# Power Dissipated in a Maximum Phone Load During Ringing

I am writing a specification this morning and I realized that I have never calculated the maximum power drawn by a phone during ringing. I was surprised by the number – 3 W. This is a lot for unit that must operate from battery backup when AC power fails. Figure 1 summarizes the calculation, which were performed in Mathcad.

Figure 1: Ring Power Calculation.

To check the accuracy of this result, I measured the power into the ringer power supply. This power supply has an efficiency of ~85 % ($\epsilon \triangleq \frac{{{P}_{Out}}}{{{P}_{In}}}$) and I measured 3.4 W into the supply.

Figure 2: Measured Ring Power at the SLIC Power Supply.

My measured result of 2.9 W and my computed result of 3.0 W are close enough considering the potential errors in my load circuit and my efficiency estimate for the power system.

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