I have a 15A, 0-265VAC Variac I got a while ago. This is a 4000VA device; and naturally, it trips the 32A CB whenever I turn it on because of the inrush. Usually, after a few flips of the breaker, I can turn it on again, however it worries me the breaker will be damaged by this repeated tripping (the breaker is only rated for something like 500 trip cycles.)
So, I've designed a circuit to solve this problem. This is a similar circuit I've seen for charging up large capacitor banks in audio equipment and plasma TVs which typically have >500µF of bulk PFC capacitors.
Essentially it consists of connecting the load in series with a resistor, and then after a pre-determined time, the resistor is shorted or removed from the circuit (typically a relay is used) and the device powers up as normal. I believe this is called pre-charging. It's needed for inductors because the core has no magnetic flux at that point in time and inductance is nearly zero, so it looks like a short circuit to the breaker.
The problem I'm having is determining the value of this resistor. My circuit can pre-charge any time from 100-500ms (for now, 200ms is achieved with 220µF cap.) Too low resistance, and it still trips the CB. Too high, and insufficient pre-charge is provided, so it trips the CB too. I can't measure the inductance of the variac unfortunately, but I can provide a datasheet if necessary.
Does anyone know what the calculation might be for this inrush resistor? I've attached a schematic of my design. The circuit is powered from a mains transformer ripped from a cheap wall wart (there should be no safety concerns with capacitive based power supplies, etc.)
So, I've designed a circuit to solve this problem. This is a similar circuit I've seen for charging up large capacitor banks in audio equipment and plasma TVs which typically have >500µF of bulk PFC capacitors.
Essentially it consists of connecting the load in series with a resistor, and then after a pre-determined time, the resistor is shorted or removed from the circuit (typically a relay is used) and the device powers up as normal. I believe this is called pre-charging. It's needed for inductors because the core has no magnetic flux at that point in time and inductance is nearly zero, so it looks like a short circuit to the breaker.
The problem I'm having is determining the value of this resistor. My circuit can pre-charge any time from 100-500ms (for now, 200ms is achieved with 220µF cap.) Too low resistance, and it still trips the CB. Too high, and insufficient pre-charge is provided, so it trips the CB too. I can't measure the inductance of the variac unfortunately, but I can provide a datasheet if necessary.
Does anyone know what the calculation might be for this inrush resistor? I've attached a schematic of my design. The circuit is powered from a mains transformer ripped from a cheap wall wart (there should be no safety concerns with capacitive based power supplies, etc.)
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