How to solve Transformer 0-22V ac Flat top in the Sinewave - Reg

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prakashvenugopal

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Hi all,

Transformer Details:

Primary : 0-200-220-240V / 2A

Secondary1 : 24-22-20-0-20-22-24V / 72VA

Secondary2: 0-16.5 / 16.5VA


I had Found Primary Waveform 0-220v ac is Pure sine-wave and Found Flat Topping in the Sine-wave of 0-22Vac Secondary Tx as we attached in the image.

I had measured the current Waveform also and it is attached in the Video for your reference.

Kindly let us know how to avoid this Flat topping in the Transformer Secondary Waveform.

Regards,
V. Prakash
 

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Hi,

Clearly a core saturation effect.(as long as you did not connect anything at the secondary... like a rectifier)
--> just reduce the primary voltage.

It seems the transfomer has several primary input options. But you did not say which one you used.
--> Use the 240V one.

Klaus
 
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Hi,

Thanks for the reply.

1. I had used bridge rectifier at the Transformer secondary.
2. I had used 0-220V option in the Transformer Primary Input.

Kindly let us know how to avoid this Flat topping in the Transformer Secondary Waveform.

Regards,
V. Prakash
 
Last edited:

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It isn't a problem and it wont stop it working. What is happening is the load current is causing a voltage drop. This is normal and the only way to reduce it is to use a larger transformer. I suspect you have a reservoir capacitor after the bridge rectifier and also some DC load on it. As the load current drops the capacitor voltage and the transformer voltage starts to exceed it, current is drawn to recharge the capacitor. This is why the current is only at peaks of the AC cycle. As the current is drawn from the transformer, the resistive and magnetic losses cause a voltage drop and hence the flat topping of the waveform.

Brian.
 
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KlausST said:
Hi,

1) read post#2
2) remove the bride rectifier

Klaus
Click to expand...
Hi,

Thanks for the reply.

1. I had Converted this secondary Tx AC Voltage --> Bridge rectifier --> LM 317 regulator --> +18V DC
without the bridge rectifier, i cant generate +18V DC.
In this case, How to remove the bridge rectifier.

2. If i use the Primary Transformer tapping 0-240V ac, this will solve the flat topping?

Regards,
V. Prakash
--- Updated ---

Hi,

Thanks for the reply.

Larger transformer in the sense?

Existing transformer design:

Primary : 0-200-220-240V / 2A
Secondary1 : 24-22-20-0-20-22-24V / 72VA
Secondary2: 0-16.5 / 16.5VA

How much Transformer VA can be increased to avoid this flat topping?

Regards,
V. Prakash
 
Last edited:

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Hi,

Asking the same question again will not bring any new information. Why not simply try what was recommendend, then you immediately get the result.

******
* in post#1 you did not mention any load.
Without load, the most probable reason for flat top is core saturation.
(although it rather is expected to happen "after" the top.)
You may reduce core saturation with using the 240V tap or reducing the input voltage

* now you say you have a load, that draws "nonlinear" current. (how could we know before?)
So - if you want to reduce the flat top - you have to remove the "nonlinear load".

It could be so simple: just do what we recommended and see what happens.

If you neither want to do the one nor the other recommendation ... then a discussion is useless.

Klaus
 
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It is NOT POSSIBLE to completely remove your observed flat-top, only to reduce it. As Betwixt pointed out, this voltage droop is caused by IR drop of internal resistance of transformer. It will ONLY dissappear if you have NO LOAD.

What you CAN do is to take the load current (for your DC supply) from one winding, and this voltage readout from a different winding - you *do* have two windings. This might solve your quandary.
 
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How much DC current does the load take?
The RMS current rating for the transformer should be at least twice the DC load current, due to the high RMS current the rectifier-capacitor draws.

Why are you concerned about the flat-top in the transformer output waveform?
 

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Core saturation happens - in case - at Vdtmax rather than Vmax.

Slightly distorted waveform with flattened top is often observed already at the mains terminals. At transformer secondary it's by rectifier load with filter capacitor.
 
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Hi,
Ok. I will try with 0-240V ac Primary tapping and give the feedback.

Yes. In the Post #1, I did not mention any load details. Sorry for that.
My AC Peak Current is 800 mA. load
In the +18V DC - 200 mA
In the -18V DC - 200 mA

Tx Primary(0-220V)--> Tx Secondary(22-0-22V) --> Bridge Rectifier --> Regulator (LM317 & LM337) --> +18 V and -18V DC

Can you please explain how to remove the non-linear load?

Regards,
V. Prakash
 

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KlausST said:
Disconnect.
At least for a test.
Click to expand...
Hi,

I had disconnected the Load, it is showing the Pure sinewave in the Transformer Secondary.
Flat topping occurs when the Load is connected.

How to resolve this Flat topping in the Transformer Secondary when the load is connected?
Kindly let us know.

Regards,
V. Prakash
 

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Hi,

Again, Again: What´s the problem with the flat top?
Why don´t you answer this question?
We are now at post#15 .....

******
How to remove flat top with load connected?
--> My idea is to use some kind of "PFC" stage at the secondary.

Klaus
 

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Someone needs to explain Ohms law to this guy. And current flow through full-bridge rectifiers.

Or just close this thread since OP seems incorrigible.
 

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KlausST said:
Hi,

Again, Again: What´s the problem with the flat top?
Why don´t you answer this question?
We are now at post#15 .....

******
How to remove flat top with load connected?
--> My idea is to use some kind of "PFC" stage at the secondary.

Klaus
Click to expand...
Hi,

Please refer the link of Flat-topping relates to the Power quality and the harmonics:

https://www.quora.com/What-is-the-flat-topping-with-regards-to-power-quality

Flat topping is affecting my system.
Non-linear power supplies draw current in high-amplitude short pulses that create significant distortion in the electrical
current and voltage wave shape—harmonic distortion, measured as total harmonic distortion (THD)

THD of Transformer Secondary shows high when comparing to Transformer Primary and also making my system malfunction.

Is there any readymade device for PFC to use in Transformer secondary (22-0-22V) you have mentioned?
Kindly let us know .

Regards,
V. Prakash
--- Updated ---

FvM said:
Core saturation happens - in case - at Vdtmax rather than Vmax.

Slightly distorted waveform with flattened top is often observed already at the mains terminals. At transformer secondary it's by rectifier load with filter capacitor.
Click to expand...
Hi,

Input Primary Waveform is Pure Sinewave. I have tested it. Secondary Transformer (22-0-22) is having the
Flat topping.

Regards,
V. Prakash
 
Last edited:

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I can't work out what the problem is here. What you are seeing is perfectly normal and expected behavior in a LINEAR power supply like yours. You ignore the comment in the link that states "If the system supplying the voltage is incapable of providing enough current, the voltage waveform will be distorted, and the sine wave top will be "clipped off." The flat tops are due to the current being drawn from the transformer and the only way to minimize the effect is to use a transformer with higher current rating.

Brian.
 

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prakashvenugopal said:
Non-linear power supplies draw current in high-amplitude short pulses that create significant distortion in the electrical current and voltage wave shape—harmonic distortion, measured as total harmonic distortion (THD)
Click to expand...
Distorted input current and flattened output voltage are both caused by rectifier filter capacitor and are unevitable with this power supply circuit topology.

There is a dependence on transformer series impedance X% and R%. The higher the impedance the stronger output voltage flattening but the lower current THD.

Since power quality regulations are constraining current THD, simple single phase rectifier circuits above a certain power level became problematic. These days, most EMC aware single phase AC/DC are using active PFC, some older devices are scarcely complying standards with a series choke.
 

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prakashvenugopal said:
Please refer the link of Flat-topping relates to the Power quality and the harmonics:
Click to expand...
Those harmonics have nothing to do with the quality of the DC out of the regulator.
prakashvenugopal said:
THD of Transformer Secondary shows high when comparing to Transformer Primary and also making my system malfunction.
Click to expand...
How is it making you system malfunction?
What does the DC signal look like from the regulator?
 

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