VOM Voltage

The most common AC voltage systems on aircraft use 115, 26, & 5 VAC. Cycles are 400Hz. Normal power sources are 3 phase engine driven generators or inverters.




Standard checks are basically verification of power being present or not. Heavy duty components such as pumps and electric actuators use 3 phase power. One phase missing or having low voltage will cause failures of these devices. Numerous electrical and avionics related components require 115 VAC as a primary power source. Signal voltage standard is 26 VAC. Sensors and transducers modify this voltage in relationship to their position or angle as feedback to associated computers/indicators. 5 VAC is a typical voltage for instrument and panel backlighting. Some external lights use 10 VAC for power. Step-down transformers are used to change 115 VAC to the lesser voltages.




28 VDC is used for both power and control functions. Transformer rectifiers are used as a power source on AC generator aircraft. Some aircraft have DC as primary power coming directly from engine driven generators.
The majority of relays and solenoids use 28 VDC.




One helpful tip when checking power with a meter in AC or DC, is the scale at the bottom of the display. The sample rate for the digit display is not fast enough at times. If voltage is varying or intermittent, the scale is a better source for observing this problem.

Do you use the MIN/MAX Record function much? The Flukes are pretty good, some can catch a 100millisecond glitch when the meter is configured correctly. In automotive, one of the best uses for a voltmeter when chasing electrical gremlins is voltage drop tests.I found lots of techs stray from it, because they don't understand how to perform the test....but it lets you test for resistance in a system while its powered and operating, which can't be done with a ohm test.

On occasions, I'll use the min/max function. I'm not to familiar with that many systems that vary voltage as a means of control. Although...... I believe servo control and direction commands "do" use variable inputs to the valves.

A/C signal synchos certainty have a variable voltage output on each leg, but monitoring would be difficult without previously "mapping" outputs to know voltage ranges for each syncro system.