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Regulated Low Power Sine Wave

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FvM said:
A simple explanation for excessive magnetizing current is that a 220V transformer isn't designed for 30% overvoltage. But you should also verify that the amplifier doesn't show parasitic high frequency oscillations.
Click to expand...

I have checked during the testing and there was no parasitics at that time but i dont remember how much current transformer draws when the output is 220V. It should be pretty lower than when the output is 265V. Eventhough, magnetising, no load current must be negligible in value.

So, even using secondary 3V as primary, magnetising current should be negligible as i understand.
 

Hi,

Eventhough, magnetising, no load current must be negligible in value.
Click to expand...
The curent will not be negligible. But the power loss in the transformer should be very low, because of low cos(phi).

Unfortunately the power loss of a class AB amplifier depends on current and doesn't care about cos(phi).

If you use a class D amplifier with analog post filter, then you can expect very low power consumption.
This is my recommendation for a portable device.

Klaus
 

I have checked some equations and transformer equivalent circuits. For example, i have 220V primary and 3V secondary transfomer but i am using this transformer as 3V primary in order to get 265V. In previous post i said magnetising current must be same but when i rechecked the transformer equivalent circuits, i realized that magnetising current would be inversely proportional to the turns ratio so i caused increase in magnetising current by using 3V as primary and i think thats why there is much current draw from TDA output even in no load condition.
 

Unfortunately the power loss of a class AB amplifier depends on current and doesn't care about cos(phi).
Click to expand...
It does care for cos phi in so far that losses with reactive load are higher than with similar resistive load. But reactive load isn't unusual, it also happens with speakers.

i dont remember how much current transformer draws when the output is 220V. It should be pretty lower than when the output is 265V. Eventhough, magnetising, no load current must be negligible in value.
Click to expand...
The problem with 30% overvoltage I'm referring to is possible core saturation. Respective current increase isn't negligible. In fact you should measure the load current instead of guessing about it.
 

What you could use might be a push pull tube type audio output transformer.
That should have enough inductance to provide plenty of output voltage at 50 Hz without saturating.

Another possibility might be a small transformer designed to be operated between two phases of a three phase supply.
These are quite common inside switchboards to provide 24v for control wiring.
Here is one 380V primary to 24v (50Hz 40vA)
**broken link removed**
 
Yea now i realized that when i use 220V side as primary, magnetising current is about 7.457mA but if i use 3V side as primary to get 265V output, magnetising current gets about 570mA. So due to using transformer as reversed, transformer magnetising current rises about 76 times. Therefore, its normal that TDA gets hot even without no load at output of the transformer and i also observed that transformer reaches to saturation when i was trying to get 265V at output.
 

You need to avoid saturation at all costs, and most mains transformers are designed by penny pinching misers to have minimum copper and iron, and will be hurting pretty badly at 265v

Only way to get more turns (and more iron) will be to use a much higher voltage rated transformer.
Not by a bit, but by a lot, like 400v or more.
That will have an amazingly beneficial effect on flux density at "only" 265v

The other issue is your amplifier. That will be much more efficient if run at a higher voltage and a lower current, if that is possible.
 

Warpspeed said:
You need to avoid saturation at all costs, and most mains transformers are designed by penny pinching misers to have minimum copper and iron, and will be hurting pretty badly at 265v

Only way to get more turns (and more iron) will be to use a much higher voltage rated transformer.
Not by a bit, but by a lot, like 400v or more.
That will have an amazingly beneficial effect on flux density at "only" 265v

The other issue is your amplifier. That will be much more efficient if run at a higher voltage and a lower current, if that is possible.
Click to expand...

Yes you are right. There must be more turns for using the transformer with less magnetising current so less current draw from TDA output. Now i have 265Vrms at output with load. Without load it becomes about 263V but i still need to get it stable as i can.

And i need to fix that point transformer reaches almost saturation about 290V. While reaching 265V, output sinusoidal signal changes little bit.
 

0.65 volt may not sound a lot, but in fact, you are overdriving the transformer 20%.

The solution is simple: a larger turns ratio.

Why don't you get a transformer with a 240 volt secondary, or better still, with a 277 volt secondary?
 
schmitt trigger said:
0.65 volt may not sound a lot, but in fact, you are overdriving the transformer 20%.

The solution is simple: a larger turns ratio.

Why don't you get a transformer with a 240 volt secondary, or better still, with a 277 volt secondary?
Click to expand...

Actually 0.65V difference must sound a lot since output difference becomes 73.33 x 0.65V = 47.66V.

Now i am trying to reduce overshoot and undershoot but its gonna be difficult i guess.
 

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