Alternator field circuit breaker

[ltdoyle]

My strobe power supply on the left wing died on my 1977 M20J.  I replaced it with. Whelen A490TCF unit.  After wiring it up, I checked it for operation and found it worked fine.  The next day when I went to fly, I noticed a low voltage warning, and a slight discharge on the ammeter.  I found the alt field breaker tripped, and reset it.  A few minutes later it tripped again.  Once again it tripped, so I landed,  I disconnected the power line to the strobe power supply and did another flight.  This time there was no problem with the breaker.  Wiring appears ok, bad power supply?

[PT20J]

The warning annunciator blinks for low voltage and illuminates continuously for an over voltage. If there is an over voltage, the relay in the regulator trips the alternator field breaker. Sounds like something is causing an over voltage at the regulator. 

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[jetdriven]

The OECO voltage regulator in the early J models has a crowbar circuit in it. If case of an over voltage, the crowbar trips the Voltage regulator (and field current) off. The field CB doesn’t trip, but you must cycle the breaker to reset the voltage regulator. 

[ltdoyle]

Thanks Jetdriven. That is exactly what is happening!   I just have to figure out what is causing the power supply to cause an over voltage!

[PT20J]

1 hour ago, jetdriven said:

The OECO voltage regulator in the early J models has a crowbar circuit in it. If case of an over voltage, the crowbar trips the Voltage regulator (and field current) off. The field CB doesn’t trip, but you must cycle the breaker to reset the voltage regulator. 

Good point. The service manual no longer mentions that VR, but I remember having to replace the power transistors in it years ago when I had a ‘78. Also remember it was a bear to get in and out!

[ltdoyle]

Had a broken ground on the power unit, problem solved.....

[Browncbr1]

It’s always a dang ground!

[carusoam]

Great follow-up Ltd.

Thanks for sharing the detail.

Best regards,

-a-