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# Low-pass Filter?

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That doesn't show me the math or answer the question.

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The math is in a text book or on the web.
Here's an example of a Butterworth lowpass filter showing the positive feedback:

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John

I know the math...... for a second order, the transfer function solution is quadratic, but you're the "theory expert", so can you explain, in technical terms why the gain doesn't start to roll off a decade before the corner frequency?

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The gain doesn't fall off too soon because the positive feedback boosts it near the cutoff frequency. For a Butterworth filter, the frequency response is absolutely flat to near the cutoff frequency, no matter how many orders there are.

As an example, I use a 2nd order highpass Butterworth filter with too much gain to give bass boost to my sound system. I used the same thing except with a 2nd order lowpass filter to give a 3.5kHz peak in telephone boardroom conferencing systems.

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why not use a fir filter? is there a difference?

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An FIR is a complicated digital filter. Maybe you are thinking of a switched capacitor filter, like I use in my ultra-low-distortion signal generator. I even use a switched capacitor notch filter in my distortion analyser circuit. They make great simple filters. ;D

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i belive its a fir it works really good, I downloaded it to a DSP Board. used a blackman window, its a band stop filter so when it reaches 60 hz it stops that frequency,  pluged in a couple numbers and tested...so  a fir filter will give better results? then a butterworth, or  bessel?  or is there something that makes them different?

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A FIR digital filter in a DSP board has a sharp cutoff because it effectively has many orders.
A Butterworth analog filter can also have many orders. Maxim have some Butterworth switched capacitor lowpass filters with 8 orders. Square wave in, sine wave out.

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OK...... so what your "saying" is that, in a second order Butterworth low pass filter, there is gain just prior to the corner frequency due to "positive feedback"??? The transfer function only has poles and NO zeros, how can that be???

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OK...... so what your "saying" is that, in a second order Butterworth low pass filter, there is gain just prior to the corner frequency due to "positive feedback"??? The transfer function only has poles and NO zeros, how can that be???

The opamp provides the majical gain for some positive feedback and boost.
In my example of a Sallen-Key lowpass 2nd-order filter that has equal component values:
1) At low frequencies, C1 and C2 have a high impedance and don't affect the response which is flat.
2) At high frequencies, C2 rolls-off the response so the opamp's output is lower than its input, then C1 also rolls-off the response.
3) Near the cutoff frequency, C2 doesn't roll-off the response very much so there is positive feedback through C1 to the input which provides a boost.
The boost is exactly the same as in my bass-boost circuit, except it is less because the opamp's gain is less.

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• 2 weeks later...

hi, Im Ren from the philippines I just want to ask if the 500 watts power inverter
is really working? and if so is it applicable to Television sets?
Becuase I want also to build the power inverter if you wuold mine.
Pls. give an assurance that the inverter will work?
Thanks a lot :).......

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• 5 weeks later...

can i upgrade thats 500w to 5000w with a 110 and a 240 output ac

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Hi Ren,
The modified 500W inverter is used in the Philippines to power fluorescent lights and TVs that work with an input from 85V to 250VAC.
If you used many paralleled batteries, an enormous transformer, 8 to 10 times as many transistors, 8 to 10 times the size of the heatsinks and a few other things then it could be made for 5000W. Its custom-made transformer could have any output voltage you want.

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• 1 month later...

Can i use the 1N5401 for the diode ?
the voltage at the primary side of the transformer should be 12V AC right ?

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Can i use the 1N5401 for the diode ?

For the modified 500W inverter project the 1N5401 diode is rated for 100V/3A so will be fine for D1 and D2.

the voltage at the primary side of the transformer should be 12V AC right ?

No. The primary has 24Vp-p so the transformer should be rated at 12V - 0V - 12V which is 24V center-tapped.
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I measured the voltage between "12V to 0" and i get 24VAC
and same result for the "0 to 12V"

I didn't connect to a transformer yet for safety purpose....

so is the reading correct ? or should be 12VAC instead of 24VAC?
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I don't know what your transformer is connected to. The mains? The project?
I don't know what was not connected to a transformer yet.
The project produces a square wave output that most meters won't measure accurately.

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sorry for confusing, I built this 500W inverter but not yet connect the transformer to that circuit yet. and the 24VAC is come from the circuit which should be connected to the transformer.

sorry for drawing it wrongly.
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The project doesn't have a regulated output voltage. It will change with the load.

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the voltage will change with the load?
the project is using 12v-0-12v to 220 transformer, so the circuit circuit should generate 12VAC at the primary side of the transformer right ?

My problem is why i get 24VAC instead of 12VAC...
Thanks

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With a 14V fully charged car battery, one 12V wire of the transformer will be driven to about 2V and the other 12V wire will swing to 26V. Then the oscillator reverses the voltages. So each transformer wire has 24Vp-p when measured from the 14V center tap.

If you connect the high voltage winding to a 220VAC sine-wave, then each wire of the low voltage winding will be 12VAC which is 34Vp-p when measured from the center tap.

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I see... thanks for your explanation .. ;D

btw I have a 3A step down transformer from 240VAC (primary) to 12v-0-12v (secondary) for the rectifier system, can i use it as the step up transformer for this circuit? 12v-0-12v become the primary side while the 240VAC as the secondary side?

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If your transformer's 24V output is rated at only 3A then the transformer's max rating is only 72 VA. Using such a small transformer in a 500W circuit wastes many parts. Just the operating power of the circuit without a load will probably overload the transformer.

Here is the modified project stripped down for a 100W load. With your small transformer its max is 72W.

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actually the transformer that i am using now is a 240V to 12V  step down transformer for my rectifier circuit.

I need this inverter for my UPS project, so this 500W inverter is suitable...
so what kind of transformer i should get to support 500W output? 20A transformer?

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actually the transformer that i am using now is a 240V to 12V

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