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 12/120V inverter again
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tim
Mad Scientist

198 Posts

Posted - Apr 13 2004 :  8:23:58 PM  Show Profile  Reply with Quote
well if i were you i would quit while i was ahead because this inverter circuit sucks, i have been working on this for months with very little luck meaning only getting 67 volts out of this project. no the the caps go in like it shows and so does the diodes.if you can get this to put out some serious AC then you should get a medal. and also the resistors have to be wire wound. good luck...... tim..

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n/a
DELETED (Inactive)

1 Posts

Posted - Apr 26 2004 :  06:21:12 AM  Show Profile  Reply with Quote
Well just a few comments on the circuit being talked about here. One thing I can tell you all for sure is that Tantalum caps do explode very spectacularly when in a circuit reverse polarity. During my time at an electronics mfg. being involved with testing and troubleshooting, we had regular incidents of tanty caps blowing up like firecrackers when they were in backwards. (Very dangerous as flying hot bits of cap come at you, and the stink !!!)
As for the circuit itself, it does look rather simplistic for an inverter circuit and even I had a bit of trouble trying to analyze how it functions. I haven't had a whole lot of time to go over it but it seems to me like there may be problems with it. If those caps are blowing up on a regular basis then they are definitely in backwards as they are shown in the circuit. But without doing more circuit analysis I can't say for sure what it is but that would be my first guess at this point. Electrolytics like to blow up when enough juice is run through them backwards or even forwards too.
Will do somemore analysis and see if I can't find another schematic for comparison. Chow-fer-now.
12:12


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n/a
DELETED (Inactive)

13 Posts

Posted - May 03 2004 :  4:31:11 PM  Show Profile  Reply with Quote
Hi,
I was just wondering if anybody has ever tried using gate insulated bipolar transistors (IGBT's) in place of the 2N3055's. I have about 10 of these things laying around and they are rated at 900vDC at 60A. I have been thinking about using these in place of the 2N3055's. By the way if the 68UF caps are blowing up wouldn't it be better to use capacitors that can withstand high current DC pulses of electricity?
I just happen to have a few high voltage pulse capacitors that are rated at 40UF, 3KVDc I think they just might be suitable if I add some reverse voltage blocking diodes.


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tim
Mad Scientist

198 Posts

Posted - May 03 2004 :  7:01:36 PM  Show Profile  Reply with Quote
i have tried hi amp bipolar npn transistors, also all differant kinds of caps. i put a cap bank of 5 50 volt 10uf caps together per side and these are nonpolar caps and under a load i still get about 67 volts no load i get 110 volts. its definitly in the type of cap used and i think strongly that it has to be a certain type of tantalum for sure and this circuit will kick ass. thetre are certain types of tantalums but i dont know if its wet or solid but i do know it can not be dipped they blow like firecrackers and this is either way you put them in, ive been there i did it all tring to get this circuit to work.

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n/a
DELETED (Inactive)

13 Posts

Posted - May 03 2004 :  8:00:46 PM  Show Profile  Reply with Quote
Well it doesn't hurt to experiment so I will see what I can come up with. I have one hell of a nice transformer with a 110-120v input and a 26.5v output at 20A.

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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - May 05 2004 :  10:50:20 PM  Show Profile  Reply with Quote
We discussed your Inverter circuit on another site and I have made a detailed analysis which explains the problems that this circuit has:
General
1) The circuit cannot produce anything near 300W.
2) If the parts values and transistor gains are identical, then when it is turned-on both transistors may conduct at the same time and latch-up. Then it will blow the fuse.
3) The capacitors are shown backwards.
4) The capacitors cause the reverse-bias maximum voltage rating of the base-emitter junction of the transistors to be exceeded, causing damage. The capacitors have huge current flow causing them to blow-up.
I have a lengthly, detailed explanation if anyone is interested.
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YS
Nobel Prize Winner

USA
1132 Posts

Posted - May 07 2004 :  08:44:24 AM  Show Profile  Visit YS's Homepage  Reply with Quote
Of course, this circuit is very simple and not ideal - or if so, why should not industry use it?
But it would be interesting to hear detailed explanations. So far, the only good point you made is the second half of #4, currents through caps were discussed.
Others :
#1 - theoretical limit is close to 300W, as the thing produces something close to square wave - and battery voltage is not 12V, as we all know
#2 - invalid. You probably used SPICE for analysis; what about astable multivibrator then? this circuit is just a variation. There are never two identical transistors unless you take special care to make them.
#3 - was discussed already, see the beginning of the topic.
As for reverse voltage, you are right. 7V is the limit and here we may have more - so emitter junction serves as zener diode. It does not necessary hurt transistor, as the current is not too huge, but maybe diodes should be used to protect transistor and reduce heat.

And the last: from the professional point of view, almost any schematic on this site may be criticized for hours and hours.. but they are quite good for amateur purposes. They (almost all of them) may be improved; so, if you see the way to do it, please share with the rest of us.

Regards, YS


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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - May 07 2004 :  09:54:26 AM  Show Profile  Reply with Quote
YS,
Thank-you for your interest, here is my analysis:
Detail and Recommendation
1a) In order to obtain 300W from a 12V battery then the output transistors must conduct 25A each. But the 2N3055 listed transistors have a maximum rating of only 15A and are spec'd to only 10A. Therefore paralleled transistors are required, with emitter resistors to equalize transistor gains.
1b) The 2N3055 transistor has a guaranteed gain of only 5 at 10A of collector current and even less at higher currents. Since the 180 ohm base-drive resistor gives only 62mA, then the gain of the transistors must be at least 403. Therefore driver transistors are required.
1c) Actually, in this circuit, using 2N3055 transistors having minimum gain of about 50 at 3A of collector current, the 62mA of base-drive from the resistors results in 3.1A of collector current. Therefore the power of the circuit is 37.2W, much of which is spent heating the capacitors and damaging the base-emitter junctions.
2) I have seen it happen, when I selected parts to make a simple multivibrator like this one produce 50-50 symmetry. The oscillator locked-up occasionally. The use of a 4047 oscillator/divider is recommended along with pre-driver transistors.
3) The collectors of the transistors are mostly positive, and the bases are near ground potential. Reverse the polarity of the capacitors.
.4) Most silicon transistors have a maximum reversed-biased voltage rating of about 7V. At that voltage, the base-emitter junction avalanches, like a zener. The avalanching burns holes in the junction, reducing its gain. The amount of damage is related to the size of the junction and the transistor's gain.
Capacitors have a "ripple current rating" which is related to their internal impedance (resistance) and physical size (to dissipate power) similar to a power resistor. The ripple current rating can be exceeded with high AC current.
In this project, the capacitors are charged to nearly 23V with a very high current flowing through a forward-biased base-emitter junction on one side, and the +24V (center-tapped transformer action) from the transformer on the other side.
The capacitors are partially discharged with a very high current through a saturated collector on one side, and an avalanching (-7V) base-emitter junction on the other side. When the capacitor voltage discharges to less than 7V then it can continue a slow dicharge through the resistor, for its timing period.
Therefore the capacitors have very high currents flowing in them which causes "ordinary" capacitors to explode. Separate oscillator and output drive circuits are recommended.


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wasssup1990
Nobel Prize Winner

A Land Down Under
2261 Posts

Posted - May 09 2004 :  03:59:44 AM  Show Profile  Visit wasssup1990's Homepage  Reply with Quote
Shyt! Some very long replies hear. I remember when i first saw this site, i was looking for an inverter schematic on google, and it came up with this site. I looked at the schematic and said to myself, "Its to simple? how could it work." Even if you did get it to work it would take a long time, thats why i'm building my own.

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YS
Nobel Prize Winner

USA
1132 Posts

Posted - May 12 2004 :  9:36:28 PM  Show Profile  Visit YS's Homepage  Reply with Quote
Not too easy, eh?

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wasssup1990
Nobel Prize Winner

A Land Down Under
2261 Posts

Posted - May 13 2004 :  01:57:09 AM  Show Profile  Visit wasssup1990's Homepage  Reply with Quote
Certainly not!! Trying to get an energy level out of another enrgy level will allways be hard.

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shivick21
Apprentice

Philippines
57 Posts

Posted - May 25 2004 :  08:39:34 AM  Show Profile  Send shivick21 a Yahoo! Message  Reply with Quote
heheheh hi again! so figured it out how does thing work? i am very intriuge about that coz i want to have that thing to!! wahehehe if anyone has/have a working diagram, pls post. hehhe so i can use my pc even the power went out like i posted earlier. (converting atx pc power supply) thats all again. thanks.

Aki-kun
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n/a
DELETED (Inactive)

56 Posts

Posted - Jun 02 2004 :  03:44:27 AM  Show Profile  Reply with Quote
Well, I built one, and I have a couple of dead 3055 transistors to show for it.

I don't have the tantalum caps (I remember thinking that they looked like popcorn kernels when I soldered them onto my prototype) because I never bothered recovering all of the fliff when they exploded.

Actually, I'm not looking for 120V from it, more like about 30, and no more than a couple of amps. What I want to do with it is to make a symmetric power supply, +/-15V for some audio circuits.

Eventually, I'd like to try something that's more along the lines of +/-35V and about 8 or 9A for a small amplifier, but that's a "down the road" idea, so to speak. Not much more than the 100W range, in any case.

The oscillator locking up would be a problem, but the power limitations should not be.

I may try this again with a 2N2222 as a driver for a darlington pair.

Or, I may just use an admittedly much more complicated SMPS based on an SG3524 controller chip...

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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Jun 02 2004 :  09:10:01 AM  Show Profile  Reply with Quote
SupraGuy,
Sorry to hear your experience with this bad circuit.
For your applications, why not keep it supra-simple:
1) A CD4047 oscillator/divider/opposing-outputs chip as the oscillator-driver. It needs only 1 resistor and 1 capacitor. Its outputs are perfectly symmetrical, and are direct and inverting.
2) A pair of power MOSFETS with built-in zener protection diodes as the output transistors.
3) A regular 50Hz or 60Hz center-tapped transformer, or a smaller high-frequency one.

300W would be easy.
I would post a circuit but I don't need another power supply. Why don't you?


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n/a
DELETED (Inactive)

56 Posts

Posted - Jun 02 2004 :  4:32:55 PM  Show Profile  Reply with Quote
Hmmm... Reading through, I was looking at wassap's inverter schematic, and came up with a variation. I removed the temperature regulation, and the push-button start, and added in some noise rejection, and overvoltage protection (I know what happens at the alternator when you turn off the headlights!) Hopefully this would be quiet enough for audio circuits.

I've put up the schematic at http://www.abstractconsulting.com/~dan/images/Inverter.gif

Please pardon my cheapo schematic drawing program. :)

The entire oscillator section was grabbed wholesale. I want to adjust the switching frequency upwards, and I plan on adding some large capacitors to the supply, which should also allow me to eliminate the input choke.

I took a brief look, but could not find a CD4047 part available. Ah. Nevermind. I just checked TI. :)

Most of the trouble that I seem to have when dealing with SMPS for a car supply is sourcing the transformers, or even just a core that I can wind myself. Nowhere local carries them, and I can't seem to order them from digikey, either. I know that such things exist, I just can't find 'em.

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