Replacing old capacitors - finding suitable replacements

Need to replace a failed capacitor? No component lasts forever. Especially electrolytic capacitors wear out over time and may need to be replaced after a few decades. This is especially true of the slightly larger capacitors that are located in the power section of almost all consumer electronics.

It is often obvious that a capacitor is broken – they may bulge out on the top, or in extreme cases they might ”exploded”, exposing their innards.

That the power supply capacitors have failed doesn’t mean that all the electrolytic capacitors in the device are bad. Often you can save e.g. a TV or amplifier by just replacing a few capacitors, and it is neither difficult nor expensive to try.

How to find an appropriate replacement:

Start by reading on the capacitor body, and look up the two most important values: capacitance and rated voltage. The unit of capacitance is farad (F), but the values are usually given in microfarad (µF, or uF) on electrolytic capacitors. On really old capacitors, you have sometimes see capacitance written as MFD. Voltage is simply written in volts, V.

A capacitor with 220uF capacitance and 50V voltage rating.

Once you have found the values, you can start searching, e.g. for 220uF, 50V. You can do free text search with 220uf 50v or go to the capacitor category and use the parameter filters.

On very small surface mount capacitors, the units are to small to print, and the values are then written without V or uF. In such cases, you have to guess based on the context. The normal voltages – 6.3, 10, 16, 35, 50 V – do not occur as capacitances, so it is usually easy to figure out what is what. Most often, the capacitance is the higher value, especially on small capacitors that tend to have low rated voltages.

Capacitance

The new capacitor should have the same capacitance as the one being replaced. Some older values are no longer used, then you have to replace with a nearby value. For example, 50uF isn’t a common value (except for motor capacitors), and may be replaced by the standard value 47uF. In many cases a higher value may work, but you should understand the circuit in order to decide. It's always best to choose a value as near the original as possible.

Rated voltage

The voltage of the new capacitor must be the same or higher than the one being replaced. The voltage rating is a measure of what the capacitor can withstand, and so here you are free to go with a higher value. A higher voltage rating can give a longer service life in normal use, as well as better resistance to fault conditions. Higher voltage capacitors are physically larger, though, so keep that in mind. The bigger size is often offset by modern components being smaller for a given capacitance and voltage, so is less of an issue when repairing older gear.

Temperature

If a capacitor is marked with a temperature, it is best to ensure that the replacement has the same or higher temperature rating. This also affects the life of the capacitor. 105°C is better than 85°C. Much like with voltages, you can always choose a capacitor with a higher temperature rating, and often get a much-improved service life. Often only the maximum temperature is indicated, but sometimes the entire range is indicated, e.g. -40° - +85° C.

Physical dimensions

When you’ve found a few capcitors with the right electrical characteristics, it is time to think about the physical characteristics. The size is often written in the product description and name. Otherwise, consult the datasheet. Older capacitors are often much bigger than modern ones (for the same capacitance and voltage). Fitting a smaller capacitor is rarely a problem.

Size

Electrolytic capacitors are cylindrical, and the size is indicated as the diameter and length of the body. As long as there is space, it doesn’t matter if a new capacitor is bigger (or smaller).

Leg spacing

Another important measure is the leg spacing, which needs to be tuned to mount a capacitor on circuit boards. If this is not the case, you may need to install the condenser lying down or find other solutions. Capacitors with higher rated voltage normally have greater leg spacing. This is so that the tension potential between the legs should have a greater physical distance.

The leg spacing is never written on capacitors, but needs to be measured on the body or PCB.

Mounting - radial, axial, surface mount or snap-in

Through-hole electrolytic capacitors are available in two main mounting formats: radial and axial. Radial capacitors are by far the most common today. Axial capacitors are only produced in small quantities, with a limited selection, so can be difficult to find replacements for. If there is enough room and the legs can reach, you can also install a racial capacitor lying down, but you should insulate the exposed legs with shrink tubing or some insluation from a piece of wire. There is no electrical difference between the two.

A third type of through-hole capacitor is the snap-in capacitor. The type is mainly used for higher voltages and capacitances. The leads are designed so that the capacitor is held in place before soldering, which makes manufacturing easier. This is less important when making repairs, so unless a snap-in capacitor can be sourced, regular radial capacitors will work fine.

Surface mount capacitors are made in a few standardised sizes, increasing with higher voltage and capacitance. Hence, replacements with the same values are commonly similarly sized. If finding a surface mount replacement is proving difficult, it’s possible to ”surface mount” regular through-hole capacitors by simply soldering their leads to the SMD pads. In that case, it may be a good idea to also secure the capacitor body with e.g. cable ties or some type of adhesive.

Bipolar Capacitors

Regular electrolytic capacitors are polar – they have a positive and negative terminal. There is also a special type of electrolytic capacitor aren’t polarised: bipolar capacitors.

Bipolar capacitors are recognized by the fact that they lack the marking of the negative pole. Sometimes they are also marked BP (for Bi-Polar) or NP (which stands for Non-Polarised) on the body. (One leg usually is usually longer, just like on ordinary capacitors, but that is irrelevant).

Since bipolar capacitors cost more and take up more space than conventional capacitors, they are only used where really needed, and so should not be replaced with conventional, polarized, capacitors.