This posting gives an analysis of the INDICATOR AC/
PMG VOLTS instrument that can be found on the ELECTRICAL panel of the Lockheed L-1011. The picture below shows the location of the instrument. The large number scale shows the AC VOLTS and the small number scale shows the Permanent Magnet Volts. The
PMG scale is activated by pressing the
PMG momentary switch on the Electrical panel.
The instrument that I picked for the L-1011 project is actually a Boeing instrument Model 1822 P/N 60800113-3 made by
AMETEK.
The instrument enclosure, which consists of a tempered glass front and tube is held in place by four screws.
One of the screws is covered by an original temper seal put there by
Ametek after probably the last service the instrument received in January of 1992.
Once the instrument core has been pulled out of the tube, we can take a look at the forward facing elements:
Most instruments used on Boeing aircraft use a 3 bulb lamp that can be replaced by detaching the front part and the power cables that connect the 3 bulb lamp attachment (see image below).
The actual indicator needle is an electromagnetic indicator. The picture below also shows the lamp that goes between the electromagnetic core and the actual needle.
The needle indicator can be adjusted with two potentiometers that are located on the backside of the basic electronics board of the instrument.
Instrument Pinout
Instrument illumination using a 5V reference and the original incandescent light source.
In order to control the instrument, a variable power source has to be supplied to PIN3 and PIN4 of the instrument. The following pictures show the volts needed for specific indicator locations on the dial.
The following example shows the instrument indicating 120V AC or 100V
PMG by applying 2.4V on PIN3.
The idea is to drive the the instrument via a basic
Digital to Analog Converter (DAC).
