Hughes 269A Ammeter Shunt

[Helofxr]

I have a Hughes 269A with the 24 volt system. The ammeter shorted out and caused an open in the circuit board on the instrument cluster itself. The maintenance manual shows a "shunt wire" on the ammeter, however I am unable to locate any other information about it.
It is obvious that a shunt should be there to protect the instrument cluster, however I have no idea where it is physically located, nor can I find any part number listing for a replacement.

Need some help keeping the magic smoke in the cluster!

[MurphyWasRight]

Shunt wire might refer to the current sensing resistor.
Although I dont know specifics of this particular unit a typical meter movement is 1ma fs (Full Scale).

The shunt converts this fs to 50A (or whatever) by "shunting" most of the current past the meter.
The value of the shunt is determined by the ratio of coil resistance of meter to shunt and meter fs sensitivity to the desired fs.

The ideal ammeter would have 0 ohms resistance, the shunt for a 50A meter will be lower than many ohmmeters can resolve so I am not sure what "the ammeter shorted out" means.

Now if the "meter" was on the pcb and the shunt was external and -opened- then the high current might have melted traces on the board depending on circuit design.


BTW: The above is for an analog meter, in a digital meter the (very low) voltage drop accross the shunt is used to measure current.

[Lone_Ranger]

Ammeter shorting out would'nt cause open circuit to other instruments unless its tripped/blown something else, investigate further

[Helofxr]

Ammeter

Thanks for the input.

All I have to go by is the maintenance manual. I will have to find me someone with some 269A time and see if they can point me to the correct location of the shunt or current sensing resistor. The circuit board melted (shorting out).

[phiggsbroadband]

Hi Helofxr, lets assume your meter reads 50A FSD, and that it requires 0.1V to move the neddle to FSD. Then your Shunt Resistor will be V/I=R..
or 0.1/50 = 0.002 Ohms.
This is an extremely small resistance and could be achieved by say 8 inches of 100A wire....


So don't go looking for ceramic insulated resistors that have colour codes...

[jxk]

Out of interest.....

On the Piper PA28's the shunt was made by tapping into the alternator/battery cable, the connections about 4 ft apart.