Instruction manual

OM2  18

The AC signal shown is what standard 120 volt house AC power ‘looks’ like.

The voltage of 120 volts is what is called RMS (Root Mean Squared) voltage.

Notice that the voltage goes up to a peak voltage of about 170 volts, goes

down to 0 and then goes to 170 volts in the opposite direction. The RMS

value is a complicated value determined with calculus and represents the

equivalent DC voltage of the peak voltage of 170 volts. In other words a DC

voltage of 120 volts would produce as much power as a 170 volt peak sine

wave voltage.

Now that you are totally confused all you need to know is that if 120VAC(rms)

is applied to the input of the OM2 the calculations need to use 170VDC as the

input voltage, Vin. To get the 170 volts simply multiply 120V(rms) times

1.414. So the formula is:

Vpeak = Vrms * 1.414

Vpeak = 120 * 1.414 =~ 170 volts peak

To calculate the resistance required for a 120V(rms) input use the formula for

the DC signal described above as follows:

Rt = (Vin – 1.25) / I

Rt = (170 - 1.25) / 0.05 = 168.75 / .05 = 3375 ohms = 3.375Kohms

A standard resistor value that is very close to 3.375Kohms is 3.3Kohms. If we

use this value the current is calculated as:

I = (Vin - 1.25) / Rt = (170 - 1.25) / 3300 = 0.051A = 51mA

We are well below the maximum allowable current so this is a good choice

and is how the values shown in the schematic were determined.

If you decide to include C2 in the input circuit it gets more complex and since

you have determined you need C2 we are going to assume you know a good

deal about electronics and physics and provide a very basic explanation of

how to calculate a value for C2.