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.
To check the accuracy of this result, I measured the power into the ringer power supply. This power supply has an efficiency of ~85 % () and I measured 3.4 W into the 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.