October 30, 2012

This month we are going to look at ways to measure power simply and accurately. Let us begin with a little background review. Energy is supplied in the form of chemical or mechanical energy. Energy is measured in joules and is dissipated in the form of power. The unit of power is the watt and is defined as the use or generation of one joule of energy per second. Power is also defined as one volt of potential pushing one ampere of current through a resistance. Therefore,

P = E x I Where:

P = Power in Watts

I = Current in Amperes

E = Electrical potential in volts or the RMS value. The RMS value is the AC waveform that produces the same heating value as the comparable DC voltage. Example: a 2.8 volt peak-to-peak sinusoidal signal has the same heating capacity as 1 volt DC .

Substituting Ohm’s law into the equation I = E / R yields P = E^{2} / R or E=√PR

Substituting Ohm’s law into the equation E = I x R yields P = I^{2} R

And lastly substituting ( 0.707 x Peak ) for E yields P = 0.5 E^{2}Peak / R or P = E^{2} Peak/ 2R or E Peak = √2RP

This last equation is handy for measuring power when using a scope. By reading the peak value, the calculation is simplified.

In our industry we often measure simple audio signals, two common ones being speaker and phones. Historically we have used devices such as the General Radio Series of audio wattmeters i.e. 583A,1840A. While these devices perform well they require an initial capital outlay (they cost money!) , they are limited by their frequency and power capabilities, and they require calibration on a regular schedule. By keeping a ready supply of resistors on hand and the formulas above you can perform the same measurements to a much higher degree of accuracy . Important note: Because you are using these resistors to test and return an aircraft component to service their values must be accurately measured with a calibrated DMM and those values used in the calculations. Be sure to use resistors with adequate power ratings for the intended test. And if testing and loading a high voltage power supply like that found in a radar indicator or radar modulator be sure not to exceed the breakdown voltage of the resistor. Typical 0.5 watt resistors should not have more then 400 volts across them. On the next page is a source listing for resistors, including high voltage types.

Example 1.

Manufacturer X recently let me inspect a sample of their new maintenance manual for a product soon to be introduced. It called for adjusting an audio amplifier for 12 watts across a 4.0 ohm resistor. With a voltmeter and our 4.0 ohm 1% resistor rated at 25 watts (i.e Dale RH-25 series) we calculate:

E= √PR | E ≈ 6.9 volts RMS |

or if measuring with a scope so that you can check distortion:

EPeak √2RP | E ≈ 9.8 volts Peak |

Example 2.

The ARC RT385A maintenance manual (serial numbers through 39,999) lists communications phone output as 3.2 Volts RMS into 500 ohms. Since it is power we really work with when troubleshooting (i.e. audio levels too high or low, or you are getting unwanted crosstalk) it is nice to know what power the unit produces.

The answer: 0.065 Watts or 65mw.

Example 3.

The Northern Airborne Technology AA-80 Intercom has in its maintenance manual procedures for measuring headset power. The manual calls for a wattmeter and the specification is 80 mw into 150 or 600 ohms from each headset tested one at a time. Let us do this the inexpensive way. Grab a 150 ohm resistor and accurately measure its value ,

attach it to the four headset outputs one at a time and record the voltage with an audio signal applied. You can then repeat using a 500 ohm resistor. The 150 ohm resistor is checking the amount of current the amplifier can source while the 500 ohm resistor is checking the amount of voltage the amplifier can source.

Minimum Voltage for 80mw into 150 ohms: ≈ 3.45 Volts RMS

Minimum Voltage for 80 mw into 500 ohms: ≈ 6.3 Volts RMS

Example 4.

The customer reported that his speaker is weak. You listened and it seemed loud but was not overly so. Not a clear cut confirmation either way. The audio panel is a Bendix/King KMA-24 rated for 7 watts speaker audio in 14 volt aircraft and 12 watts in 28 volt aircraft. To check the integrity of the aircraft wiring you can replace the speaker with a 4 or 8 ohm resistor and perform your power check. Beware that this unit has a 4 or 8 ohm selectable output on the secondary of transformer T203. Set this for 4 ohms and drive an 8 ohm speaker and you won’t get rated power.

Example 5. Use resistors to check circuit breakers in aircraft. The 0.5 and 1 amp values can be especially troublesome. You should be able to draw the circuit breaker’s rating indefinitely.

Below are some sources of resistors that I am familiar with. This list is not an endorsement but supplemental information only.

Caddock Electronics Riverside, CA. (909)-788-1700, www.caddock.com *All types of resistors, good selection of high voltage types.*

Dale (Vishay) Distributed by Newark Electronics Chicago, Il. 1-800-463-9275 www.newark.com *All types of resistors, good selection of high quality 1% power types.*

Next Month- more on power