You probably see videos of technicians changing capacitors in old electronics in order to restore proper operation. They may use the term ‘leaky’ in those videos. But, what exactly is a leaky capacitor and what causes a capacitor to be ‘leaky’?

First, only certain capacitors have the potential to become ‘leaky’. Those capacitors are usually ‘electrolytic’ type capacitors. Other capacitors, like ceramic or tantalum capacitors rarely become ‘leaky’.

A leaky capacitor is one that conducts current when energized with a steady DC voltage. A capacitor should never conduct very much current when a constance DC voltage is applied. If too much current flows in this situation, it can affect the circuit in which the capacitor is used.

Electrolytic capacitors are constructed in such a way that the ‘dielectric’ material that separates the plates can become defective over time and due to excessive heat. This results in the characteristics of the dielectric changing and possibly becoming more conductive.

Other older style non-electrolytic capacitors were constructed with paper dielectrics and they would deteriorate over time, as well. In some older electronics, these capacitors are often found to leak and should be changed more often than not during a restoration or repair.

Electrolytics

Usually constructed with higher capacitances that other types of capacitors and often are polarized. They must be connection in a certain way with positive potential on the appropriately marked lead or damage will occur. Electrolytic, both polarized and non-polarized are prone to degrade over time and with heat.

Ceramic

Very stable capacitors and usually in smaller capacitance values. They rarely degrade and leak, but are susceptible to moisture and can change characteristics in different environmental conditions.

Tantalum

Usually in capacitance value in the mid to high range of values and very stable. They are used in many applications, including decoupling. They are unlikely to degrade and leak over time.

Mylar

Extremely high isolation and capacitance values in the low to mid range. They withstand high heat and humidity. They are unlikely to degrade and leak over time.

You don’t often think of a transistor as a switch or relay, but they are very useful for switching power on and off to a load, where higher current is needed. When used as an ON/OFF switch, the transistor is not operated in linear mode, but operated in ‘saturation’ or in ON/OFF mode. For some applications, a cheap transistor is more suitable than a mechanical relay for ON/OFF switching.

As an example, your Raspberry Pi or Arduino General Purpose I/O (GPIO) only provides a limited output current. Usually in the range of 10 to 50 milliamps, but it could vary. If you wanted to drive a small 5 volt buzzer or motor, that was rated at 500 milliamps, you could not directly drive it, even if the voltage output of the GPIO was 5 volts. The voltage would be sufficient, but the current demand of the load would burn out the GPIO driver.

Since a relay can accomplish the same thing, you might ask, why use a transistor instead of a relay? Let’s explore the differences between a transistor operated as a switch and a relay. The differences will affect your choices when designing digital electronics. (This comparison is between mechanical relays and transistors. There exists a class of relays that are marketed as ‘solid state’. This just means that they are transistor based relays or switches.)

Isolation

The transistor will not provide isolation of the GPIO output to the load device. Sometimes isolation is needed, such that the ground and drive lines are isolated electrically. The relay provides some isolation if wired using a separate power supply with a separate and isolated ground from the ground used by the GPIO. This is because the relay contacts are not physically or electrically connected to the input drive lines that energize the relay coil.

Speed

Transistors will be much faster when turning on and off. The action of moving a mechanical device, like a relay, inherently takes some time. Whereas, transistors react much quicker. The speeds are different by orders of magnitude in most cases.

Longevity & Durability

Transistors are much more durable and have longer life spans than mechanical relays. Mechanical devices tend to wear out over time. This does not mean transistors do not have expected life spans, but they are much greater than the lifespans of mechanical relays.

Cost

When a transistor can be used, the cost is usually lower than a relay. This is not always the case, but in general and for some applications the cost will be lower.