# Purpose of high pass filter for AC Mains?

I wanted to ask what the purpose of the high pass filter does in the below circuit. Is it necessary?

• Is it from a LED lamp by any chance, because that is a common technique to ballast them? – winny Feb 27 at 7:05
• As winny mentioned, it is used in LED circuits. Check out "transformerless power supply" – C K Feb 27 at 8:26
• I hate to state the bleedin' obvious, but why would they fit it if it wasn't? – Finbarr Feb 27 at 9:30
• Could I call it a capacitive dropper without discharge resistor but with a current limiting resistor? – Max Ried Feb 27 at 13:21

## 1 Answer

Is it necessary?

If it isn't there you have full mains voltage on the bridge rectifier.

The capacitor has an impedance to AC given by $$\ Z = \frac {1}{\omega C} = \frac {1}{2 \pi f C} \$$ where f is the mains frequency. The result is that it will drop the voltage to the load and, since it is a capacitor, there will be very little power dissipated in it and this eliminates the heating problem associated with a resistor voltage dropper.

Please be aware that this circuit provides no isolation from the mains and that the low voltage side must be considered live.

• Thanks. Since it drops the remaining voltage efficiently, this circuit is a current limiting circuit - not a high pass filter - right? Also, can I get rid of the 27 ohm resistor since it doesn't really affect the overall impedance? – Alex Feng Feb 28 at 1:48
• Yes, it is current limiting. The 27 ohm resistor may be required to limit the current to a safe value in the case when the mains is at a high voltage at the moment of switch on. I advise you to study the theory behind these very carefully before building one. – Transistor Feb 28 at 7:09