Capacitors are crucial elements in so many electronic devices from your household appliances to your smartphone to your computers.
A capacitor’s main function is to store electrical energy to assist the electronic device’s operation in different ways. What is the purpose of this stored electrical energy?
In a nutshell, the capacitor can be slowly charged to reach the necessary voltage and then discharged quickly to provide the energy needed by the electrical device.
The thing is, a charged capacitor left by itself will retain this charge for a long time, even years. When a capacitor is disconnected, the instant voltage 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.
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.
Before we can discuss how to safely discharge a capacitor, we have to first understand how a capacitor works.
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.
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 resistance 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 resistance load, and the capacitor will illuminate the light bulb, effectively discharging the energy stored in the capacitor.
Once the bulb is turned off, then 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:
Below we will discuss more specific ways to discharge a capacitor with various tools.
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:
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 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 charged condition and so always hold the body and don’t touch the plates/terminals of the capacitor for safety reasons.
Also, check the screwdriver’s condition whether the insulated material is 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 on the screwdriver’s insulation might be threatening for you.
Then, you can follow the following steps:
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:
If the terminal is showing zero voltage, the capacitor is completely discharged.
Yes! You can use a capacitor discharge pen such as the Sparkpen Battery Capacitor Discharge Pen.
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).
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