In most phantom power supply schematics or documentation (see for example https://en.wikipedia.org/wiki/Phantom_power#Technical_information), I see:
- 6.81 kΩ when powered by 48V
- 1.2 kΩ when powered by 24V
- 680 Ω when powered by 12V
- 470 Ω when powered by 10V
- sometimes (but not sure if correct) 180 Ω when powered by 9V
Question: how to find these values, what is the formula being used?
Attempt 1: Obviously, it's not a direct application of Ohm's Law, because:
48 / (6810 / 2) would give 14 mA to the microphone (I divide by 2 the resistance value because there are two same resistors in parallel)
12 / (680 / 2) would give 35 mA to the micrphone, which is very far to what the linked page above states:
The IEC standard gives 10 mA as the maximum allowed current per microphone
Attempt 2: We could imagine there's an "equivalent" resistance for the mic (maybe impedance).
I tried to find the value Req
such that V_i/(R_i/2+Req)
is close to each other for each of the 5 cases (i=1..5) mentioned above. 650 Ohm seems to work:
48/(6800/2+650)=11.9 mA
12/(680/2+650)=12.1 mA
10/(470/2+650)=11.3 mA
9/(180/2+650)=12.2 mA
but unfortunately:
- it's not coherent with the 24V case: 24/(1200/2+650)= 19.2 mA! which is too much
- 12 mA is higher than the IEC standard mentioned above, so I can't imagine they created a standard schematic based on a too high current