[SOLVED] High Voltage AC to low voltage DC

[CataM]

Hello everyone,

I know that the usual way of going from the 311 V (220 RMS) peak given from the mains to some low voltage DC is to use a transformer before the rectifier.
Another way used in power supplies is to rectify the mains and then convert it to low voltage DC using DC to DC converters (SMPS).

However, why a capacitive divider at the input (just before the rectifier) is not used ? What are its drawbacks so that nobody uses it ?

See e.g. an example below.

SCHEMATIC:


Output Voltage:


Thank you for your time !

 

[betwixt]

1. no isolation from the incoming AC lines so there is a safety issue.
2. the reactance of the capacitor is what causes the voltage drop and it is constant for a given value of capacitor and frequency. That means it drops a constant voltage IF THE CURRENT IS CONSTANT. So if the load current changes, so does the voltage. At zero load, the full 311V will appear at the load.

Brian.

 

[treez]

Yes i second what Betwixt says, that is why they are not at all common. The "capacitive droppers" that the OP allures to (but does not draw) do get some use at very low powers. Also, the only other way i can think caps get used in offline smps (other than snubbing filtering etc) is when you get "voltage doubler" capacitors used in series for USA offline SMPS's.....but this is rarely done today.
You get a switch so that you have it one way for conxion to UK mains, and the other when on USA mains.

 

[kumarpradeep]

The current you need is low, you could wind your own coil and tap power by placing it within the magnetic field of the overhead line. You may not have to get too close to the 10KV line (that depends on how much current is passing down the 10KV line and consequently the magnetic field strength).

 

[betwixt]

Direct wiring to 220V AC is dangerous.
Wiring close to 10KV is suicidal.

I very strongly recommend you ignore that suggestion immediately. Especially as you seem to be wanting to draw 12V into a 200 Ohm load (60mA) which would require a substantial line transformer if you were to keep say 50mm separation from the line wire and secondary. Note that you need isolation not only for 10KV but also enough for lightning strikes and the secondary voltage would depend on the load on the AC line.

Apart from the obvious technical and safety issues, it would constitute theft from the electricity company and possible legal problems for tampering with their equipment.

Brian.