# What's the name of this circuit, and how does it work?

I was reading Hack Into a Timer AC Socket and when I got to this paragraph:

The design firstly have a RC buck? (not sure how to call correctly in English) circuit to step down the voltage from 220V AC to 5V AC, a 0.33uf safety capacitor (yellow one) used here, accompanying with a 1M discharge resistor. And after these two there is another current limited big resistor to prevent pulse and shock.

I was curious - I didn't even realize you could change AC voltage levels without a transformer (or at least not cheaper than with a transformer).

I understand that a resistor will lower voltage for a given current, but I would have expected AC mains to be too much power for a cheap resistor. And I don't get what the capacitor does - even if it is helpful here to get smoother DC power, wouldn't it interfere with anything else on the same AC line?

WARNING - ALL PARTS OF THIS CIRCUIT SHOULD BE CONSIDERED TO BE AT AC MAINS POTENTIAL AT ALL TIMES.

Capacitor C1 MUST be an "X RATED" capacitor specified by its manufacturer
for "across mains" use. NB NOT 'Y' rated.
Examples of X & Y rated capacitors

• Do you have a readable version of the schematic? Clicking the thumbnail in that article gets me a server error. – gwideman Aug 15 '14 at 1:06
• A resistor by itself does not "lower voltage". The concept you need to study is "voltage divider". en.wikipedia.org/wiki/Voltage_divider – gwideman Aug 15 '14 at 1:08
• Actually, I do - change the subdomain to WWW and the links work: electrodragon.com/wp-content/uploads/sites/7/2014/08/… – Nathan Friedly Aug 15 '14 at 1:14
• That looks somewhat like a very poorly laid out bridge rectifier with an input filter. – sherrellbc Aug 15 '14 at 1:17
• @RussellMcMahon Are there cases where a Y capacitor would be less safe than an X capacitor? – Spehro Pefhany Aug 15 '14 at 2:49

This is a capacitive dropper power supply. The bulk of the voltage is dropped across the 'safety' rated capacitor C1. It acts a bit like a resistor but does not dissipate significant heat. The reactance is Xc = $\dfrac{1}{2\pi f C} \Omega$.