as seen in picture, what sort of circuit is it? does it have a name? what should I google for to find out more about it? and how do I do the calculation etc..

this circuit is use to sense the voltage of a AC source, that all I know

 

The intent is probably creating a DC offset AC signal. Looks very unsafe to me.

 

How's that work?

The intent is probably creating a DC offset AC signal. Looks very unsafe to me.

 

See the attachment - but don't try to build it as a real circuit. You could be electrocuted if something goes wrong or you make an error.

 

How's that work?

The large capacitor you can treat as a voltage source as long as R*C (capacitor time constant) >> 1/f_ac_signal.
So in your digram capacitor is charge to
Vc = Vdd * R4/(R3+R4). So simply replace capacitor with Vc voltage source.

 

which resistor/s do I use to calculate the time constant?

The large capacitor you can treat as a voltage source as long as R*C (capacitor time constant) >> 1/f_ac_signal.
So in your digram capacitor is charge to
Vc = Vdd * R4/(R3+R4). So simply replace capacitor with Vc voltage source.

 

Well I think that the time constant is equal to resistance seen from the capacitor terminal. t = R * C ---->R = R3||R4 + R1||R2

 

ok, this confused me more, I simply don't understand why R3||R4 and R1||R2, if || here means parallel.

Well I think that the time constant is equal to resistance seen from the capacitor terminal. t = R * C ---->R = R3||R4 + R1||R2

 

ok, this confused me more, I simply don't understand why R3||R4 and R1||R2, if || here means parallel.

Simply you must find a equal resistance seeing from the capacitor terminal when the other remaining voltage source replace with a short-circuit.