and a relay… but the relay and wiring is selected so that…. When the relay loses a wire in the middle of the flight… it reverts to a state that still supplies power to the electronics… European standards have taken that to the next level and supply another switch and relay to the system… in case the power wire flat out falls off… or the relay fails… For more info on how the system works in modern Mooneys…. The explanation is around here somewhere… Modern wiring diagrams will also have the layouts… Of course, this is more critical if you fly in IMC… Best regards, -a- 1 Quote
Yetti Posted January 19, 2022 Report Posted January 19, 2022 (edited) I think I figured that the relay in the 75 F was under rated amp wise with the old power hungry radios. 10Amps per radio Which is why someone by passed it at some point. You will find the diagram has the relay normally closed and then power is supplied when the master in turned off opening the relay. When the radio mast switch is turned on power is removed from the relay and the normally closed condition is met and the radios are powered. Using a relay allow a wire from the start circuit with a diode in it to turn off the radios when starting. Probably not a thing now with modern avionics as the Dynon has one display on the Master circuit to monitor the engine during start up. You can note that a standard bosch relay has a NC circuit in it. It also has a diode to keep the voltage spikes at bay when switching. Edited January 19, 2022 by Yetti 1 1 Quote
AIREMATT Posted January 19, 2022 Report Posted January 19, 2022 (edited) Not sure if it’s OEM or not but this is along the bottom of pilot side switch row on my 67C. Edited January 19, 2022 by AIREMATT Add photo 1 Quote
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