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How to discharge capacitors safely
Capacitors

Written by Brian Bell

10/07/2022

Introduction

Capacitors are in devices such as consumer electronics, HVAC devices, and even medical devices. However, these are just a few of the devices that use capacitors. Believe it or not, capacitors are used in almost every device out there. But, did you know many capacitors can be dangerous? Therefore, knowing how to safely discharge capacitors is important for anyone who works in electronics.

What are capacitors?

First off, capacitors store energy. Consequently, this is done by storing charges on two conductive plates, separated by a non-conductive material (aka as the dielectric).

Additionally, there are different types of capacitors. Capacitors come in different shapes, sizes, and colors. However, the most important things to know about your capacitor are the capacitance and the voltage rating.

Electrolytic Capacitor

Now, capacitance is measured in Farads and the voltage rating is displayed in volts. Moreover, common values for capacitors are often in the μF and nF range but can get much larger for certain applications. In addition, common voltage values can range anywhere from 10 V to 1000 V. 

How can capacitors hurt you?

First, you may wonder are capacitors safe? Second you may ask, “whats the worst that can happen?”. 

Well that all depends on the amount of energy stored in the capacitor, at what voltage the energy is stored, and the path of the current. Capacitors can discharge energy very fast. Consquently, if you are the path of discharge you may experience tingling, shock, burns, and heart arythmias. 

To summarise, charged capacitors can discharge current through your body and cause harm. 

Learn how to discharge a capacitor safely

Therefore, one of the best ways to learn how to discharge capacitors safely is to understand how capacitors work. If you want to learn more about capacitors, including what not to do, watch this video. 

Furthermore, the general outline of steps include:

  1. Understand the circuit
  2. Unplug the power
  3. Locate the capacitor
  4. Determine the best way to discharge
    • If the capacitor is small (under 24 V) and you are not worried about damaging the electronics, you can short out the capacitor leads to discharge it quickly using a tool such as a screwdriver.
    • else if, the capacitor is small but you do not want risk damageing the electronics use a correctly sized and rated resistor to discharge the capacitor
    • Otherwise, the capacitor is larger than 24 V consider using a discharge tool to safely and slowly reduce the charge in the capacitor.
  1. Wait till the capacitor is fully discharged
  2. Verify discharge with a voltmeter before removing

To summarize, these are general guidelines – the most important step is step 1. If you understand the circuit it is possible to safely and quickly discharge larger capacitors with other methods. To conclude, these are just guidelines and the best solutions is going to depend on your specific situation. 

Note: 24 V is a very conservative number most capacitors that are less than 100 V will not cause serious injury. In the past, when I was a kid, I use to shock my friends with 250 V low capacitance camera flash capacitors all the time – and while it did hurt it was not very dangerous. That is why it is best to know your situation and understand all the hazards present. 

 

For What tools can I use? 

As mentioned above, the best tool you have is your mind. Therefore, make sure you understand your situation and basic electrical safety methods.

Next, make sure you have a quality meter to test voltage of capacitors. Further, always verify that you have discharged a capacitor before you try to take it out of the circuit. 

Next, if you are dealing with high voltages, I recommend using a tool specifically designed for discharging capacitors. One example is this High Voltage Discharge Stick. You can also build your own discharge jig with a correctly sized resistor and insulated wire. For example this is a High Wattage Resistor that can be use to discharge certain capacitors.

Finally, if you find yourself using your screwdriver to quickly discharge capacitors, make sure they are designed for electrical work. For example, these drivers can be used around high voltage.

How long to discharge? 

Moreover, it is critical to know how long it takes to discharge a capacitor. Because, you may discharge a capacitor partially and it can still remain dangerous. Therefore, you need to have some sense of how long it takes to discharge.

The discharge time of a capacitor is defined by the time constant. The time constant is: T = C * R. Where, T is the time constant in seconds, C is the capacitance in Farads, and R is the resistance in Ohms. Furthermore, for a capacitor to be considered fully discharged we must mulitply the time constant by 5. For more infomration on discharging a capacitor check out this video

General Tips

Finally here are some general tips when working with capacitors

  • After discharging a high voltage capacitors – short the leads out with a resistor or alligator clips.
  • Most capacitors discharge when there is no external power, but always verify with a voltmeter before assuming the capacitor is discharged. 
  • Never assume circuits have bleed resistors always check voltage
  • Use one hand when working with high voltages do reduce the chance of current going through your heart
  • Discharging a capacitor too fast can be dangerous for you and the circuit – therefore its always better to err on the side of to slow
  • Do not wear jewelry when working on high voltages
  • Keep liquids aways from high voltage circuits 

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