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Simple capacitor question

Mustwin351

Apr 10, 2013
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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
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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)
C = Capacitance (Farads)
 

davenn

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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
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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.
 

Mustwin351

Apr 10, 2013
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Great link Dave. Thanks!

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?

http://www.learnabout-electronics.org/PSU/psu11.php
 

Mustwin351

Apr 10, 2013
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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
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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?

http://www.learnabout-electronics.org/PSU/psu11.php
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
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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

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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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