Notes

How to Discharge a Capacitor
How to Discharge a Capacitor

Forewords

Why would you need to discharge a capacitor? Capacitors can hold an electrical charge much long after a device is powered off, making them an unexpected danger (i.e. shock hazard) to anyone who might service the device. It’s very important to always treat capacitors as if they have a charge until you prove otherwise. For safety and precision requirements of general projects, it’s imperative to discharge a capacitor before implement it to the circuit.


Detect and Determine a Charge









You don’t want to be kept in the dark that the capacitor you use remains charge until you get electrocuted by it. Besides, without knowing the specific charge, you cannot decide what resistive material/tool you are going to apply. Thus, detect and determine a charge of the capacitor should be the first thing to settle in the discharge process. Generally, a multimeter is all you need to get it done.

- Step 1. Unplug all power supplies to the capacitor.
- Step 2. Set up a multimeter to its highest DC limit to get an accurate reading.
- Step 3. Connect the multimeter probes to the capacitor leads.
- Step 4. Read and take down the voltage reading that the multimeter displays.


Discharging with a Screwdriver





It should be stated that if you use a screwdriver to do the discharging job, especially when you are confronted with a capacitor of high capacitance or carrying over 50V, there might be considerable sparks and surge that can damage the screwdriver, the capacitor, and even yourself. It is a less recommended method while it’s probably the simplest way to conduct, though.

- Step 1. Measure the charge on the capacitor’s terminals to see if it is suitable to be discharged with a screwdriver.
- Step 2. Inspect your screwdriver carefully in case of any damage to the insulated rubber-plastic handles. If there are any tears on the handle, don’t hesitate to change it with another one.
- Step 3. Grab the capacitor with one hand stably (or wherever the cap can stand steadily) and point the screwdriver to the terminals of the capacitor at the same time. It’s likely to see instant sparks occur which is a normal phenomenon so try don’t panic.
- Step 4. Once there are no longer sparks happening, move off the screwdriver and measure the capacitor again to avoid the residual charge.


Discharge a Capacitor with a High OHM Resistor

By means of a high ohm resistor (for example, a 20k ohm resistor), the discharging process can be convenient as long as safe and more “gentle”. When it comes to choosing the appropriate resistor, a table is for you to refer to.












Of course, you’d prefer reaching out to the online capacitor-discharge calculator which can not only figure out how high the ohm should the resistor has but also how long it’s going to take for the discharging process. Now follow me through these steps.





- Step 1. Check the charge of your capacitor.
- Step 2. Choose the properly sized resistor using the discharge calculator mentioned above.
- Step 3. Use an isolated plier to hold the resistor. Be aware that you should never touch any metal terminals with your bare hand.
- Step 4. Connect the terminals of the resistor with the leads of the capacitors. Some capacitors may have more than two leads; in that case, find out the negative and the positive leads and connect the resistor onto them.
- Step 5. After a while, re-measure the capacitor according to section 2 instructions to ensure it has been safely discharged.

While ac capacitor is discharging the capacitor, you can attach meter leads to measure the real-time charge simultaneously, which can save your time cost in step 5.


Use a Light Bulb to Discharge a Capacitor













Another method is to use an incandescent bulb with a voltage rating higher than the capacitor. Easybom suggests a 75W enabled light bulb that can be used to discharge a 100V ~ 300V capacitor.

- Step 1. As we did with the resistor method, check the capacitor charge using a multimeter.
- Step 2. Connect the light bulb to the capacitor. Be careful not to touch any metal plates directly with your hands.
- Step 3. The light bulb may emit a dim or bright radiance depending on how much the charge is.
- Step 4. Wait for the radiance to fade out and re-detect the capacitor’s charge.


Final Words











Except for the three methods listed above, you may also create your discharging tool to meet your custom needs for your PCB design or houseware use. Keep in mind that the re-measure step shall not be skipped.


URL1: https://www.easybom.com/
K1: Easybom

URL2: https://www.easybom.com/c/capacitors
K2: capacitors

URL3: https://www.easybom.com/c/resistors
K3: resistors

Website: https://www.easybom.com/c/capacitors