# Simple capacitor question

#### Mustwin351

Apr 10, 2013
49
When you have a capacitor between your two inputs on an alternating current circuit (i.e. possibly a power supply) why does that not create a direct short?

What am I missing? If a capacitor passes ac voltage why is that not like a line to line short?

#### Gryd3

Jun 25, 2014
4,098
When you have a capacitor between your two inputs on an alternating current circuit (i.e. possibly a power supply) why does that not create a direct short?

What am I missing? If a capacitor passes ac voltage why is that not like a line to line short?
A capacitor can appear as anything from a short to a very high value resistor depending on the frequency and capacitance.
There is a formula for it...

Xc = 1 / (2*Pi*f*C)

Xc = Capacitive reactance (Ohms)
Pi = 3.14159.....
f = Frequency (Hz)

#### davenn

Moderator
Sep 5, 2009
14,241
What am I missing? If a capacitor passes ac voltage why is that not like a line to line short?

A capacitor doesn't pass AC as such .... as in NO current flows directly between the 2 plates
Rather each plate alternately charges and discharges

And as Gryd3 said, it will "act" as a short circuit dependant on the info he gave above

#### Mustwin351

Apr 10, 2013
49
Thanks guys I recall that now.

For a power supply the capacitor goes on the "dc" side or the circuit and no matter the reactance of the cap you won't have a short. Instead it just "charges" the capacitor.

It's been sometime since school. Thank you.

#### davenn

Moderator
Sep 5, 2009
14,241
The capacitor after the rectifier smoothens out the AC to something that resembles a DC voltage ( but with ripple (the remaining AC))

Have a look here and ask about anything you have a problem understanding
cheers
Dave

#### Mustwin351

Apr 10, 2013
49

It states that in order to use a bridge rectifier for full wave rectification that a center tapped transformer must be used...but it then refers to figure 1.1.3 for this which does not have a center tapper transformer.

Is this just a mistake in figure 1.1.3?

#### Gryd3

Jun 25, 2014
4,098
This is certainly a mistake
The Label for each image is below the image, not above
You are looking at figure 1.1.4 The Bridge Rectifier that has no center tap and 4 diodes.

Figure 1.1.3 certainly does have a center tap on it's secondary and is using two diodes.

#### Mustwin351

Apr 10, 2013
49
You are looking at figure 1.1.4 The Bridge Rectifier that has no center tap and 4 diodes.

Figure 1.1.3 certainly does have a center tap on it's secondary and is using two diodes.

So figures:
1.1.4
1.1.5
1.1.6 should all have center taps on the transformer secondaries connected between the two diodes correct?

I guess without a center-tapped transformer the best you can achieve is half wave rectification?

#### Gryd3

Jun 25, 2014
4,098
So figures:
1.1.4
1.1.5
1.1.6 should all have center taps on the transformer secondaries connected between the two diodes correct?
Nope.
1.1.4
1.1.5
1.1.6
Are all using a Bridge Rectifier which does tot require a center-tapped transformer secondary.

1.1.3
Is using 2 diodes, and requires a center-tapped transformer secondary to rectify a full wave.

It states that in order to use a bridge rectifier for full wave rectification that a center tapped transformer must be used...but it then refers to figure 1.1.3 for this which does not have a center tapper transformer.

Is this just a mistake in figure 1.1.3?

This was a misunderstanding. The page correctly refers to figure 1.1.3 which had 2 diodes and a center-tapped transformer, but you had accidentally looked at 1.1.4 instead which is the first figure to use a bridge rectifier.

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#### Mustwin351

Apr 10, 2013
49
I gotcha now. Sorry a little further reading would have helped. The center tapped transformer with a rectifier is just more efficient than one without a center tap.

Thank you very much!

#### davenn

Moderator
Sep 5, 2009
14,241
I gotcha now. Sorry a little further reading would have helped. The center tapped transformer with a rectifier is just more efficient than one without a center tap.

Thank you very much!

you still might be misunderstanding ... the way you just wrote that is misleading and not what the www page said

they said ....
If a transformer with a centre tapped secondary winding is used, more efficient full wave rectification can be used. The centre-tapped secondary produces two anti-phase outputs, as shown in Fig 1.1.3.

they are telling you its more efficient that a single winding transformer and only ONE rectifier diode as referenced to in Fig 1.1.2

A single winding secondary and a bridge rectifier is still efficient. and has the advantage of full wave rectification without the need for a centre tapped secondary

Dave

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