How to Test an AC Capacitor with a Multimeter?

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What is an AC Capacitor?

AC Capacitor

An AC Capacitor is a temporary storage unit for electrical energy in an alternating current circuit. It is made up of two conducting plates separated by an insulating substance called dielectric. When a voltage is supplied, the capacitor plates create and store opposing electrical charges. The charge on the plates varies as the voltage does, allowing current to flow through the circuit.

What Does an AC Capacitor Do?

AC capacitors play an important role in a variety of appliances.

Power Factor Correction: They increase the efficiency of alternating current circuits by reducing reactive power, resulting in decreased energy consumption and cheaper electricity costs.
Signal Filtering: Capacitors filter out undesirable low-frequency AC frequencies while allowing high-frequency signals through. This is necessary in electrical circuits for noise reduction and proper signal transmission.
Motor Start and Run: Capacitors supply the initial surge of current required to start a motor and then maintain smooth functioning.

How AC Capacitor Works?

An AC capacitor's primary principle is the ability to store and release electrical charge. When the voltage is applied:

Charging: A positive voltage attracts electrons to one plate and a negative voltage repels electrons from the other. This produces an electric field across the dielectric, which stores energy within the capacitor.
Discharging: As the voltage changes, the electric field diminishes, allowing the stored charge to flow back into the circuit. The capacitor discharges, releasing the previously stored energy.

How to Test AC Capacitor with a Multimeter?

Before testing a capacitor, make sure the appliance is unplugged and fully discharged (see the safety section below). Here's how to test an AC capacitor using a multimeter: Tools:

Multimeter with capacitance measurement capability.
Screwdriver (to access the capacitor).

Steps:

Discharge the Capacitor: This is critical for safety concerns. See the "How to Discharge an AC Capacitor" section below.
Locate the Capacitor: Locate the capacitor within the appliance. It is usually a cylindrical or rectangular component with two terminals.
Set Multimeter: Adjust your multimeter's capacitance (uF) range. If you're unclear about the range, start with a higher setting and modify as needed.
Touch the Probes: Carefully connect the multimeter probes to the capacitor connections. Important: Do not touch the metal sections of the probes while they are attached to the capacitor.
Observe the Reading: A good capacitor will usually exhibit a short deflection of the needle toward infinity (open circuit), followed by a gradual increase to a certain value and then a gentle drop back to zero.
Interpret the Results:
- Open Circuit: If the meter indicates no deflection or an "OL" reading, the capacitor is most likely open and requires replacement.
- Shorted: If the meter deflects but remains at zero (overload), the capacitor is shorted and must be replaced.
- Low Capacitance: If the displayed value is much lower than the capacitor's rated capacitance (written on the body), it may be losing its ability to hold charge and should be replaced.

Video related to How to Test an AC Capacitor

How to Discharge AC Capacitor?

AC capacitors can hold harmful amounts of voltage even when the appliance is turned off. To safely discharge a capacitor before testing or replacement:

Unplug the Appliance: Disconnect the appliance from its power source.
Shorten the Terminals: Using an insulated screwdriver, carefully touch one of the capacitor's terminals and then briefly touch the other. A spark may appear, indicating that the capacitor is discharging. Caution: To avoid overheating the screwdriver or capacitor, hold it by the insulated handle and make brief contact.
Repeat and Verify: Touch and hold the screwdriver tip to one terminal before briefly touching the other. If there is no spark, the capacitor has likely discharged. However, it is recommended that you perform this process several times to ensure accuracy.
Wait a Few Minutes: Allow enough time for any leftover charge to dissipate fully.

How Long Does AC Capacitor Last?

The lifespan of an AC capacitor is determined by a variety of parameters, including operating temperature, quality, and usage patterns. AC capacitors typically last between 5 and 20 years, with higher-quality capacitors lasting longer.

AC Capacitor Symptoms

Several signs may suggest a malfunctioning AC capacitor:

Reduced Appliance Performance: The appliance may struggle to start, operate inefficiently, or produce less electricity.
Unusual Noises: Malfunctioning capacitors might cause the device to emit humming, buzzing, or clicking sounds.
Breaker Tripping: When a capacitor fails, it might consume too much current, causing circuit breakers to trip more frequently.
Overheating: The device may become unusually hot to the touch due to excessive energy consumption caused by a damaged capacitor.

How to Troubleshoot AC Capacitor?

If your appliance shows signs of a failing capacitor, here's a simple troubleshooting method:

Isolate the Problem: Check for other possible culprits besides the capacitor. Ensure that the appliance is correctly plugged in and receiving power. Check for any loose connections or damaged wiring inside the device.
Visual Inspection: Visually inspect the capacitor for bulging, leaking, or discoloration on the body. These are indications of a damaged capacitor.
Test with a Multimeter: To measure the capacitor's capacitance with a multimeter, follow the instructions given before.

How to Replace AC Capacitor?

To replace an AC capacitor, you'll need some technical knowledge and soldering skills. If you are uneasy with this technique, you should obtain assistance from an experienced appliance repair technician. Here's a general outline of the steps involved:

Gather Information: Determine the particular capacitor model required for your appliance. For information on replacement parts, consult the appliance handbook or visit the manufacturer's website.
Safety First: Ensure that the item is unplugged and discharged.
Access the Capacitor: Locate the capacitor within the appliance and gently remove it with the required tools (this may require soldering or desoldering connections).
Install a New Capacitor: Solder the replacement capacitor's leads to the appropriate terminals, ensuring proper polarity (positive and negative leads match the previous capacitor).
Reassemble and Test: Carefully reconnect the appliance and plug it back in. Check the appliance for correct operation.

How Much is an AC Capacitor?

The cost of an AC capacitor varies with its capacitance (microfarads), voltage rating, and brand. AC capacitors typically cost between a few and tens of dollars.

Where to Buy AC Capacitor?

AC capacitors can be acquired from the following sources:

Electronics Stores: Local electronics stores may carry a variety of common AC capacitor kinds.
Online Retailers: Many online stores sell a large range of AC capacitors at cheap costs. Make sure you find a reliable seller with a fair return policy.
Appliance Components Suppliers: These companies specialize in providing replacement components for various appliances, such as AC capacitors.

Conclusion

Understanding AC capacitors and their testing techniques will allow you to maintain your equipment and troubleshoot potential faults. Following the safety standards and techniques given in this article will allow you to effectively test AC capacitors with a multimeter and make informed replacement decisions. Remember, if you are unsure about any step of the process, seek assistance from a qualified appliance repair specialist.