AC/DC Components

[HRS]

I understand the principles of AC and DC current. Fine. But somehow until now I never thought of the reasoning behind components specifically for either. What is the difference? Is it just that in AC components that current has to be able yo travel in both directions, and DC components current can only travel through one direction? Then in essence, wouldn't a DC switch, for example, be the same as an AC switch with two diodes facing the same direction in both input and output? I am probably mistaken in my reasoning but a little education here would help.

[Sensei]

Imagine two roads:

1. cars can travel just in one direction. Speed of all cars is pretty much constant.

2. cars can travel half of time in one direction, then another half of time in opposite direction. Speed of cars is constantly changing using sin(2*PI*frequency*t) function.

 

For 230 VAC with 50 Hz frequency, 1/100 s = 0.01s electrons flow in one direction, yet another 0.01s electrons flow in opposite direction.

 

I made OpenOffice spreadsheet for you:

 

2 Hz AC result (it's using sin(2*PI*frequency*t))

 

 

5 Hz AC result (it's using sin(2*PI*frequency*t))

 

 

When you will pass AC through rectifying bridge or 4 rectifying diodes, 2 Hz AC will change to this (it's using abs(sin(2*PI*frequency*t))):

 

 

When you will pass AC through 1 rectifying diode, 2 Hz AC will change to this (it's using max(sin(2*PI*frequency*t)),0):

 

 

I have attached OpenOffice documents in ZIP archive, so you can play with it by yourself.

Small frequency is used to better visualize effect.

 

In real rectifying AC->DC there is also rectifying electrolytic capacitor(s) needed after diodes/bridge.

So instead of "bumps" like on two last screen-shots, there is smooth flat line of Voltage (pretty much constant velocity of electrons in just one direction).

ac 2 hz.zip

ac 5 hz.zip

dc 2 hz.zip

dc 2 hz 1 rectifying diode.zip

Edited March 31, 2014 by Sensei

[HRS]

Ah interesting. But if I were to use a bridge to convert AC to DC, then wouldn't I then need to reconvert it as the DC current flowed to an AC power source? I wouldn't think you cab have AC flow out and DC flow in. Or did I get tripped up somewhere?

[studiot]

You can connect any componeent to AC or DC but

 

The interaction with any component is, in general, different for AC and DC.

 

For instance a capacitor can pass AC, but not DC. However it can store DC, but not AC.

 

As to switches and their ratings.

 

You will often find the same switch with a lower rating for DC than for AC.

This is again because of the different response (yes even humble switches have different responses).

 

When a switch closes some arcing inevitable occurs. This damages the contacts by pitting.

Perhaps you remember the old fashioned pre electronic ignition system of points in a car?

If this passage of current is always in the same direction (DC) then this can lead to corrosion and enlargement of these pits and eventual failure of the switch.

Continuous reversal as in AC does helps even out the situation.

 

As a matter of interest

Ask yourself 'What is the meaning of AC' and then ask yourself 'What, therefore, is AC current?'

[HRS]

While I am not quite old enough to recall such pre electric ignition, the concepts hit home. Thanks for the help