How to Test an Electrolytic Capacitor with a Digital Multimeter

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Any specific electronic circuit might involve electrolytic capacitors that are vital parts of electrical designs. They provide low impedance values, high capacitance per volume, not to mention they are the best to use for high energy capacity and storage.

But did you know what they are and their major purpose? 

How is an electrolytic capacitor used?

Not all are aware of this but, electrolytic capacitors are normally seen in inverters and power supplies and can be used in any electronic system. They are extremely practical because of their efficiency for lowering unwanted frequencies and efficiency for energy storage.

That technology keeps transforming because of new improvements and new materials within the production stage. Through the years, they have become more dependable and stronger, as good quality units could last at least twenty years. 

One of the major uses of a capacitor is to store energy in an electric field until the current is started. It is composed of two electrodes of electrically conductive material divided by an insulator made out of paper and a conductive liquid referred to as an electrolyte. 

Take note that an electrochemical reaction produces an oxide layer in the anode electrode that functions as an insulator or dielectric. As an insulator, charges don’t flow through its material. Simultaneously, it can move the electric force.

A key advantage to an aluminum electrolytic capacitor is it offers enough capacitance per unit of volume for the voltage rating of the application. 

Other uses of electrolytic capacitors are the following:

  • to offer time delays concerning two functions in a circuit
  • keeping energy in low power applications
  • coupling signals among amplifier stages
  • noise filtering or decoupling in power supplies
  • lowering voltage fluctuations in filtering devices

How does it work?

Electrolytic capacitors are at all times polarized. They could only be utilized for DC voltage. Every time the voltage source is wrongly polarized or the AC voltage is applied, the insulating oxide layer is ruined. Hence, the electrolyte evaporates, and the capacitor bursts open.

Nevertheless, in most applications, there’s a need for a greater capacitance that can’t be reasonably accomplished with classic bipolar capacitors such as polypropylene or polyester capacitors. With that in mind, bipolar capacitors have been created that, in principle, are composed of two capacitors attached in series. 

On the other hand, bipolar electrolytic capacitors are often utilized as coupling capacitors in amplifier circuits wherein the polarity isn’t clear or in crossovers. Instead, the current load is secondary in such applications. Still, the signal quality or filter function is more the criterion.

What’s crucial here is a small design and high capacity instead of the load capacity. The capacity of the first capacitor is created by the inner aluminum foil and the electrolyte. The aluminum foil’s oxide layer is dielectric. 

The second capacitor is created by the outer aluminum foil and the electrolyte. The anode’s oxide layer is dielectric as well. Hence, the outer aluminum is not just used for contact but is a capacitor coating its right. 

Finding failures on electrolytic capacitors

Have you ever worked on old gear? Then you are probably aware that electrolytic capacitors are highly susceptible to failure. What’s the reason behind these failures?

Well, the reason such units are susceptible to failure is that the flip side of what people like about the component itself. Remember that an electrolytic capacitor has a high capacitance in a tiny package. 

In a standard parallel plate capacitor, the capacity increases as the distance among the plates decreases. In an electrolytic capacitor, one plate is a rolled-up spiral. Meanwhile, the other plate is a conductive fluid. Also, the insulator between the dielectric is a small layer of oxide forming on the spiral.

Later on, the oxide worsens. However, that degradation regenerates itself with the capacitor. The oxide will degrade beyond the point of self-repair, especially if the capacitor is not powered up for a longer time. 

Further, the manufacturer creates the oxide layer by a thorough application of a forming voltage into a newly made capacitor. That not just forms the dielectric layer, but that also sets the anticipated current course. Sometimes, it’s possible to use the same method to regrow the oxide and revert a capacitor back. 

So, how do you test an electrolytic capacitor?

The first step on how to test an electrolytic capacitor is to compute the value. Without understanding the capacitor’s value, you won’t realize if a capacitor is good or bad. Keep in mind that these values come in the unit of a microfarad, nanofarad, and picofarad. 

Each capacitor comes with a working voltage, and the standard values are 100v, 250v, 400v, 1000v, 1600v, and etch. 

To test an electrolytic capacitor, here’s what you need to do. 

  1. Grab your digital multimeter and connect any capacitors to the meter. 
  1. Make sure you connect the black probe to the negative lead if you like to check your electrolytic capacitor while attaching the red probe to the other lead. 
  1. Read from the LCD the capacitance value. 
  1. Set your multimeter to a higher range if you like to check the capacitor. For instance, the meter should be set to 2000 microfarad if you like to test the 470 microfarad capacitors. If you like to measure on the non-polarity capacitor, then you can attach the test probes on either capacitor’s leads. Read from the display of your meter.
  1. If the display gives a result of 330 microfarads when testing a 470 microfarad capacitor, that suggests the capacitance value has altered and requires an immediate replacement. There you have it. That’s a basic way to determine if an electrolytic capacitor is bad or not. 

Just a tip. Make sure you have discharged the capacitor before testing it with a resistor and not a screwdriver. 

Final thoughts

We have discovered that testing the cap tops is the most efficient way to determine a degrading electrolytic capacitor. In most scenarios, the cap might still work fine but show signs of degradation. It’s probably best to change the cap while you have the PCB in hand.

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