How to discharge a capacitor

How to Safely Discharge a Capacitor?

Capacitors have many electrical and electronic applications from your household appliances to smartphone to computers. A capacitor’s main function is to store electrical energy to assist an electronic device’s operation in different ways. They can be used to provide different types of electronic filtering, to protect components, and provide a means to store and discharge energy.

Simply put, a capacitor gradually accumulates charge to reach the necessary voltage level and then discharged quickly to provide the energy needed by an electrical device. A charged capacitor left by itself will retain this charge for a long time, even years.

Why Do We Need to Discharge Capacitors?

Capacitors store electric charge. When a capacitor is disconnected from its supply voltage or power supply, the voltage (and current) it carries is maintained across its terminals, which can be dangerous. This excess electrical energy needs to be safely dissipated.

This is why it’s very important to discharge a capacitor before you disconnect it to remove all its stored energy.

Will a Capacitor Discharge On Its Own?

Yes, a capacitor automatically discharges on its own. In theory, a capacitor will gradually lose its charge. A fully charged capacitor in an ideal condition, when disconnected, discharges to 63% of its voltage after a single time constant. Thus, this capacitor will discharge up to near 0% after 5 time constants.

All capacitors have leakage so we can imagine that we have a very high-resistance (mega ohm) resistor parallel to the capacitor. When the capacitor is disconnected, the voltage will be discharged via this imaginary resistor. This is what causes the gradual discharge.

However, every capacitor has a different capacitance and will need a different time period to fully discharge. If it’s a really big capacitor, then the charge might stay for months and even years. Not to mention, things can always go wrong even in smaller capacitors and these charges would remain in the capacitors.

The problem is, these capacitors can’t notify you about these charges until they cause damages, which can be life-threatening. This is why the ideal practice is to properly discharge the capacitors manually for safety reasons.

How to Safely Discharge a Capacitor?

Image of a circuit board with power supplies, output capacitors and other electronic components

Before we get into our discussion of how to discharge a capacitor, we have to first understand how a capacitor works.

How does a capacitor work?

Capacitors are made of two electrodes that are separated by a dielectric material. The capacitor stores electric charge of the same value and opposite potentials are accumulated within it.

There are actually several different types of capacitors, but the simplest of them is made of two metals with a dielectric material (ceramic, impregnated paper, or even air) in between. These conductive plates are used to store energy.

When the capacitor is connected to a main power supply, the capacitor begins the process of storing electric charge through electricity accumulation in the capacitor poles. When the power supply is disconnected, the electrical charge remains on these capacitor plates.

The accumulated charges between the two capacitors always have equal value but with opposite potentials, just like in a battery. Now, to safely discharge the capacitor, we can simply follow a similar process to charging this capacitor, but it will vary depending on the type and capacitance of the capacitor, as we will discuss below.

Safely discharging a capacitor

In general, safely discharging a capacitor is about connecting a resistive load that will be able to dissipate the electrical energy stored in the capacitor. As a general rule of thumb, a capacitor charge of more than one farad should be discharged carefully and we’d recommend using special capacitor discharge tools (see below).

Is there a Special Capacitor Discharge Tool?

Yes! You can use a capacitor discharging tool such as this Capacitor Discharge Pen.

Supplying Demand CAPDIS Capacitor Discharge Pen Tool with LED Indicator 14 Inch Lead 440 VAC Max

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Having a proper discharging tool in your toolbox is an essential component when working with capacitors. This will provide visual indication that the capacitor is discharged completely.

How Do You Discharge a Capacitor With a Multimeter?

Image of a multimeter with a good power rating

You cannot discharge a capacitor with a multimeter, per se, but a multimeter is useful to check the voltage stored in a capacitor so that we can choose an adequate resistive material to actually perform the discharge.

First, make sure you are using a proper multimeter to ensure your safety and accuracy, and you can use our previous guide on the best multimeters available in the market to help you get the right multimeter for the job.

We can use either an analog multimeter or digital multimeter to perform this job, simply turn the multimeter into a voltage reading and check the voltage of the capacitor:

  1. Set up a multimeter to its highest possible DC voltage setting
  2. Connect the multimeter problem to the plates of the capacitor
  3. Read the voltage reading in the multimeter display, make sure it’s accurate

How Do You Discharge Capacitors With a Screwdriver?

An insulated screwdriver is a perfect DIY discharge tool.

As discussed, you can use an insulated screwdriver with a decent power rating (voltage rating) to safely discharge a capacitor if the voltage stored is relatively low (below 50 V).

First, make sure you are using a good-quality insulated screwdriver and we recommend you also wear a pair of electrical gloves to prevent accidental electrical shocks. Choose one with rubber plastic handles or other non-conductive materials on the handles to prevent yourself from getting electrocuted.

Always assume all capacitors are in a charged condition and so always hold the body and don’t touch the plates/terminals of the capacitor for safety reasons.

Also, check your gloves and screwdriver’s condition to see whether or not the insulated materials are damaged before you perform the discharge. This might seem like a simple thing to do, but if you are discharging a high-voltage capacitor, even a small tear in your glove or on the screwdriver’s insulation might be threatening for you.

Then, you can follow the following steps:

  1. Hold the capacitor’s body with your active hand. Again, make sure you don’t touch the capacitor’s terminals. Make sure you have adequate control over the grips.
  2. Carefully touch the insulated screwdriver with the two plates/terminals of the capacitor at the same time. The discharge process should now happen. Warning, you may see small to medium sparks when performing this step.
  3. After a few seconds, remove the insulated screwdriver from the capacitor.
  4. Reconnect the screwdriver to the plates, if there are no sparks, the capacitor has been fully discharged. Repeat the process as needed.

How to Discharge Capacitor with a Bleeder Resistor?

Image of capacitor probes (resistor probes) holding a bleeder resistor by only its tip

If the capacitor’s stored electric charge is higher than 50 V, then don’t discharge it with a screwdriver. You’ll risk damaging the capacitor, the screwdriver, and even yourself.

Instead, you can use the light bulb method (discussed below) or use a high-voltage bleeder resistor to do the job:

  1. Use insulated pliers, and hold the high-voltage resistor in the middle. Don’t touch the resistor with your hands, as it may get very hot during the discharge process.
  2. Place the terminals of the resistor across the two plates of the capacitor. Don’t touch any metal part with your hands or you may get electrocuted.
  3. Use the multimeter and re-check the voltage of the capacitor. If it’s not yet zero, repeat the process to properly discharge the capacitor.

If the terminal is showing zero voltage, the capacitor is completely discharged.

How to Discharge a Capacitor with a Light Bulb?

Image of light bulb to ensure the capacitor is appropriately discharged

You can discharge a capacitor with a light bulb. The bulb acts as a bleeder resistor in the circuit, allowing the stored charge in the capacitor to gradually flow through and dissipate as heat and light in the bulb.

To discharge a capacitor with a light bulb, you would typically connect the capacitor and the bulb in series. Here’s a basic procedure:

  1. Ensure that the capacitor is disconnected from any power supply.
  2. Connect one terminal of the capacitor to one terminal of the light bulb.
  3. Connect the other terminal of the light bulb to the other terminal of the capacitor.
  4. Monitor the capacitor’s charge. The bulb will start glowing as the capacitor discharges through it. Once the light is completely out you will know the capacitor has been properly discharged.

Special Considerations for High-Voltage Capacitors

High-voltage capacitors, due to their increased energy storage capacity, require a heightened level of caution and specific safety measures when discharging. The potential risks associated with high-voltage large capacitors, such as electric shock, arc flash, and equipment damage, necessitate additional precautions for those working with these components.

Isolation and Lockout/Tagout Procedures

High-voltage capacitors often exist within systems with multiple power sources. Implementing isolation procedures and lockout/tagout protocols is crucial to ensure that the capacitor is completely de-energized and cannot be accidentally re-energized during the discharge process. This involves physically disconnecting power supply, using lockout devices, and tagging equipment to inform others that maintenance work is in progress.

Before starting the discharge process, technicians should verify the isolation of the capacitor by using appropriate voltage detectors or meters. This step is essential to prevent any potential catastrophic events that may occur if the capacitor remains energized.

Using a Discharge Tool

High-voltage capacitors require specialized discharge tools designed to handle the unique challenges associated with their discharge. These tools often have higher voltage and current-handling capabilities, ensuring safe and efficient discharge without risking damage to the tool or compromising the safety of the operator.

Discharge tools for larger capacitors may include heavy-duty discharge sticks, insulated discharge probes, or other devices specifically engineered for high-energy systems. It is crucial to select tools that match or exceed the voltage and current ratings of the capacitor being discharged.

Additionally, the discharge tools should be properly insulated to prevent any accidental contact with live components during the discharge process. Insulation integrity must be regularly checked to ensure ongoing safety.

Controlled Environment

Discharging high-voltage capacitors should take place in a controlled environment to minimize risks. This environment should be free of flammable materials, with proper ventilation to dissipate any heat generated during the discharge. Adequate personal protective equipment (PPE), including insulated gloves and safety glasses, becomes even more critical when dealing with high-voltage systems.

The workspace should be organized and well-lit to facilitate a clear view of the capacitor and associated components. Emergency response equipment, such as fire extinguishers and first aid kits, should be readily accessible.

Emergency Procedures

High-voltage capacitor discharges should be accompanied by well-defined emergency procedures. In the event of an unexpected incident, technicians should be trained to respond promptly and effectively. This may involve having an emergency shut-off procedure, clear communication protocols, and evacuation plans in place.

Emergency response drills can help personnel become familiar with the steps to take in case of an unforeseen event, fostering a quick and coordinated reaction to mitigate potential risks.

Capacitor Inspection and Maintenance

Regular inspection and maintenance of high-voltage capacitors are paramount to identify potential issues before they escalate. Inspections should include a thorough examination of the capacitor’s casing, terminals, and associated connections. Any signs of damage, leakage, or deterioration should be addressed promptly to prevent potential hazards.

Frequently Asked Questions

Why do capacitors need to be discharged before handling?

Capacitors store electrical energy and can retain a charge even after being disconnected from a power source. Discharging capacitors before handling reduces the risk of electric shock and ensures a safe working environment.

Can I discharge a capacitor by shorting its terminals with a screwdriver or other metal object?

While it’s technically possible, shorting capacitor terminals directly with a metal object can lead to sparks, damage the capacitor, and pose safety risks. It’s recommended to use controlled discharge methods like resistors or discharge tools for a safer process.

How do I choose the right resistor for discharging a capacitor?

The choice of resistor depends on the capacitance of the capacitor and the desired discharge time. You can use the RC time constant formula (? = R * C) to calculate the required resistance (R). Select a resistor with an appropriate resistance value based on this calculation.

Can I discharge a capacitor without any external components?

In theory, yes, but it’s not recommended. Discharging a capacitor without external components can lead to rapid and uncontrolled discharge, resulting in sparks and potential damage to the capacitor. Controlled discharge methods using resistors or discharge tools are safer alternatives.

Is it necessary to discharge low-voltage capacitors?

Yes, even low-voltage capacitors, such as those used on circuit boards, should be discharged before handling. While the risk of electric shock is lower with low-voltage capacitors, it’s a good practice to ensure safety and prevent any unexpected discharge.

Can I use a light bulb to discharge any type of capacitor?

In general, yes. The light bulb discharge method is versatile and can be used with various types of capacitors. However, ensure that the light bulb’s voltage rating is suitable for the capacitor being discharged to avoid damaging the bulb.

How long does it take for a capacitor to discharge completely?

The discharge time depends on the capacitance of the capacitor and the resistance in the discharge circuit. It can be calculated using the RC time constant formula (? = R * C). Keep in mind that a practical approach is to monitor the discharge process and wait until it is visibly complete.

Are there any risks associated with discharging high-voltage capacitors?

Yes, discharging high-voltage capacitors comes with increased risks. It’s crucial to follow additional safety precautions, use specialized tools designed for high-voltage systems, and work in a controlled environment to minimize potential hazards.

Can I discharge a capacitor by disconnecting it from the circuit?

Disconnecting a capacitor from the circuit is a good first step, but it may still retain some charge. To ensure complete discharge, use one of the recommended discharge methods such as resistors, discharge tools, or the light bulb method.

Can I reuse a capacitor after discharging it?

In most cases, yes. Once a capacitor is discharged and confirmed safe, it can typically be reused. However, always inspect the capacitor for any signs of damage or degradation before incorporating it back into a circuit.