Low cost mains power supply

Arlet Ottens wrote:

>> I consider it useful to mention mains voltage (and frequency) when
>> presenting such calculations.
>
>They were mentioned (170V peak, 60Hz, half-wave) in the part that you 
>snipped.

correct, sorry!

Oliver
-- 
Oliver Betz, Munich http://oliverbetz.de/

On 4/9/2013 3:17 AM, Stef wrote:
> In comp.arch.embedded,
> Pete<pjetson@yahoo.com>  wrote:
>> Nomen Nescio wrote:
>>> In most DIY stores you can find these cheap Chinese digital timers to
>>> turn you lamps or appliances on at specified time. The thing I'm
>>> wondering is, how do they power the digital circuitry from
>>> 110VAC in a device costing only $5-$10? Are they using a transformer or
>>> are they directly powering the digital circuitry from the AC voltage?
>>
>> They don't use a transformer.  They usually use an X2 rated capacitor to
>> drop the voltage, then some rectification and regulation.  This might be
>> as simple as a single series diode, a resistor, a zener, and a smoothing
>> capacitor.
>
> Google for [capacitor dropper] and you will find lot's of examples.
> Efficiency is not that bad, power factor is terrible.
>
> And keep in mind that this type of circuit is not isolated, so your 'low
> voltage' side is live AC!

It may not be isolated, but it can be current limited and so not lethal.

-- 

Rick