Tips on How To Discharge A Capacitor With A Multimeter

A capacitor will store excess electrical energy which it supplies to the central appliance or device in case of power stoppage or shortage. However, when you want to work with a capacitor for a new device, it is imperative that you first discharge the capacitor for safety reasons.

And today’s write-up we will discuss how to discharge a capacitor with a multimeter. Our guideline will also include suggestions on discharging an AC capacitor, from the circuit board and the use of a capacitor discharge tool.

So, don’t miss the chance to know the safe discharging technique of the capacitor.

Table of Contents

How To Discharge A Capacitor With A Multimeter

When we talk about discharging a capacitor with multimeter, there’s often a misconception that arises among us. So, let us first clear one thing-

The multimeter isn’t used directly to discharge the stored energy of a capacitor. Instead, people use it to measure the voltage and power of the capacitor to know whether it is fully released or not.

You can use different tools such as a light bulb or a DIY discharge tool for the process. But everything starts with checking the actual charge of the capacitor.

The First Step: Checking the Capacitor Charge

Step 1: Unplug from the Power Source

Make sure you have disconnected the capacitor from its power source. If you work with a capacitor in the car, disconnect the battery. And for home appliances, unplug the device from the power outlet.

Step 2: Setting Up the Multimeter

You need to set up the multimeter at its highest DC voltage capacity. Since different multimeters come with various DC voltage limit, read the user manual to know the specific DC voltage limit of the multimeter.

Then turn the knob to set the DV voltage in its peak. The highest DC setting ensures you will get the most accurate readings from the capacitor.

Step 3: Connecting the Multimeter with the Capacitor

Connect the two probes of the multimeter with two heads of the capacitor. It doesn’t matter on which capacitor head you connect the black or red probe or vice versa. Carefully hold the probes on the capacitor lead and check the readings on display.

Step 4: Understanding the Readings

Depending on the capacitor size, the multimeter display may show readings between a few volts to a few hundred volts. If the reading is above 10V, it is considered dangerous since anything above 10V can provide you with an electrical shock.

The few critical notes for the capacitor voltage readings are:

  • A capacitor doesn’t need discharging if the reading is below 10V.
  • You may use a screwdriver or light bulb to discharge a capacitor with a voltage between 10V to 99V.
  • If the capacitor has a voltage above 100V, use a specific capacitor discharge tool for safety reasons.

Now we will discuss different discharging methods of the capacitor using light bulbs, screwdrivers, and discharging tool.

Method 1: How To Discharge A Capacitor With A Light Bulb

If you have a capacitor bank with a higher voltage value, you may use a light bulb to discharge it securely. The three standard DC capacitor voltages are 100V, 200V, and 300V. You can use a 75W enabled light bulb to discharge these capacitors.

These steps include:

  1. Open all switches on the power board. Then bring up the voltage supply on the capacitor to 100V with all switched adequately opened.
  2. When the voltage reached 100V, turn off the charging switch of the device.
  3. Wait for a few moments until the digital multimeter (DMM) display shows the result strictly at 100V.
  4. When you see the readings at 100V, open the circuit for charging switch.
  5. After that carefully close the adjacent discharging switch of the device.
  6. As soon as the discharging switch is closed, the connected light bulbs will start glowing with a dim light.
  7. After a few moments open up the discharging switch again.
  8. You need to repeat the steps with 200V and 300V for the capacitor to burn out the used light bulbs properly.
  9. Finally, close down the shorting switch securely to safeguard the capacitor from electrical shocks.
  10. Use the multimeter to check if the capacitor has been discharged appropriately or not.

Method 2: Use a Screwdriver

Another easy way to discharge the capacitor is the use of a screwdriver. It is less complicated and doesn’t require any superior skills too.

  1. At first, pick an insulated screwdriver. The rubber or plastic handle of the screwdriver works as an insulated barrier to prevent shocks.
  2. Once you have the insulated screwdriver at hand, closely inspect the handle. If the handle’s plastic or rubber seems worn out, don’t use the screwdriver. The screwdriver handle should be free from any cracks, or damages on the rubberized section.
  3. Now, you need to hold the capacitor with your active hand. Make sure you don’t touch the capacitor terminals. It is suggested that you handle either side of the capacitor body for safety purposes.
  4. When you hold the capacitor body focus on gripping it at a lower end. When you use your non-dominant hand to grab the capacitor, your hand and finger will make a ‘C shape”. It offers you maximum control over the grips.
  5. Now carefully touch the screwdriver with the two poles of the capacitor at the same time. Once you connect the screwdriver with the bars, it will discharge the unit soon.
  6. After a few seconds remove the screwdriver from the capacitor. Then, connect it with the capacitor poles to check if it has been appropriately discharged or not. If there are no sparks, the capacitor is discharged correctly.

Method 3: Use of a Discharging Tool

When you want to discharge a large amount of voltage from the capacitor, applying a discharging tool is recommended. You can DIY create the device.

Things you will need:

  • Two alligator clips
  • 12 gauge wire
  • Electrical tapes
  • A 50W 20k OHM resistor

The steps include:

Step 1: Assembling the Required Tool

You should remember that a discharging tool is simply a small resistor. The wires are connected with the alligator clips to soak the voltage from the capacitor. So, organize all the required items in one location to create the discharging item.

Step 2: Preparing the Wires and Alligator Clips

Firstly, wrap the two alligator clips with the electrical tapes. Wrap one clip with black tape and the other one with red tape to quickly identify which is for what ends of the resistor. Then, use a scissor to cut the wire in two equal parts, each 6-inch long.

When you use longer wires, it offers you easiness to connect the ends with the capacitor poles.

Step 3: Clipping off the Wire Insulation

Use a wire stripper to clip off about ½” from the wire insulation to bring out the copper wire. Alternatively, you can use a razor blade or sharp knife to cut the insulation carefully. Then, you may use fingers to pull the remaining part of the wire.

Whatever process you follow, make sure you don’t damage the wire.

Step 4: Connect the Wire with the Resistor Probes

Once you have brought out the metal portion of the wire, you need to connect its one end with the resistor. You can use solders to touch one end of each wire with one pole of the resistor. Once you have connected the cables with the resistor poles, the other side of each wire is free to attach with the capacitor.

Step 5: Wrapping the Soldering Joints

This step is pretty simple. Use the black and red electrical tapes to wrap each soldering points of the wire. It secures the connection from any loose activity. Also, it acts as a safeguard against any accidental electrical shocks.

Step 6: Connecting the Alligator Clips with the Wire

This time, use your soldering skill to connect the free ends of the wire with the alligator clips. After soldering the alligator clips with the wire, wrap them using the electrical tapes for security.

Step 7: Connecting the Alligator Clips with the Capacitor Poles

Place the capacitor on a plain woody surface and make sure it doesn’t move. Now, connect each wired alligator clips with one pole of the capacitor. When the poles are associated with the resistor through the wires, it will quickly discharge the entire capacitor.

Step 8: Check the Discharging Correctness

Now connect the multimeter with the capacitor probes and check the readings to find out whether or not the capacitor has been correctly discharged. As you set the multimeter at its highest voltage and connect the capacitor, the reading should come below 10V.

If the reading is above 10V, connect the alligator clips correctly and continue the discharging once again.

You can follow these guidelines to accomplish knowing how to discharge an AC capacitor too.

Final Words

Discharging a capacitor doesn’t need any rocket science knowledge. Neither does it require any expert skills. Nonetheless, when you work to discharge the capacitor with a multimeter, you must be attentive and operate with caution to prevent electric shocks.

Also, note that the multimeter is only used to measure the voltage of the capacitor; it doesn’t have an active role in the discharging project itself. You will need a proper discharging tool or a screwdriver to complete the whole process.

Peter Gong
Peter Gong