Somebody school me on amp meters.

[Timbeck2]

I want to replace the amp meter in my plane to one with a choice between volts/amps with discharge and overcharge warning lights. I've found the one I like but I don't know what internal vs external shunt means or how to tell.

1970 Cherokee 140

Link to Spruce amp meter choices:

http://www.aircraftspruce.com/catalog/inpages/ei-digitalgraphic4.php


There are other reasons why I want a new amp meter. The old one looks like it has rust on/in it, it isn't internally or externally lit and rather than running a Nulite over to it I'll just buy one that is internally lit.

 

[bnt83]

I want to replace the amp meter in my plane to one with a choice between volts/amps with discharge and overcharge warning lights. I've found the one I like but I don't know what internal vs external shunt means or how to tell.

1970 Cherokee 140

Link to Spruce amp meter choices:

http://www.aircraftspruce.com/catalog/inpages/ei-digitalgraphic4.php


There are other reasons why I want a new amp meter. The old one looks like it has rust on/in it, it isn't internally or externally lit and rather than running a Nulite over to it I'll just buy one that is internally lit.

Installation manual page 8

http://buy-ei.com/wp-content/uploads/2013/06/OI-II-VA-1A.pdf

 

[bnt83]

 

[Dan Thomas]

The ammeter's movement (or digital processor) itself is incapable of measuraing large amperages, so a shunt is used to carry the big current and the meter measures the voltage drop across it. That drop is proportional to the current it is carrying. That shunt might be built into the meter (internal) but the big wire feeding the bus has to pass through it. An external shunt is usually installed somewhere more convenient and small wires bring the voltage drop reading to the ammeter.

The shunt is just a metal bar having a very low, calibrated resistance, mounted on an insulating block.

 

[Timbeck2]

An external shunt is a strip of metal, usually mounted on a bakelite base.

Well that certainly narrows the search doesn't it?

In my defense though, I didn't read the installation instructions because I didn't know which one to install; and I still don't. Thanks Brian.

 

[bnt83]

Well that certainly narrows the search doesn't it?

In my defense though, I didn't read the installation instructions because I didn't know which one to install; and I still don't. Thanks Brian.

I can't find the piper maintenance manual handy so I can't really say which you have. I'd guess its an internal shunt. You can tell by looking at the back of the instrument for two big ass wires. Two big wires means internal shunt.

Think 8 gauge wires. Most of the wiring coming off circuit breakers is smaller than 16 gauge. Battery cables are around 2 to 0 gauge.


If external shunt, you'll need to find the shunt to match the new instrument to it. Only small wires on the instrument = external shunt.

 

[Timbeck2]

Thanks again Brian. I'll do just that.

 

[kgruber]

Old cars with points had "ballast resistors," which was just a shunt used while starting to keep the points from burning.

 

[Timbeck2]

On our 70-something Plymouth Duster, that 99 cent item cost us an overnight stay in Memphis. After that, Dad kept a couple in the glove box just in case.

 

[Dan Thomas]

Old cars with points had "ballast resistors," which was just a shunt used while starting to keep the points from burning.

No, that ballast resistor was to aid starting, not to save the points. It was in line with the power to the coil (which is switched by the points, after the coil, to ground), and the coil was designed for an operating voltage of about 8 volts. The resistor dropped the voltage to that level for normal operation. When starting, the resistor was either shorted by a separate terminal on the starter contactor, or it was bypassed by a start line from the ignition switch so that the coil got the full battery voltage, which often dropped to 8 volts or so during the heavy loads of cranking the engine. That way, the spark plugs made nice hot sparks during start.

 

[BigBadLou]

No, that ballast resistor was to aid starting, not to save the points. It was in line with the power to the coil (which is switched by the points, after the coil, to ground), and the coil was designed for an operating voltage of about 8 volts. The resistor dropped the voltage to that level for normal operation. When starting, the resistor was either shorted by a separate terminal on the starter contactor, or it was bypassed by a start line from the ignition switch so that the coil got the full battery voltage, which often dropped to 8 volts or so during the heavy loads of cranking the engine. That way, the spark plugs made nice hot sparks during start.

+1

Don't ask me how I know. Okay, okay! I still got one on the boat.