How to discharge a capacitor

How to Safely Discharge a Capacitor?

Capacitors are crucial elements in so many electronic devices 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, and provide a means to store and discharge energy. 

Simply put, a capacitor can be slowly charged 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.

When a capacitor is disconnected, the voltage (and current) it carries is maintained across the previously connected terminals, which can be dangerous. This is why it’s very important to discharge a capacitor before you disconnect it to remove all charges and corresponding voltage.

Will a Capacitor Discharge 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 discharge the capacitors manually for safety reasons.

How To Safely Discharge a Capacitor?

Before we can discuss how to safely 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 will store an 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 metal plates are used to store electrical energy.

When this capacitor is connected to electricity, the voltage supply begins the process of electricity accumulation to these capacitor plates.

When the voltage source is then disconnected (due to electrostatic attraction), 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

As a general rule of thumb, capacitors with more than one farad should be discharged carefully and we’d recommend using special capacitor-discharge tools (we’ll discuss more on this below).

In general, safe discharging a capacitor is about connecting a resistive load that will be able to dissipate the electrical energy stored in the capacitor.

For example, if it’s a 200 V capacitor, then a 220 V light bulb can act as a resistive load, and the capacitor will illuminate the light bulb, effectively discharging the energy stored in the capacitor.

Once the bulb is turned off, the capacitor is now completely discharged. You can use a resistive receiver for this purpose, not exclusively a light bulb, but you should get the idea.

Thus, the basic steps of discharging a capacitor are as follows:

  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 accurate 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 (i.e. screwdriver) 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 need.

Below we will discuss more specific ways to discharge a capacitor with various tools.

How Do You Discharge a Capacitor With a Multimeter?

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:

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

How Do You Discharge a Capacitor With a Screwdriver?

As discussed, you can use an insulated screwdriver 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 ensure your safety. 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 screwdriver with the two plates/terminals of the capacitor at the same time. The discharge process should now happen
  3. After a few seconds, remove the 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 a Capacitor With a Resistor?

If the capacitor’s stored voltage 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 as discussed above or use a high-voltage 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 as needed.

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

Is there a Special Capacitor Discharge Tool?

Yes! You can use a capacitor discharge pen such as the Sparkpen Battery Capacitor Discharge Pen.

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When using a capacitor discharge pen, you don’t have to worry about voltage, resistor values, and so on. Simply check the pen’s box what size capacitors it can safely deal with.

Sharkpen Discharge Pen, for example, is safe for capacitors between 5V-1000V. To use the pen, simply connect the black lead to the capacitor’s cathode terminal (the – symbol on the capacitor body), and the red lead/probe to the capacitor’s anode terminal (+symbol).