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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
For what it's worth, if you want to reduce 230Vrms to 110Vrms with just a phase-angle controller, you need to set it to pass approx 35.5% of each half-cycle. (That's 3.55ms out of every 10ms in 50Hz countries, or 64 degrees of conduction in every 180 degrees.)
(You can get this answer mathematically by writing an expression for the instantaneous power dissipated in the heater and then integrating it over an adjustable interval of a mains half-cycle!)
It doesn't matter whether it is the first 35.5% at the start of each half-cycle, or if it's at the back end of each half-cycle, because half-cycles are symmetrical about their middle. So leading-edge modulated TRIAC dimmers or trailing-edge IGBT ones will both work with equal mediocrity.
Since the load is only connected to the line for 35.5% of the time it will see a very sharp spikey voltage waveform. In this case voltage peaks at 292 volts (almost twice what the 110Vrms appliance was actually designed to see!!!) It will draw a lousy current waveform, radiate shed loads of EMI and most likely buzz audibly too!
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> You can also use a capacitor in series (motor run etc.)
Indeed you can. An AC rated capacitor in series with the heater is probably a better option.
A cookie for the first person who can calculate the required series capacitance to make the 250W 110VAC heater dissipate 250W from a 230VAC 50Hz line...
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
GeordieBoy wrote ...
> You can also use a capacitor in series (motor run etc.)
Indeed you can. An AC rated capacitor in series with the heater is probably a better option.
A cookie for the first person who can calculate the required series capacitance to make the 250W 110VAC heater dissipate 250W from a 230VAC 50Hz line...
-Richie,
With 250W off a 110 element, the current equals 2.272727A
there is 120V across capacitor, so reactance is 52.8 ohms
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Finn Hammer wrote ... With 250W off a 110 element, the current equals 2.272727A there is 120V across capacitor, so reactance is 52.8 ohms at 50Hz, this comes out to 60uF
Is the cookie for the first CORRECT answer? V_cap and V_heater are 90 degrees out of phase, so for 230 volts total and 110 volts across the heater, we need 202 volts (RMS) across the cap. Thus a reactive impedance of 88.9 ohms. Use 36 uF. Respectfully, Rich F.
p.s. In a similar application, I have in front of me an iron-free circuit powering a 12-V DC fan from 120 V 60 Hz. There's a 3.3-uF series capacitor, a bridge rectifier, 4700 uF 16V filter cap, and a Zener diode as shunt regulator. Anybody want it for the cost of postage?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Yeah, Klugesmith is correct. The same current flows through the heater and the capacitor as they are in series. The voltage vectors are at 90 degrees. I calculated the capacitance to be 35.8uF, in order to admit the correct current through the heater.
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Making 110V out of 240V
