Making a DIY Capacitor Discharge Tool

How to build a DIY capacitor discharge tool

When working with capacitors, especially high voltage ones, a capacitor discharge tool is essential. It is a device that helps you drain capacitors safely before handling them further.

While you can buy a capacitor discharge tool, they are just as easy to make. It is a quick, simple project that only requires a couple of components and a bit of your time.

In this article I will teach you how to make a capacitor discharge tool for yourself and show you exactly how to use it.

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Capacitors can be extremely dangerous! The content in this article is for informational purposes only. Any actions you undertake based on this information are your own responsibility.

What is a capacitor discharge tool?

In its most basic form, a capacitor discharge tool is simply a resistor with two wires attached to it, one on either end. It is used to create a path for the charge in a capacitor to drain safely through.

Some capacitor discharge tools come with one or more LEDs that indicate the current charge of the capacitor that it is connected to. Other than that, they are all pretty much the same.

You can also find them under the names capacitor discharge pen, discharge tool, or some other combination of these words.

Why do you need one?

When desoldering and replacing a broken capacitor, salvaging components or otherwise working on electronics, you will regularly encounter capacitors that still contain a charge. If the charge is large enough and is not handled correctly, it can be dangerous.

Powerful capacitors with a sufficiently high charge can shock, burn, injure and even kill you. Because of this, you must drain any potentially dangerous capacitor that you come across.

Discharging capacitors with other methods can be unsafe. For example, a commonly used method is to short out capacitor terminals with a screwdriver. This creates a path with very little resistance, releasing a lot of charge in too short of a time.

While this does achieve the goal of discharging the capacitor, it also can damage the capacitor, along with other components on the circuit board, the screwdriver and you.

To avoid this, it is recommended to use a proper discharge tool that drains charge from capacitors more slowly.

How to make your own capacitor discharge tool

Making your own discharge tool takes a small amount of time and tends to be cheaper than buying one. Especially if you are like me and you have most of what you need already lying around.

The design I used here is as simple as it gets. It is also possible to make a capacitor discharge tool with LED, but I prefer to keep devices like these as simple as possible.

By adding more components like an LED, additional resistor and diodes, you introduce more points of failure. Both in the additional components themselves and in their solder connections.

When discharging high voltage capacitors, it is safer to rely on the voltage measurement of a quality multimeter than on an indicator LED. After all, if the LED is off, you can’t be sure whether the capacitor is actually discharged or if something is wrong with the LED or its circuit.

What you’ll need

Materials

  • For discharging small capacitors: ~2kΩ 5W resistor. If you want to discharge large high voltage capacitors (>400V), you are better off using a ~20kΩ 5W resistor.
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Don’t use regular through-hole resistors! These are typically only rated for 1/4 or 1/2 Watt, which is not enough for discharging capacitors.
  • Electrical wire with 16AWG thickness or thicker. Alternatively, you can use a set of insulated probes. These are easier to handle and come with multiple probe/clamp options, like alligator clips. Just make sure that they are rated for 600V or higher voltages.
  • Heat-shrink tubing that is a bit larger in diameter than the wire you plan on using.

Tools

Step-by-step

A pair of wire clippers being used to cut a blue wire.
Create two wire segments with a wire clipper of about 20 cm / 8″ in length. This is long enough to let you comfortably drain capacitors on a variety of circuit boards, but is not so long that the discharge tool becomes impractical to store.
A pair of quality wire strippers being used to remove the insulation from the end of a wire.
Use wire strippers to strip 5mm (1/4″) of insulation from one end of each wire. If you use a set of probe leads for the discharge tool, only strip the wire side of the leads.
A pair of wire clippers being used to shorten the leads on a 2.2kOhm 5W resistor.
Shorten the resistor leads. They need to be about the same length as the exposed copper on the wires.
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If you use a set of probe leads, slide a piece of heat shrink over each of the probe wires at this point. Heat shrink is too narrow to slide over the probe end, so it has to be slid over the wire before soldering.
A soldering iron soldering a strand of wire to the lead of a 5W resistor.
Solder the wire to the resistor leads. Make sure to use plenty of solder, when you use thick wires there are a lot of gaps to fill between the copper strands.
An upside down resistor for a capacitor discharge tool flipped upside down to show an imcomplete solder joint.
Flip the resistor and check whether the underside of the wire is properly soldered. If it still has exposed copper, continue adding solder on this side of the solder joint.
A 5W 2.2kOhm resistor with two blue wires attached to it.
Repeat the previous steps for the wire on the other end of the resistor.
A hand sliding a piece of heat shrink tubing over a solder joint.
Slide the heat shrink over the solder joints. The pieces should be long enough to cover the entire length of the exposed metal.
A piece of heat shrink tubing being shrunk by the heat from the nozzle of a rework station.
Heat the heat shrink. You can use a lighter, heat gun, hot air rework station, or even hold it above your stove. Just gradually heat it until it completely shrinks.
A piece of thick copper wire in the mouth of a wire stripper.
If you used plain wires (instead of probe leads), strip and tin the ends. This ensures that the ends keep their shape over time.

 

The soldered end of a wire being sharpened with a wire clipper.
To make the tool more accurate, you can cut the ends into a sharp point with a pair of wire clippers.
Front view of a DIY capacitor discharge pen
That’s all there is to it! You now have your very own DIY capacitor discharge tool.

Tips

  • Make sure that all wires and leads are properly insulated. You don’t want the discharge tool to have any exposed metal that you can accidentally touch.
  • Double check that the wires (or probe leads) are rated for the maximum voltage that you will be working with. Typically, 600V rated wires are more than sufficient.

How to use a capacitor discharge tool

For the instructions on how to use the DIY capacitor discharge tool I will refer you to the dedicated article on how to safely discharge capacitors. It has all the do’s and don’ts of properly handling capacitors when working with electronics.

Conclusion

As you can see, it is pretty straightforward to build a DIY capacitor discharge pen. If you are into electronics, chances are that you already have most of the required parts and tools lying around.

What matters is that you use the right resistor (resistance and wattage) for your desired application. On top of that, you need to make sure that the tool has no exposed metal anywhere. Aside from at the tips, of course.

If you find this article useful, please share it or leave a comment. I love to hear your feedback and questions!

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