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Incandescent lamps would appear to be a good candidate for power output measurement. Since they come in all power levels from "grain of wheat " types that draw less than 30 milliwatts (1.5 V @ 20 mA) to giant 1kW line operated lamps, we have the ability to match our lamp to the expected output of the device under test "DUT".
Lamps not only provide a true rms function by integrating pulses into heat, no matter what the crest factor and regardless of positive or negative value. They also provide a quick visual assesment of output level.
We need first to calibrate the lamp bulb envelope for temperature vs. power input using a known source e.g power supply or mains depending on type. A thermocouple fastened (e.g. high temperature epoxy or ceramic cement or taped firmly) to the top surface of the lamp can read temperature which is correlated to power input at several input power points for characterization. Ambient must be norted and subtracted from each reading for normalization. Plotted on a graph we can now look up the temperature over ambient and convert the reading to power when hooked to our DUT.
Another method is to feed the output into a sensitive D'Arsonval movement meter the scale of which is directly calibrated in power. For this it is good to use a second ambient sensing thermocoupe in inverse series which nulls out ambient. In this way we need not consider or subtract out ambient (as in the prior method) since it is automatically subtracted electronically.
For quick and easy calibration from the mains, we can use full power direct connection from the mains and half power with a diode in series to determine two points and characterize by drawing a straight line between the points. Also, considering "no power input" as a point we have a total of three points that can be curve fitted. The more points that can be used the better. Lamps can be wired in series to reduce power e.g. 4 lamps = 1/4 power in each lamp. Always measure the current and voltage to determine the actual power while characterizing your "standard" lamp
The same methods can be used with a resistor in place of the lamp and extra sensitivity at low levels can be had if our resistor is insulated from ambient.
IR thermometers can be used to measure bulb or resistor temperature, but I recommend that the test spot on the top of the emitter be painted flat black with high temperature paint to normalize emissivity. Then a test stand should be used to mount the IR gun and emitter such that they remain at a fixed distance and spot position.
Attached is a sample graph for a 60 Watt 120 volt lamp measured with a thermocouple taped to the top of the bulb in free air (unstirred) and corrected for ambient.
I will be updating the graph with measurements using an IR thermometer for comparison.
« Last Edit: 2013-12-08, 17:39:03 by ION »
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Topic: Simple Thermal Power Measurement (Read 939 times)
