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 12/120V inverter again
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 01 2008 :  11:24:25 AM  Show Profile  Reply with Quote
There is only one oscillator, not two. Tt is a multivibrator. Look at it in Google. It probably operates at exactly 60Hz.

The polarity of the capacitors is backwards.

The transistors have avalanche breakdown of their emitter-base junctions (max allowed voltage is 7V) because the capacitors are charged to about 23V then they try to drive the base of the transistor to negative 22V. Therefore the capacitors have a very high discharge current which overheats them even if their polarity is correct.

The transistors do not have enough base current for a powerful inverter so the output power is very low.
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gim
Apprentece

5 Posts

Posted - Oct 02 2008 :  10:24:25 AM  Show Profile  Reply with Quote
Thanks for the detailed analysis.

According to Wikipedia, for a multivibrator, "The period will also depend on any current drawn from the output..."

So maybe it won't run at exactly 60 Hz?

The circuit of this inverter is slightly different from the basic multivibrator, because the +V feed to the collectors of the transistors passes through the load, which is variable.
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gim
Apprentece

5 Posts

Posted - Oct 09 2008 :  09:01:45 AM  Show Profile  Reply with Quote
quote: Now the schematic is confusing.
D3 and D4 connect to pin 5-and-a-half on the CD4017.

EDIT:
Thanks again Juan for fixing the schematic.



Can someone please direct me to the schematic referred to here? I can't find it. Thanks.
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gim
Apprentece

5 Posts

Posted - Oct 09 2008 :  09:12:34 AM  Show Profile  Reply with Quote
I have built a modified square wave inverter according to the schematic I found at http://www.discovercircuits.com/C/co-dctoac.htm (Third circuit in the list).

This inverter works fine, at low power. I have not tried it at high power yet. It does not require a centre tapped transformer. I have used 12amp MTP3055E MOSFETs rather than the 0.5amp BS170s.

Audioguru, if I may ask you a question. In a previous post you stated that the gates of MOSFETs should be fed through a (47 ohm) resistor, to prevent high frequency oscillation. The schematic I have used does not have any such resistors. How necessary are they?
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 09 2008 :  10:45:34 AM  Show Profile  Reply with Quote
quote:
Originally posted by gim

I have built a modified square wave inverter according to the schematic I found at http://www.discovercircuits.com/C/co-dctoac.htm (Third circuit in the list).

This inverter works fine, at low power. I have not tried it at high power yet. It does not require a centre tapped transformer. I have used 12amp MTP3055E MOSFETs rather than the 0.5amp BS170s.

Audioguru, if I may ask you a question. In a previous post you stated that the gates of MOSFETs should be fed through a (47 ohm) resistor, to prevent high frequency oscillation. The schematic I have used does not have any such resistors. How necessary are they?


The BS170 is a tiny low power Mosfet. Its input capacitance is only 60pF max so it probably doesn't oscillate without a resistor in series with its gate.

Real power Mosfets have an input capacitance almost 200 times higher so they oscillate at a high frequency if the gate doesn't have a series resistor mounted closeby.
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4396459saad
New Member

Pakistan
1 Posts

Posted - Oct 11 2008 :  11:22:02 AM  Show Profile  Send 4396459saad a Yahoo! Message  Reply with Quote
hi i am making a ups for my project
i need a ckt diag of a ups that could drive a load of 100w for 10min

SAAD SARDAR
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yunusu
New Member

Rwanda
1 Posts

Posted - Oct 12 2008 :  11:17:39 AM  Show Profile  Reply with Quote
It's possible to adjast the output fo an inverter
by using a simple variable resustor in it's
oscilator ? how

Hello
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 12 2008 :  3:08:18 PM  Show Profile  Reply with Quote
quote:
Originally posted by yunusu

It's possible to adjast the output fo an inverter
by using a simple variable resustor in it's
oscilator ? how


Adjust what? A variable resistor can adjust the frequency.
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forlan12
New Member

4 Posts

Posted - Oct 13 2008 :  03:45:41 AM  Show Profile  Reply with Quote
can u guys help me.

in my project, i have to replace the inverter IC into PIC microcontroller.

1)i want to find the replacement component for 2sj471- pnp mosfet.
2)what specification should i look in the datasheet?
3)is it irf9450 suitable for replace 2sj471?


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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 13 2008 :  12:06:51 PM  Show Profile  Reply with Quote
You found a low power Japanese inverter circuit. Of course it uses Japanese parts.
Its output is a simple square-wave that will not drive many electronic products properly.
It uses 4 Mosfets because it doesn't use a center-tapped transformer when it needs only 2 N-channel Mosfets.

Its performance is horrible because it does not have voltage regulation. With no load then its output voltage is 10% high. With a 40W load then the output voltage is correct. With a 140W load then the output voltage is 10% low.
If you use a center-tapped transformer then the output power would be double with only 2 of those Mosfets.
http://hobby_elec.piclist.com/e_ckt30.htm

what will a PIC do? If it produces a high frequency PWM signal to make a stepped sine-wave then the transformer must be different and the transistors must switch quicker.



Download Attachment: 110W inverter.PNG
50.02 KB


Edited by - audioguru on Oct 13 2008 12:07:58 PM
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forlan12
New Member

4 Posts

Posted - Oct 13 2008 :  2:44:23 PM  Show Profile  Reply with Quote
quote:
If you use a center-tapped transformer then the output power would be double with only 2 of those Mosfets.
what will a PIC do? If it produces a high frequency PWM signal to make a stepped sine-wave then the transformer must be different and the transistors must switch quicker.


PIC produce PWM signal. i plan to make sine-wave inverter 12/240V.

plz corect me if i wrong.
the transformer should be 12V-0V-12V and its center-tap connects to the positive supply. The sources of the N-channel Mosfets (2sk2956)connect to ground.

btw what mosfet should i use? is it IRF9540 suitable for replacement 2sj471?

Download Attachment: power invert edit.GIF
9.5 KB
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 13 2008 :  6:40:28 PM  Show Profile  Reply with Quote
The IRF9540 is a weak low power P-channel Mosfet. It is not needed with the center-tapped transformer.

Your new circuit shows a low frequency square-wave inverter, not a high frequency PWM sine-wave one.

The peak voltage of a square-wave is the same as its RMS voltage. But the peak voltage of a sine-wave is 1.414 times higher so the transformer must be about 9V-0V-9V to make the higher peak voltage for a sine-wave. The PWM transformer must be a small high frequency ferrite one.
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forlan12
New Member

4 Posts

Posted - Oct 14 2008 :  12:30:15 AM  Show Profile  Reply with Quote
is it ok, if i change the circuit by changing the placement of mosfet into full-bridge.
all the mosfet are 2sk2956 npn.
any recommend , what mosfet should i use other than 2sk2965?
i prefer mosfet IRF brand



Download Attachment: power invert2.GIF
10.35 KB
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Oct 14 2008 :  10:58:30 AM  Show Profile  Reply with Quote
quote:
Originally posted by forlan12

is it ok, if i change the circuit by changing the placement of mosfet into full-bridge.

It won't work because the upper N-channel Mosfets need a gate voltage that is 10V higher than the source voltage. Then the gate voltage must be +22V to +24V.

Usually a full-bridge Mosfet driver IC is used that has a voltage doubler circuit inside for driving the upper (high side) Mosfets.

The Mosfets are selected for the amount of output power you want from the inverter.
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mleiss
New Member

2 Posts

Posted - Oct 18 2008 :  01:02:18 AM  Show Profile  Reply with Quote
This is in regards to simply multivibrators like the original schematics shown. With switchers and inverters it is wise to know if there is a "dead band" between the switching pulses, especially if there is more than one transistor in the switching circuit. When one transistor switches off, the other one is turning on and there is a moment in time where both are on, though very brief. This has a detrimental effect on your efficiency of the circuit. Also, since both transistors are on for that brief moment, guess what this does to the current... All of the current in the circuit goes to ground, essentially removing the stored energy that is in the system and effecting efficiency because now your supply has to provide additional energy to make up for the losses. So to make this work, and I haven't tried this circuit out myself either, I believe that a more sophisticated clocking source would be required to allow about 200ns to 400ns of a dead zone between each triggering pulse of the transistors.
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