What are the effects of leaky capacitors.

I had a question on my test which require me to troubleshoot the problem of an amplifier circuit. The answer was a leaky capacitor causing a higher than normal current to flow through a resistor. This creates a larger voltage drop across the resistor which in turn lowers the transistor collector voltage.

Can anyone explains the effects of a leaky capacitor. As I read once before a leaky capacitor acts like a large resistor (100k plus) in a circuit. If that is true why would that cause a higher current draw? that should be the opposite. The higher the resistance, the lower the current.

 

A good capacitor acts like an infinite number of ohms. A leaky capacitor at 100,000 ohms allows more current than infinite ohms.

 

Well, a capacitor is supposed to block DC, and pass the effects of AC.
If a capacitor is good, it will have a very low leakage rate. Aluminum electrolytic caps have a higher leakage rate than most other types; and the allowable leakage for a given capacitance increases with the voltage rating.

As electrolytic capacitors age, their dielectric can weaken; particularly if they have been out of use for a period of time, causing the leakage rate to increase. As leakage increases (internal resistance decreases) the current flow through the capacitor increases, and so does power dissipation within the capacitor. If the power dissipation becomes too high, the package can rupture forcefully.

Anyway, high leakage rates cause increased power consumption due to the increase in current flow through the leaky capacitor(s).

 

The type and degree of the ill effects caused by a leaky capacitor depend upon its circuit function, as well as the amount of leakage. This may be less noticeable in low-impedance, high current situations. For instance, slight leakage in a capacitor across a power supply may occur without obvious effects, unless it eventually escalates to a level causing it to fail catastrophically.

In some more sensitive higher impedance circuits, only a very small amount of leakage may be required to cause a malfunction. This is one of the reasons why electrolytic capacitors do not suit all applications. Examples include RC timing circuits, where the time delay may be incorrect or the circuit may fail to operate altogether. RC and LCR filter circuits can have incorrect responses if the capacitors become more lossy.

One quite common problem occurs when current from a leaking capacitor upsets DC bias conditions in circuits such as amplifiers - the sort of thing you saw in your transistor experiment. The situation may be worse with very high impedance devices like FETs or FET-input op-amps, (historically, and in specialist modern applications, vacuum tubes) where very little current may be needed to upset things. A leakage resistance of many tens of megohms or more may be too low in some such cases.