Some circuits drop mains voltage with a capacitor and a resistor. I presume it is cheaper, How do you work out which c and r to use and what the disadvantages and advantages of this as opposed to a transformer.

yours needing help

Philip Gordon

 

Yes, it's cheaper, but it's also much more dangerous, and the values of the capacitor don't seem to match those calculated by the usual formulas.

A lot depends on your exact usage, it also isn't good for a device which has variable power demands. Mostly only cheap crap items do this.

Advantages:

Cheaper, smaller, cooler.

Disadvantages:

Not isolated, much more dangerous, limited to a narrow current range, difficult to work out.

 

"Mostly only cheap crap items do this."

Such as the Tektronix 211, 212 and 214 handheld oscilloscopes of the 1970s and 80s, anywhere from $1800 to $2500. These are called "reactive dividers" and Tek used two hefty Mylar capacitors in series for the divider, used both to power the little scopes and to charge the internal battery packs.

The reactive divider (if fully capacitive) does offer line isolation, but that isolation is limited. If the cap tied to the line shorts, the world literally goes up in smoke. The little 200-series scopes were double-insulated, so the isolation problem was a moot point. A reactive divider has little actual power dissipation as compared to a resistive divider. In fact, the divider you mentioned with one cap and one resistor will dissipate power through the resistor, so isn't as economical.

Dean

__________________
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

What do you think 'mostly' means

However, a country which still uses 'screwit' connectors for electrical wiring is hardly a good example of safe practice - they were declared illegal in the UK back in the 60's.

 

I think that the main disadvantage of using resistors and capacitors (linear devices) is that they don't provide extensible management of the circuit. You just have to add'em in series or/and in parallel to get to your needs: you need more than one device in order to get to the preffered result, AND the descriptive equation, though simple at the beginning, may get quickly :twisted: ....

 

What are these "screwit" connectors?

 

Sounds like technobabble to me...

 

think he means twist-on connectors...

 

Yes, for twisting over bare mains wires, they were called 'screwit connectors' in the UK - I cringe every time I see them on 'This Old House'

Funnily enough my boss has just come back from New York, he was in a big department store (waiting for his wife!) and a guy opened up a panel on the wall and added an extra wire - by removing one twist connector, and fitting a larger one.

 

Yeh -- we call them "wire nuts" on this side of the pond...

 

You can still buy those in Australia, highly illegal to use on mains wiring though...