Baphomet said:
It's easier, simpler, cheaper and much more reliable to use just a step down
transformer. For example
http://www.electronics.50g.com/cdcac.htm uses 2
transistors, 5 resistors, 2 diodes, plus the need for a transformer in any
event. While this isn't the exact circuit you need, I think you can get an
idea of the added complexity of going the inverter route.
Yes; unless the weight of a transformer would be a consideration,
it would (at 50 or 60 cycles) probably be the cheapest, simplest
and most reliable (fewest parts involved) solution. About the
only extra 'component' I would suggest would be a fuse in the
input! At 400 hertz (e.g. aircraft) a 3 VA transformer could be
quite small.
An ordinary mains transformer with a 6 volt 0.5 amp transformer
secondary should be pretty small; also the inductance of the
transformer might provide some protection against sudden voltage
spikes (transients) that occur on the commercial mains supply.
You might be able to salvage such a transformer from something
small such as a desk calculator/adding machine, old printer or
something similar. Even an old model railway transformer or one
of those that operates residential front door chimes (although
those latter often don't like a continuous load and and can burn
out). Even some wall warts can supply 500 milliamps at around six
volts. A transformer also nicely isolates the 'load' from the
main supply; often required and good safety design.
Only other suggestion; if you really DON'T want to use a
transformer, and assuming the load being is say close to a pure
and unvarying resistance, would to put it in series with an AC
rated capacitor across the mains supply. Fuse this at around one
amp?
This will produce a leading current across the resistance portion
of the circuit.
This is a vector diagram situation; but in this case the
resistance will be small (less than 10% or so) in comparison to
the capacitive reactance that within practical limits the 'R' can
be ignored!
Have done this several times to power the filament heater of a
single tube rated at 6.3 volts 0.3 amps. (Very similar to the 6
volt at 0.5 amp requirement here!) from 230 volt mains supply.
[6.3/0.3 = resistance of tube heater, hot = 21 0hms].
Using the formula Xc (capacitive reactance) = 1/(2pi x frequency
x capacitance);
That is Xr = One divided by the total of (two pi times frequency
in cycles, times capacity in farads).
Thus at say 60 cycles a one microfarad capacitor (10 to the minus
6 farads) will work out to;
Xc Ohms = 1/(2pi x 60 x 10^-6) = 10^6/ 375 = 2667 ohms or approx.
2700 ohms. [One microfarad].
So a 6 microfarad cap will have one sixth of the reactance and
will 'pass';
At say 230 volts then; 230/450 = 0.5 amps.
At 115 volts then double the capcitance to 12 mfd for 115/225 =
0.5 amps.
It's rough and ready but it works!
Disadvantages of the series capacitor method:
1) One side of the mains supply is connected to the load circuit.
2) The size of capacitors (and possibly their weight) to
withstand these voltages and the cost if buying new), will be
greater than that of a simple cheap transformer.
2) If the capacitor ever breaks down full line voltage will
impressed on the load circuit. Pose a possible fire and safety
hazard and will most likely destroy the load. Whatever it happens
to be?
I.e. KISS. (Keep it simple, Simon). Why complicate it?