KEggemeyer
- Jan 15, 2013
- 12
- Joined
- Jan 15, 2013
- Messages
- 12
I received a circuit design from a friend that looks for the presence or absence of an AC signal and can’t figure out exactly how the capacitor in the circuit works…
The circuit has a capacitor (0.22 uF) with the upper end attached to the positive 12 v DC supply and the lower end attached to ground through a 1 mega ohm resistor. The 12 v AC signal (pulled from the same transformer before being rectified and filtered) passes through a forward biased diode for half wave rectification and attaches to the MIDPOINT between the capacitor and the resistor. The midpoint is also attached to the inverting input of a comparator. (Imaging a plus sign, capacitor on top, comparator on the right, resistor on the bottom, diode with incoming signal on the left.)
I know the capacitor is a filter, but I can’t wrap my head around how the capacitor is acting. This is nothing like any standard capacitive filter circuit I’ve seen (albeit my experience is limited). This circuit has the capacitor with the upper end continually at 12 v DC and the lower end bouncing from 0 to 12 volts. Obviously the difference in potential will form a capacitance, but how is this modeled to determine the filtering effect?
The circuit works, but I just can’t understand it and don’t know what to search for to find more information.
Thanks!
Kevin
The circuit has a capacitor (0.22 uF) with the upper end attached to the positive 12 v DC supply and the lower end attached to ground through a 1 mega ohm resistor. The 12 v AC signal (pulled from the same transformer before being rectified and filtered) passes through a forward biased diode for half wave rectification and attaches to the MIDPOINT between the capacitor and the resistor. The midpoint is also attached to the inverting input of a comparator. (Imaging a plus sign, capacitor on top, comparator on the right, resistor on the bottom, diode with incoming signal on the left.)
I know the capacitor is a filter, but I can’t wrap my head around how the capacitor is acting. This is nothing like any standard capacitive filter circuit I’ve seen (albeit my experience is limited). This circuit has the capacitor with the upper end continually at 12 v DC and the lower end bouncing from 0 to 12 volts. Obviously the difference in potential will form a capacitance, but how is this modeled to determine the filtering effect?
The circuit works, but I just can’t understand it and don’t know what to search for to find more information.
Thanks!
Kevin