Film-Tech Forum ARCHIVE: Super Lumex Ammeter

This is topic Super Lumex Ammeter - Resistor Values?? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Matt Close (Member # 996) on 12-03-2002, 10:54 PM:

I have a 150A ammeter from a Super Lumex lamphouse. The mech seems to be fine, but a couple of resistors inline with it are burnt out. If you look inside one of these ammeters, there is one resistor on one pole, and two resistors on the other.... it is those TWO resistors that are toast. Does anyone know what values they should be? ... And if not, does anyone have a super-lumex meter in their spares that they can pry open and check for me? .....
Would really appreciate it .... [thumbsup]

Posted by Paul G. Thompson (Member # 655) on 12-04-2002, 12:05 AM:

I don't know the resistor values you speak of are, but one thing for sure....before you replace the meter with a good one, it would be a good idea to make sure the meter shunt is in good condition.

Posted by Ken Layton (Member # 133) on 12-04-2002, 12:45 AM:

There is a 100k half watt resistor connected to the pushbutton switch for reading VOLTS (when pushed).

Posted by Paul G. Thompson (Member # 655) on 12-04-2002, 01:08 AM:

100K? whatever it was, it would have to work at it, unless it got frapped by the igniter....

Posted by Lindsay Morris (Member # 1430) on 12-04-2002, 04:01 AM:

Matt,

Being basically an ammeter the value of the resistors would be quite low and I am just a bit puzzled as to why they would Strong would have any resistors INSIDE the meter apart from the small coil of wire type resistor inserted by the manufacturer as part of the calibration process. Most ammeters will run without that resistor as it is only there to trim the movement to absolute accuracy and often was made of material that had the opposite characteristics of the shunt so that as the temp varied the meter accuracy stayed the same.
In the Super Lumex with the capability of reading volts as well as amps I would only expect to see the makers resistor and nothing else.
In the ammeter mode the shunt provides the loading for the movement so it reads amps and then when you press the Volts button R3 which is 150K ohm becomes the multiplier so the unit can read volts. R3 was mounted externally unless later models have 3 terminals on the meter... which I would hardly think would be the case as the amps side of things has to be disconnected to enable volts to be read.
This was a very common setup in a lot of telephone exchange power supplies when I used to work there some years ago.IE Normal position read amps (or milliamps as reqd) and press for volts etc.
So for my money I would just rewire the movement so it came out on the applicable terminals and hook it up and see what happens.
I have a Lumex lamp in bits right now and even though it does not have the Volts Press button the meter just looks like a stock standard Edge reading meter. The two leads just wander off to the shunt as I would expect to see... standard practice.
Without the small makers trimming resistor your meter may become a litte inaccurate as the temperature varies but not enuff to actually worry you about what you are seeing as amps on the scale. The inaccuracy maybe in the order of 5%.
Lindsay

Posted by Matt Close (Member # 996) on 12-04-2002, 06:37 AM:

Thanks for 'reading' my post Lindsay .... The resistors in this meter (inside this meter) are just 1/4W carbon resistors inline with the shunt.. i.e. outside terminal > resistor > shunt

On one side there a resistor, on the other side there are 2 in parallel. If they were a very, very small value, I guess I could bypass them and live with a little inaccuracy .... but if they are higher in value, I got problems [Wink]

So, I think I really need to find out what these values should be. As before .... if anyone has one of these meters floating around, could you maybe just pry off the top and take a look for me? .. pleeeeeease? [Big Grin]

Careful with that needle though!

Posted by John Eickhof (Member # 323) on 12-04-2002, 07:43 PM:

I believe they are not resistors, they may be RF chokes to supress the ignition spike.
Usually an ammeter reads directly from the shunt, the usual full scale reading is 50mv across the shunt under load. If the meter shows continuity, the devices are most likely good, they may appear discolored due tor the shellac coating applied when they were manufactured.