Capacitor in a DC circuit

[Malcolm Fowler]

Hello All,

I am based in the UK, I am 57 and have recently retired. I have been a programmer for many years and now decided to start learning electronics in my spare time. I am reading Make: Electronics, Second Edition by Charles Pratt.

I have just completed the section on capacitors and have a question the book hasn't answered so here goes...

If a capcitor blocks DC then why not just leave it out of the circuit? Also, when a capacitor discharges, I assume the circuit has been turned off so what use was the charge? Can you help me with a real world example of the use of a capacitor in a DC circuit?

One other question. Can anyone recommend a kit the I should buy to carry out the experiments in this book. I can only find US sites carrying the kit recommend by the author

Thanks

 

[(*steve*)]

The "A capacitor blocks DC" is a generalization that only has useful meaning when it is in series with something.

A capacitor resists a change in voltage across it. Current must flow (in proportion to it's capacitance) in order for the voltage to change.

When a capacitor is in parallel with some signal, current must flow into or out of the capacitor in order that the voltage across it changes. In this case, the capacitor is doing something akin to "blocking AC" (or probably better expressed as bypassing the AC component to ground).

The latter use of a capacitor is used in power supplies to filter ripple from the rectified AC to create a smoother (almost) DC voltage.

 

[hevans1944]

Capacitors pass AC signals while blocking DC. Removing the capacitor would block both signals. Thus a capacitor is useful when you don't want to disturb the DC biasing conditions on either side of the capacitor and simply pass an AC signal from stage to stage... capacitance-coupled amplifier stages, for example, are quite common.

Capacitors are often charged and then discharged, in a manner controlled by external circuitry, for timing applications. The ubiquitous 555 timer chip depends on the controlled charging and discharging of a capacitor through resistors to perform its timing function. There are many other uses for capacitors, and a huge range of capacitor types. Tuning capacitors, for example, in conjunction with an inductive coil (an inductor) establish a resonance condition in a circuit at a particular frequency or band of frequencies, which is quite useful in radio frequency applications to separate a desired frequency or band of frequencies from everything else.

I am not familiar with what is available in the UK for beginners, but you could assemble your own "kit" from mail-order parts found on eBay and mostly imported from Asia. The cost is dirt cheap, so if you get a few bum parts it's not a big loss. Biggest problem is the (sometimes) long delay between ordering and actually receiving anything.