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#1
November 5th 03, 02:55 AM
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#2
November 5th 03, 07:23 AM
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The 5 amp field breaker in the 1976 Cessna 172M does NOT have the alternator field current flow through it. It has only the field relay coil current flow through it which is about 1/10 of an amp. The alternator field current comes from the output terminal of the alternator through the field relay and the switching transistor in the regulator. There is NO fuse or breaker in this circuit unless you call the 60 amp alternator breaker the field breaker. Look it up, you will be surprised. The schematic of the regulator is shown in the Cessna "Alternator charging Systems"Manual. The airframe wiring is shown in the airframe service manual. Cessna made a mistake in my opinion. Ask your self what will happen if the 60 amp breaker opens under nearly full load. Will the field switching transistor that normally operates at 14 volts be able to take the 90+ volts without shorting? Will the over voltage relay be able to break the 90+ volts DC on a relay that has contact spacing designed to break 14 volts? If the above two answers are no then you have just applied 90 volts or more to the field winding and the regulator. I can tell you this it does not take long to fry the regulator and the field into a crispy critter once this occurs. I know for sure that if this happens just after take off you will not be able to come back and land before the regulator and the alternator is fried. The 14 volt over voltage light is really bright with 90 volts applied, but not for long! When I was looking for the 60 amp breaker the largest I could find in that case size was 50 amps so I had to buy the Cessna over priced part. This circuit really needs a 70 amp breaker since the 60 amp rated alternator will generate more than 60 amps when the windings are cold. You are correct that if the breaker popped during landing then there may be something loose in the wiring or a sharp metal edge cutting through a wire's insulation. John Frerichs On 4 Nov 2003 17:55:59 -0800, (Dan Thomas) wrote: wrote in message ... You need to replace the main 60 amp breaker. This is a thermal type breaker and as it gets older the contacts build up oxidation and self heat more reducing the breaker rating. You are lucky!! When my 60 amp breaker popped one night the load dump fried the alternator field, the voltage regulator and the over voltage lamp. I measured the 60A breaker and it would pop at 35 amps. Cessna wants lots of money for the 60 amp breaker but it still is lots cheaper than a new alternator and regulator. On a C172M the 5 amp field breaker will NOT protect the alternator field winding since it is not in series with the alternator field. It protects the over voltage relay that draws only 0.1 amps. The 60 amp breaker can be bought from someone other than Cessna, like Aviall, for a lot less cash. The 5 amp breaker in the 172M feeds the regulator and therefore the field. We have two of these airplanes. The overvolt sensor disrupts the feed if the voltage gets too high. A good way to burn out an alternator field is to turn the master on when preflighting, then forget to turn it off. The field is being fed full voltage (since the alternator isn't producing anything) and gets really hot (since the alternator isn't turning and drawing cooling air through itself). If the original poster's breaker popped during a landing, I might expect to find some loose bit of hardware dancing around behind the panel and tangling with the bus bar. Wouldn't be the first time. Dan
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#3
November 5th 03, 07:35 PM
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