Discharge a Capacitor
This article explains a technique that is dangerous and potentially deadly in some situations particularly when working with high voltages. It may work with a battery powered radio, but the microwave pictured has a high voltage capacitor in the double circuit for the magnetron which may be charged to 1kv or more!
Capacitors are found in a number of electrical appliances and pieces of electronic equipment. They store excess electrical energy during power surges and discharge it during power lulls to provide the appliance with a constant, even supply of electricity. The larger the capacitor, the more charge it can store per unit voltage, even after the appliance is turned off. This is not to say however that smaller capacitors cannot be dangerous. Before working on an appliance or electronic device, you must first discharge its capacitor; here are instructions to discharge a capacitor safely.
Steps
- Learn and use proper techniques and equipment for working on live electrical equipment. Don't let your unprotected hands near anything involved.
- Disconnect the device with the capacitor from electric current. Alternating current will flow through a capacitor until the source of the current is removed. That current can add to the shock you'll get if you mishandle the capacitor, and may keep recharging it.
- Look for the capacitor. Most capacitors are constructed of 2 conductive plates separated by an insulating plate; more complex capacitors feature multiple layers of metalized plastic. Big capacitors, the kind most likely to be dangerous, are usually cylindrical and look roughly like battery cells.
- Disconnect the capacitor from the system, if it is not permanently installed. This may prevent damage to the circuitry the capacitor was connected to when you discharge it.
- If it is removable, it is probably very large and potentially very dangerous.
- Touch a device to the capacitor leads for several seconds. Doing this will provide a path for the electricity to go and discharge the capacitor. You can use a 5- to 10-watt resistor, voltmeter, test light or ordinary light bulb.
- Using a meter or light will show the progress of the discharging, either with a digital display or a progressively dimming bulb.
Tips
- Once the capacitor is discharged, keep its leads connected with a resistor or piece of wire to keep it discharged.
- Capacitors will discharge on their own over time, and most are likely to be discharged after a few days so long as no external power or internal battery is charging them -- but assume they are charged unless you have confirmed that they are discharged. The device should be unplugged the whole time, not just switched "off".
- Do not lick your fingers and then touch either side of the terminals to discharge! This will light you up!
- Do not hold the resistor in your hands, use a test lead or wire.
Warnings
- Large capacitors are extremely dangerous and others are often in the vicinity of one you might attempt to work on. Working with them probably isn't best for a typical hobbyist.
- While it is also possible to use a small screwdriver to connect the capacitor leads, the amount of current discharged may melt the screwdriver's tip or the copper on the circuit board if the capacitor is still connected to it. Particularly large sparks can fry power supplies or turn melted copper or solder into a projectile that can injure you.
Things You'll Need
- Resistor, voltmeter or light bulb (to discharge a capacitor)
- Length of wire (to keep the capacitor discharged)
Related Articles
- Build a Capacitor
- Install a Capacitor
- Test a Capacitor
- Read a Capacitor
- Check a Resistor
