How Can an LC Circuit Prevent Light Bulb Filament Damage at Turn-On?

[ekociu]

The room temperature resistance of the tungsten filament in an ordinary incandescent light bulb is about 1/15th of the hot resistance exhibited during the normal operation at 117 VAC. Therefore, when you throw the switch to turn on the light bulb there is a huge surge of current in excess of the steady-state operating current until the filament heats up. This can cause the light bulb to "blow out."
So you need an inductor to not allow current change too suddenly and a capacitor to prevent arcing.


Design an LC circuit to protect the tungsten filament of an ordinary 75-Watt incandescent light bulb from over the current-transients greater than 50% of the normal operating current at the moment of turn-on!

So can anyone help or give me a head start how to make this circuit!

 

[berkeman]

The room temperature resistance of the tungsten filament in an ordinary incandescent light bulb is about 1/15th of the hot resistance exhibited during the normal operation at 117 VAC. Therefore, when you throw the switch to turn on the light bulb there is a huge surge of current in excess of the steady-state operating current until the filament heats up. This can cause the light bulb to "blow out."
So you need an inductor to not allow current change too suddenly and a capacitor to prevent arcing.


Design an LC circuit to protect the tungsten filament of an ordinary 75-Watt incandescent light bulb from over the current-transients greater than 50% of the normal operating current at the moment of turn-on!

So can anyone help or give me a head start how to make this circuit!

Welcome to the PF. Adding the inductor in series will limit the inrush current. Can you explain how that works, and show us the equation relating the voltage across an inductor to the current through it?

And where do you think they are talking about an arc forming? Why would there be an arc, and when would it occur?

 

[Mene]

I would recommend using a simple series LC circuit to protect the tungsten filament from overcurrent transients. This type of circuit consists of an inductor and a capacitor connected in series with the light bulb.

To begin designing this circuit, you will need to determine the values of the inductor and capacitor that will provide the necessary protection. The inductor should have a high enough inductance to limit the rise in current during the initial surge, while the capacitor should have a high enough capacitance to prevent arcing.

To calculate the required inductance, you can use the formula L = V/(di/dt), where L is inductance, V is the supply voltage (117 VAC in this case), and di/dt is the desired rate of change of current. In this case, we want to limit the current to 50% of the normal operating current, so di/dt would be equal to 0.5 times the steady-state operating current divided by the time it takes for the filament to heat up (usually a few milliseconds). This will give you the minimum inductance needed to limit the current surge.

Next, you will need to calculate the required capacitance using the formula C = Q/V, where C is capacitance, Q is the charge needed to prevent arcing, and V is the supply voltage. To calculate Q, you can use the formula Q = CV, where C is the capacitance and V is the voltage across the capacitor. In this case, you want to prevent arcing at 50% of the normal operating current, so the voltage across the capacitor would be 117 VAC. This will give you the minimum capacitance needed to prevent arcing.

Once you have calculated the values for the inductor and capacitor, you can then select components with the closest values that are commercially available. It is important to note that the inductor should have a high enough current rating to handle the initial surge, and the capacitor should have a high enough voltage rating to handle the supply voltage.

In summary, to design an LC circuit to protect the tungsten filament of an ordinary incandescent light bulb, you will need to calculate the required inductance and capacitance using the formulas provided and then select components with the closest values that are commercially available. It is also important to note that this type of circuit may not completely eliminate the initial surge, but it will help to reduce it and protect the filament