# Thermal Power Meter

Posted on Dec 17, 2012

After one has built a QRP transmitter, it would be interesting to know the exact amount of output power. Without a power meter the peak to peak voltage Vss is measured at an 50 ohm resistance (dummy load) with the available oscilloscope. Now the power rate can be calculated according to the formula P = V x V / R. For V the effective voltage rate Vrms = 0,707 x Vss / 2 is to be used. With this type of power determination by a voltage measurement there are different sources of error: A dummy load resistance deviating from 50 Ohms when heating up, the basic accuracy of the oscilloscope, the amplitude drop of the Y amplifier at high frequencies and a possibly not correctly calibrated 10:1 probe. Each more or less measured volt has an appropriate influence on the result of the calculation (power) due to V x V. 50 % deviation upward or down from the real power level is quite possible under worst case conditions.

Another method of the power determination gets along without measurement of the HF voltage. It operates according to the principle of the thermal conversion. A controller ensures that the temperature at the two heating resistors Ra and Rb is identical. Rb is warmed up as long as the bridge circuit consisting of the temperature sensors Fa/b and the resistors R1/2 is ballanced. The DC power dissipated by Rb corresponds then exactly the HF power dissipated by Ra. As controller an operational amplifier with a linear amplifier (power transistor) is usually used. Rb receives thereby as correcting variable a variable voltage. Since the amount of heat is however proportional to the square of this voltage, one gets a controller loop gain proportional to V x V. The result is a nonlinear step response and thus all well-known problems concerning the controller stability at setpoint changes. The additional power dissipation at the linear amplifier of max. 0.5 x Pmax is here only mentioned by the way. A duty cycle of tp = 0,95 x T is feasible with the parallel arrangement of the switching transistors. The missing 5% are of principle due to the push-pull design of the switching regulator. Who needs a 5 W power meter should take into consideration that Ra/Rb have sufficient high power dissipation rating and the transistors in the regulator IC are not overloaded. If the 13.8 V supply voltage should be kept, it is recommended to reduce the...

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