How to Discharge a Capacitor? Using Bleeder Resistor, Screwdriver, Lamp


A Capacitor is an essential component used in electrical and electronic applications. Along with Resistor and Inductor, the Capacitor is one of the three basic passive components. Being a commonly used component, capacitors are bound to faults and damages and we have to replace them. Since Capacitors store electric charge, you have to properly discharge them before working with them so that you don’t get zapped. In this tutorial, we will learn How to Discharge a Capacitor using a couple of techniques.

What is a Capacitor?

In its simplest form, a capacitor consists of two conductors made up of conductive plates, foils, wires, or solids and are separated by Dielectric, which can be air, paper, vacuum, or any insulating material.

When there is a potential difference between the two plates of the capacitor with the help of a power supply, it stores electric charge. If we remove or disconnect the power supply, the capacitor can supply its stored charge into the circuit.

An important point about capacitors is that if a fully charged capacitor is not discharged in the circuit can hold the charge even after we remove the main power supply. So, you must be extremely cautious when working with capacitors in general.

What Does a Capacitor Do?

When we connect a DC Power Supply across the leads of a capacitor, the capacitor gradually accumulates charge between its plates until the voltage is equal to the supply voltage. Even if we disconnect the power supply, the capacitor continues to store the charge and in this way, a capacitor acts like a small battery.

We can use this energy in the capacitor in a circuit and we can also control the rate of charge and discharge with the help of a series resistor.

Apart from storing electric charge, capacitors can also block DC while passing AC, noise, audio, and other non-DC signals. We can use this property of the capacitor to smooth out the output of power supplies, to remove voltage spikes from signals that can cause the unnecessary triggering of circuits, protect components from DC and also provide clean transfer of audio signals.

Why Do We Need to Discharge Capacitors?

As mentioned before, capacitors store electric charge and they can hold the charge even if we remove the main power supply. If you accidentally touch the leads of the capacitor after disconnecting the power supply thinking that no power supply; no problem, then you can get an electric shock as the capacitor discharges through your body.

Depending on the amount of charge, you can get a small tingle or a burn on your fingers and in the worse case, it can be fatal.

So, if you planning to work with capacitors, you better discharge them properly even after disconnecting the power supply. Capacitors in large AC / DC Power Supplies are very big and can hold the charge for days or months if not discharged. If you are salvaging parts from such old devices, don’t assume there won’t be any danger.

How to Discharge a Capacitor?

Now that we have seen that discharging a capacitor is extremely important to work with them in circuits and PCBs. But the question is; How to Discharge a Capacitor? If you want to test a capacitor, see this tutorial on how to test a capacitor using Multimeter.

There are a couple of techniques to properly discharge a capacitor. We will see the details for each technique one-by-one. No matter how we discharge the capacitor, never touch the leads of the capacitor with your bare hands. Be extremely careful.

Using a Metal Object (Screwdriver)

This method is not the safest but it can discharge capacitors easily. For this method, you will need a metal object such as a screwdriver. To be on the safe side, we can select an insulated screwdriver with only its tip being the metal part.

First of all, disconnect the device from the main power supply. If the capacitor is on a PCB, desolder it and do not touch the leads of the capacitor. Once the capacitor is out of the PCB, hold it in one hand using its base.

Now, take the screwdriver in the other hand. Short the tip of the screwdriver with both the leads of the capacitor. The capacitor discharges with small to medium sparks depending on its state of charge. Do this a couple of times to make sure that the capacitor is discharged completely.

If you are uncomfortable with the sparks or worried about your safety while discharging using a screwdriver, you can skip this procedure and try any of the other methods mentioned below.

Using a Bleeder Resistor

This is a professional and safe way to discharge a capacitor. In fact, a properly design power supply uses this method to discharge the output capacitors after disconnecting the power supply.

In this method, a resistor known as Bleeder Resistor is connected across the leads of the capacitor. When the power supply to the circuit is removed, the capacitor discharges through this bleeder resistor. If your circuit already has a bleeder resistor, then the capacitor automatically discharges through it after disconnecting the power supply and the discharge rate depends on both the capacitance of the capacitor as well as the value of the resistor.

But if your circuit doesn’t have a bleeder resistor or if you are not sure whether a bleeder resistor is used or not, you can try to connect one yourself. Take a bleeder resistor and connect its leads to a couple of crocodile clips. Now, connect those crocodile clips to the leads of the capacitor. Wait for the calculated time and the capacitor will be completely discharged.

But what resistor value to select? Here is an interesting calculator by Digi-Key, which calculates the value of the resistor and its power rating after we enter the capacitance, voltage, and time of discharge.

For example, if we have a 470µF Capacitor rated for 50V and we want to discharge this capacitor down to 0.1V. If we use a 1kΩ resistor across the leads of the capacitor, it will discharge in 3s. But the important thing to remember is the power rating of the resistor. To safely discharge the capacitor, the resistor must be rated for at least 2.5W of power dissipation. So, choose a 5W 1kΩ resistor, in this case, to be on the safe side. These high-power resistors are usually expensive.

Using a Resistive Load

Another easy way to discharge a capacitor is using a resistive load such as a tungsten lamp. If you have an old tungsten lamp lying around with a decent power rating, you can use it as a “Bleeder Resistor” to discharge a capacitor.

Tungsten lamps are essentially resistive wires enclosed in a vacuum / noble gas filled glass bulb. Choose at least 10W rated tungsten lamps so that they can properly discharge the capacitor without getting damaged.

To discharge a capacitor using a tungsten lamp, take the leads of the capacitor and connect them against the terminals of the lamp. Depending on the state of the capacitor’s charge, the lamp will glow slightly while the capacitor is discharging and fades out once it is fully discharged.

Conclusion

Capacitors are very important electrical and electronic components that store energy in the form of electric charge. Due to the working of the capacitor, it can hold the charge even if we disconnect the main power supply to the circuit. So, they must be properly discharged before working with them. We learned how to discharge a capacitor using three different methods. If you have any suggestions regarding this topic, please comment below as it might be helpful to others as well.



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