bridge rectifier......

[p72]

Hi all,

I have few questions:

(1.) As i know if we put Capacitor in to DC path, it will block DC and would not allow to go through it. But it allows AC to pass it through. How can I block AC? (By Inductor?)
(2.) If I use Bridge Rectifier with the AC input (240Vac,50/60Hz), Will I get "always" DC as an output?


Thank you in advance,
p72

 

[kam1787]

1) inductor alone will not work. You can suppress the AC by using a low-pass filter

2) yes, although it is not true DC. It is pulsating DC.

 

[p72]

1) inductor alone will not work. You can suppress the AC by using a low-pass filter

2) yes, although it is not true DC. It is pulsating DC.

(1.) can you explain how?
(2.) yes, it will not be a pure DC. I wanted to make sure do i get DC or not.

 

[mouu]

hi..i have answer of your questions
(1)..as we know that frequency for DC is Zero.therefore the impedance provided by capacitance in the circuit will be infinite thus path will be completely open and any signal will not go through the circuit.in AC case it will not block the signal.
in same manner inductor have impedance {Xl=wl}

(2)yes

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hi..i have answer of your questions
(1)..as we know that frequency for DC is Zero.therefore the impedance provided by capacitance in the circuit will be infinite thus path will be completely open and any signal will not go through the circuit.in AC case it will not block the signal.
in same manner inductor have impedance {Xl=wl},here rate of change of current will mateer to partially block the signal....

(2)yes...bt will be pulsating DC

 

[BradtheRad]

(1.) As i know if we put Capacitor in to DC path, it will block DC and would not allow to go through it. But it allows AC to pass it through. How can I block AC? (By Inductor?)

A series inductor can be used in this role, where it often is called a choke.

Its Henry value must be tailored to the current and frequency going through it.

I have Youtube videos which are animated simulations of inductor behavior.

They portray flux fields building and collapsing. Also changing EMF. Current bundles are shown moving through wires.

Various waveforms are applied.

Behavior with DC:

https://www.youtube.com/watch?v=LVNxrN4jgvs

Behavior with AC:

https://www.youtube.com/watch?v=Os3jF9UeMoE

 

[Venkadesh_M]

a series inductor block AC
a parallel inductor blocks DC (by decoupling/bypassing)
a series capacitor blocs DC
a parallel capacitor blocks AC (by decoupling/bypassing)

This can be in any combination to reduce a particular frequency component in output. That is called filter.
You will use a low pass filter in power circuit to block AC..

 

[p72]

Thank you all for the response.

What is the proper inductor value to block Mains AC 265Vac, 60Hz?
What is the correct value for low-pass filter components?

Thank you,
p72

 

[Venkadesh_M]

You need not to mention the voltage 265V.. If you meant that then you are going to rectify AC directly without transformer and going to filter......

when rectification the frequency will be doubled ( for 5oHz AC the output will be 100Hz pulsating DC)..

Then you have to design a low pass filter for your Current Rating and which eliminates 120Hz ripple...

I think the following example will help you how to Do...

Q = C * V, C = capacitance in farads, Q = charge in coulombs, V = voltage potential

Q = I * T, I = current flow in amps, Q = charge in coulombs, T = time in seconds

F = 1 /T, F = frequency in hZ, T = period in seconds

Therefore,
I * T = C * V, or C = (I * T) /V

The delta “Δ” symbol indicates a change, such as a change in voltage or a change in time

C = I * ΔT / ΔV

In this case, ΔT = 1 /F e.g. the time period of a 50hZ waveform = 1 /50hZ = 0.02seconds

Therefore,
C = I / (ΔV * F)

Now including the 70% factor we get the final relationship:
C = 0.7 * I /(ΔV * F)
C = capacitance in farads, I = current in amps, ΔV = peak-to-peak ripple voltage, F = ripple freq in hZ
Note that ripple frequency in a full-wave rectifier is double line frequency. For half-wave rectification, the ripple frequency is the line frequency.

Solving for ΔV
ΔV = 0.7 * I /(C * F)

The 70% factor

This is an assumption—in cases where the capacitor is oversize, it will be perhaps 80%, but this is close enough for government work and not very critical.

Putting it to work, a practical example

Assume that we want to make a 9V, 500mA power supply using the LM7809 voltage regulator device, 12V transformer, bridge rectifier and filter capacitor. Line frequency is 50hZ. How large should we make the filter capacitor?

From the spec sheet, we learn that the dropout voltage of the LM7805 is 2.5V. Therefore, the valley of the peak-to-peak ripple should be 9V + 2.5V = 11.5V. Assuming that the average rectified DC voltage is 12VDC, the minimum negative peak of the ripple voltage = 12V – 11.5V = 0.5V. The peak-to-peak ripple voltage is double the peak or 1.0V. Plugging it into the formula:

C = 0.7 * I /(ΔV * F) = 0.7 * 0.5A /(1Vp-p * 100hZ) = 0.0035farads or 3500µf

Selecting the capacitor

Since 3500uf is not a standard value, simply scale up to 3900uf.

 

[BradtheRad]

What is the proper inductor value to block Mains AC 265Vac, 60Hz?

If you are running small current, you need a large Henry value. And vice-versa.

Example, 265VAC rectified, through a choke coil, at 1A load.

A value of 3H reduces ripple to the vicinity of 10%.



Since large coils are expensive, it is cost-effective to add a capacitor or two, allowing the use of a smaller Henry value.

 

[kam1787]

Since large coils are expensive, it is cost-effective to add a capacitor or two, allowing the use of a smaller Henry value.

Ahhhhhhhhhhhhhhh but then it is no longer a choke but a filter.

 

[BradtheRad]

Although we use the general term smoothing filter, there is choke-input and capacitor-input, and 1 or 2 or 3 stage, etc.

https://www.tpub.com/neets/book7/27f.htm

 

[p72]

Hi BradtheRad,
Thanks for the suggestion.

I am looking for 2A load at Max. What is the value I should look for cap and inductor? (I agreed that large coils are expensive so not to prefer) Can you draw rough circuit connection with can and inductor?

Thank you,
p72

 

[BradtheRad]

I am looking for 2A load at Max. What is the value I should look for cap and inductor? (I agreed that large coils are expensive so not to prefer) Can you draw rough circuit connection with can and inductor?

Screenshot showing simulation...

Choke-input (200mH), followed by capacitor (1000 uF), provide regulation within 1 or 2 percent, at 2A.

 

[kam1787]

Screenshot showing simulation...

Choke-input (200mH), followed by capacitor (1000 uF), provide regulation within 1 or 2 percent, at 2A.
[/url]

Does he want REGULATION?

for AC the cap must be non-polar with WVDC >> highest input voltage

 

[Venkadesh_M]

Does he want REGULATION?

for AC the cap must be non-polar with WVDC >> highest input voltage

He wants or not The regulation happens with series things... thats he mentioned and here, after rectifier electrolytic can be used as well, what is the need of non-polar cap....

 

[p72]

If I use this circuit to block the AC signal, then will it allow DC to pass through it? My DC is between 20-40V.

 

[nirajd]

Hi all,



(2.) If I use Bridge Rectifier with the AC input (240Vac,50/60Hz), Will I get "always" DC as an output?

The answer is "yes" to some extent!! Full rectification of the input ac voltage will turn the negative half cycle of it into positive... and most probably u know, the wave forms not containing negative parts can be considered as dc!! u can use filter circuit (capacitor in parallel with the load and inductor in series with the load, hope you know the circuit) to smoothen the waveform!! Be careful, use a proper combination of diode and make sure that you ground the proper part!! This is common but try to simulate the rectification circuit in PROTEUS and you will be able to know much about it!! This was not as easy as i thought when i simulated the same in proteus..however i got to learn a lot of things about rectification only after i simulated it!!
REGARDS
niraj

 

[p72]

Hi niraj,
Thanks for the quick response.
What is PROTEUS? I searched on google but did not find proper result. looks like some kind of simulation program.
Could you please send exact link for the same?

 

[kam1787]

He wants or not The regulation happens with series things.....

Series things? What things? I have been doing electronics for 30 years and this is the first I have heard of such a thing. Do I insert a 'thing' and get regulation? Can I get voltage regulation? Can I get current regulation? I just pop in a 'thing' and get regulation?

what is the need of non-polar cap....

Are you saying that a polarized capacitor can used in AC circuit where the voltage reference to the cap 'positive' terminal can be negative, can be a polarized cap? This is amazing. So there is no need for non-polar caps? You can use polarized caps in an AC circuit which the cap can be reverse polarized - by the applied voltage? Can you share references that state this? I would love see this stated by a reliable source.

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Choke-input (200mH), followed by capacitor (1000 uF), provide regulation within 1 or 2 percent, at 2A.
[/url]

A simple LC circuit will provide regulation? Within 1 to 2%? can you kindly provide a source for this? I never heard of this before. As far as I know it provide a frequency filter action, and not regulation. Are you sure you are using the term correctly? So you saying that current regulators and voltage regulators (ie 78NN) are not needed? One only needs a LC circuit? Does this extend to RC circuits? I fear I wasted a great deal of time and money in current and voltage regulators if this is the case.

 

[nirajd]

**broken link removed**
Yes, Proteus is a circuit simulation application where you can find almost all the electronics component and some of the electrical components too!! before implementing your idea into hardware, it provides you the best way to check whether your circuit works or not in reality!! i have send a link,, dont know if you can download it from there but try once!! other wise, this program can be easily available if you consult any electronics, computer, or electrical engineer friends of yours!!
regards.