How to Discharge a Capacitor: Comprehensive Guide

Introduction

A capacitor is an essential component used in electrical and electronic applications.  Capacitors store electric charge, so users have to properly discharge them before working with them in order not to get electrocuted. 

A capacitor in its most basic form consists of two conductors made up of conductive plates, wires, foils, or solids. These conductors are separated by any insulating or dielectric material, which can be ceramic, air, or impregnated paper.

Solid capacitors can be split into two basic subcategories: ceramic and film capacitors.

Foil capacitors consist of a three-layer foil arranged in an electrode-dielectric-electrode order. These layers are rolled up and placed in appropriate housing. They are commonly used in electronic circuits found in various types of household appliances as well as audio/video devices.

When there is a power supply that creates a potential difference between the two plates of the capacitor, it stores electric charge. This means that the capacitor gradually accumulates charge between its plates until the voltage is equal to the supply voltage.

At this point, if the power supply is removed or disconnected, the capacitor can supply its stored charge into the circuit. In this way, the capacitor acts like a small battery. This energy-storing property of a capacitor is called its capacitance, and is usually measured in Farad.

In other words, a capacitor can be gradually charged to reach the required voltage level to provide the energy needed by an electronic device. Since 1 farad is a very high value, capacitors used in electronics usually contain capacitances measured in picofarads, nanofarads, microfarads, and millifarads. A charged capacitor left unused can retain its charge for a long time, even years.

Capacitors are crucial components in so many electronic devices such as household appliances, handheld devices like smartphones, and computers. The main function of a capacitor in these devices is to store electrical energy to facilitate their operation in diverse ways. Thus capacitors can be used to provide different types of electronic filtering, as well as provide a means to store and discharge energy. 

It is pertinent to note that if a fully charged capacitor is not discharged in the circuit, it can retain the charge even when the main power supply is disconnected. Therefore, extreme caution is required when working with capacitors in general.

Apart from storing electric charge, capacitors can also block DC (direct current) while passing AC (alternating current), noise, audio, and other non-DC signals. This function of the capacitor can be used to remove voltage spikes from signals that can result in an undue triggering of circuits, smoothen out the output of power supplies, ensure clean transfer of audio signals, and protect components from DC.

Why do Capacitors Need to be Discharged?

As earlier mentioned, capacitors store electric charge and they can hold this charge even if the main power supply is removed. Discharging a capacitor means releasing the charge stored within the capacitor.

Accidentally or carelessly touching the leads of the capacitor after disconnecting the power supply, on the misconception that the absence of a power supply makes it harmless, can bear consequences ranging from mild tingling or burn to fatal electrocution and fire, depending on the amount of charge present in the capacitor. Large capacitors can store enough energy to inflict injuries, so they must be appropriately discharged.

When a capacitor is disconnected, it retains its accumulated voltage (and current) across the previously connected terminals, which is notably dangerous. This is why it is imperative to discharge a capacitor before disconnecting it to remove all charges and corresponding voltage.

A short circuit of a charged capacitor poses a great risk of burning out the electronic component and other circuit elements. The greater the capacitance and voltage of the capacitor, the greater the damage it can potentially cause.[1]

Safety Precautions to Take

Some general safety precautions when handling capacitors include:

• Maintain a low and comfortable grip on the capacitor to avoid contact with sparks.

• Use a pair of insulated pliers to hold smaller capacitors to avoid accidental electric shocks while discharging them.

• Always wear safety goggles. 

• Handle large capacitors with extreme care, as they can still hold high voltage even after the circuit is disconnected from power.

• Do not charge capacitors by a higher current or voltage than what is prescribed.

• Ensure to test capacitors with a suitable voltmeter and, if necessary, discharge them before working on or close to them. 

• A loose high-voltage capacitor should have a wire connected across the terminals once it has been discharged.

• Do not solder directly to the capacitor.

• Do not touch live capacitor wires or terminals.

• Do not discharge by force.

• Do not short the terminals, as this can cause heavy sparks and damage to capacitor terminals.

• When discharging, it is highly recommended to short the capacitor terminals through a resistance first, then short them directly by a wire. 

• An electrolytic capacitor must never be connected in the wrong polarity (reverse placement of positive (+) and negative (-) This can cause extensive damage, and even trigger an explosion.

• Do not heat or dispose of capacitors in fire. [2]

How to Discharge a Capacitor

There are a couple of techniques that can be applied to properly discharge a capacitor. All of these techniques require the use of a multimeter. This tool cannot discharge a capacitor, rather it is used to check the voltage stored in a capacitor so that an appropriate resistive material can be used to execute the discharge.

For safety and accuracy purposes, a proper multimeter should be used. It can be analog or digital. To use the multimeter, simply turn it to voltage reading and proceed to check the voltage of the capacitor.

Measuring Voltage with a Multimeter

The basic of discharging a capacitor is summed up in the following steps:

1. Cut off the power supply to the capacitor completely to ensure your safety.
2. Use a volt/ohm meter or a multimeter to determine the amount of voltage the capacitor stores. Make sure you get the precise amount of volts.
3. If the voltage is relatively low (below 50 V), then you can use an insulated screwdriver to discharge this voltage. Or else, use an appropriate resistive receiver that can handle the voltage.
4. Hold the capacitor firmly. Make sure your hands are protected from the terminals so you don’t get electrocuted. The resistive receiver (screwdriver, light bulb) must be in contact with both capacitor terminals at the same time.
5. Test the capacitor again, and if there’s any voltage left, repeat the process as required.

More precise techniques are discussed subsequently:

1. Using a Metal Object (Screwdriver)

Using a metal object such as a screwdriver as a capacitor discharge tool is one of the ways of dissipating a capacitor charge. For this method, an insulated screwdriver is usually the preferred tool, with only its tip as the exposed metal.

This procedure first requires disconnecting the device from the main power supply. If the capacitor is on a PCB, remove the soldering and hold it in one hand by its base, making sure not to touch the leads of the capacitor.

Ensure to use a good-quality insulated screwdriver and for further caution, wear a pair of electrical gloves to ensure safety. Gloves with rubber plastic handles or other non-conductive materials on the handles are recommended to prevent possible electrocution.

Extra precaution involves checking the gloves and screwdriver’s condition to ensure the insulated materials are not damaged before performing the discharge. This is important because in working with a high-voltage capacitor, a little tear in the glove or on the screwdriver’s insulation poses threatening implications.

The steps for this method are outlined thus:

1. Hold the body of the capacitor in one hand, preferably the active hand. Again, take care not to touch the capacitor’s terminals. 
2. Next, take the screwdriver in the other hand, making sure you have adequate control over the grips.
3. Carefully touch the screwdriver with the two terminals of the capacitor at the same time. 
4. The capacitor discharges with small to medium sparks depending on the amount of charge present.
5. After a few seconds, remove the screwdriver from the capacitor.
6. Repeat this process a couple of times to make sure that the capacitor is discharged completely.

Reconnect the screwdriver to the terminals, if there are no sparks, the capacitor has been fully discharged.

The insulated screwdriver method can be used to safely discharge a capacitor if the voltage stored is relatively low (less than 50 Volts).

If the capacitor’s stored voltage is higher than 50V, discharging with a screwdriver should be avoided, as it can cause damage to the capacitor, the screwdriver, and even the user.

Instead, users should explore other methods such as the use of a bleeder resistor or a DIY discharge tool. All these are discussed in subsequent sections. [3]

2. Using a Bleeder Resistor

Another safe way to discharge a capacitor is by using a bleeder resistor. This method is commonly employed with a properly designed power supply to discharge the output capacitors after the power supply has been disconnected.

To apply this method, we connect a bleeder resistor across the terminals of the capacitor. When the circuit's power supply is disconnected, the capacitor's retained energy is discharged through the bleeder resistor. In cases where the circuit already has this resistor, the capacitor automatically discharges through it as soon as the power supply is disconnected. The rate at which the capacitor discharges is determined by the capacitance of the capacitor and the value of the resistor.

However, if the circuit does not have a bleeder resistor, it can be connected manually. This is done by connecting the leads of the resistor to alligator clips, then connecting the clips to the terminals of the capacitor. After the required calculated time has elapsed, the capacitor's charge will be completely dissipated.

One crucial factor to note is the power rating of the resistor. There are technological resources that calculate the value and power rating of the resistor, deriving from the capacitance, voltage, and time of discharge.

To safely discharge the capacitor with this method, the resistor must be rated for at least 2.5W of power dissipation. High-power resistors like this are usually expensive. [4]

3. Using a Light Bulb

Another effective way to discharge a capacitor is by using a light bulb. A light bulb with a decent power rating can be used as a “Bleeder Resistor” to discharge a capacitor.

These bulbs are essentially resistive wires enclosed in a vacuum and gas-filled. You need at least 10W-rated bulbs so that they can properly discharge the capacitor without getting damaged.

To discharge a capacitor using this method, connect the leads of the capacitor with the terminals of the bulb. Depending on the amount of charge in the capacitor, the bulb will glow slightly while the capacitor is discharging, until it fades out once it is fully discharged.

The following step-by-step process is a useful guide:

1. Take a light bulb that is connected to a bulb socket with wires.
2. Hold the resistor with insulated pliers, making sure to avoid physical contact with bare hands, as it may get quite hot during the discharge process.
3. Place the terminals of the resistor across the two plates of the capacitor. Again avoid any metal part with bare hands to prevent electrocution. The bulb will start to light up.
4. Check the multimeter display for the voltage of the capacitor. If it is not yet zero, repeat the process as required.
5. When the terminal is at zero voltage, the light bulb goes out, meaning the capacitor is completely discharged.[5]

4. Using a Discharging Tool

Discharging capacitors with large amounts of voltage sometimes requires the application of a DIY discharge tool. This means you can create a device to discharge a capacitor with materials such as electrical tapes, alligator clips, 12 gauge wire, and a 50W 20k ohm resistor.

The steps to achieve this include:

• Gather the Materials to Assemble the Required Tool

A  discharging tool is a small resistor, where the wires are connected with the alligator clips to absorb the energy from the capacitor. These materials should be organized in one place to fabricate the device.

• Prepare the Wires and Alligator Clips

For this part, wrap the two alligator clips with the electrical tapes. It is recommended to wrap each clip with a unique colored tape (usually black and red) for easy identification of which goes on what end of the resistor. Then, cut the wire into two equal, 6-inch parts, using a pair of scissors. Longer wires make for an easy connection of the ends with the capacitor poles.

• Snip off the Wire Insulation

Cut back about a half inch from the wire insulation to bring out the copper wire. 

This can be done using a wire stripper, as well as a sharp razor blade or knife, after which the wire is pulled out with the fingers. This process must be applied with care, to avoid damaging the wire.

• Connect the Wire with the Resistor Probes

The now exposed metal portion of the wire will now be connected to the resistor on one end. One end of each wire should touch one pole of the resistor. After this connection is done, the other side of each wire is left to link with the capacitor.

• Wrap and secure the Soldering Joints

Here, we introduce the black and red electrical tapes again, to wrap each soldering point of the wire. This serves to hold the connection from getting loose, as well as safeguard against any accidental electrical shocks.

• Connect the Alligator Clips with the Wire

Next is connecting the free ends of the wire with the alligator clips by soldering, and once more wrapping them with the electrical tapes to be secure.

• Connect the Alligator Clips with the Capacitor Poles 

Start this step by placing the capacitor on a plain wooden surface, making sure it is stable. Then connect each wired alligator clip with one pole of the capacitor. When the poles are in contact with the resistor through the wires, it triggers the quick discharge of the capacitor.

• Check the Level of Discharge

Finally, set the multimeter to the highest voltage and connect it with the capacitor probes and check the readings to determine the extent of the capacitor's discharge. It should ideally be below 10V.

If the reading is above 10V, reconnect the alligator clips and administer the process again, until the required discharge is achieved.

Apart from the above-mentioned, a capacitor discharge pen can be used to safely discharge a capacitor. With this, there is no need to worry about voltage, resistor values, and other parameter measurements. This is because there are pens that are tailored to specific capacitor sizes, which can simply be found written on the packaging.

To use the discharge pen, simply connect the black lead to the capacitor’s cathode terminal (represented with the – symbol), and the red-colored probe to the capacitor’s anode terminal (+symbol). [6]

Key Takeaways

This article defined capacitors as essential electrical and electronic components that store energy in the form of electric charge. We further explained the current absorbing attribute of capacitors that makes them retain an electrical charge even after being disconnected from the main power source.

For safety reasons, therefore, it is imperative to discharge capacitors before working with them. To this end, this article discussed different techniques and tools that can be applied to safely and accurately discharge capacitors.

References

1. Electronics Hub. How to Discharge a Capacitor Using Bleeder Resistor, Screwdriver, and Lamp. 2021. [Cited 2022 Dec 2] Available from: https://www.electronicshub.org/how-to-discharge-a-capacitor/

2. DIY Stadium. How to Safely Discharge a Capacitor. 2021. [Cited 2022 Dec 2] Available from: https://diystadium.com/how-to-discharge-a-capacitor/

3. Electronics Hacks. 6 Steps to Discharge a Capacitor with a Screwdriver. 2022. [Cited 2022 Dec 3] Available from: https://electronicshacks.com/how-to-discharge-a-capacitor-with-a-screwdriver/

4. Electronics Hub. How to Discharge a Capacitor Using Bleeder Resistor, Screwdriver, and Lamp. 2021. [Cited 2022 Dec 3] Available from: https://www.electronicshub.org/how-to-discharge-a-capacitor/

5. SFX PCB. How to Discharge a Capacitor with a Multimeter. 2022. [Cited 2022 Dec 3] Available from: https://sfxpcb.com/how-to-discharge-a-capacitor-with-a-multimeter/

6. Electro University. How to Discharge a Capacitor: Ultimate Guide for Beginners. 2022. [Cited 2022 Dec 3] Available from: https://electrouniversity.com/how-to-discharge-a-capacitor/