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audioguru

"Power?" Inverter

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Hi Guys,
Why is this simple multivibrator inverter included in the Projects Section of this high-quality site? It is here:
http://www.electronics-lab.com/projects/power/018/index.html

The author claims that it will deliver 300W as shown.
Has anyone built it? Unfortunately, it has problems:
1) 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....., well it doesn't have a fuse, so something will get mighty hot.
2) With only 63mA of base-drive from R3 or R4 (180 ohms), if the 2N3055 transistors have their minimum-spec'd gain, then the output would be only 39W. (I know, the capacitors boost the base current, but only for a moment).
3) The capacitors cause the reverse-bias maximum voltage rating of the base-emitter junction of the transistors to be exceeded, causing damage.
4) The capacitors are shown backwards.
No, I don't want to re-engineer it, I think that it should be removed from this site.

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Ante,
How many inverters do we need on this site?
I think that the other one, the Low Cost 500W (or 700W) recently revised one, should be stripped-down (and will be explained), when someone requests a reduced power output.
As you know, I expect projects to work the first time, every time.

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We need as many power inverters as contributors want to post to this site. There is no limit set for posting schematics.

It is also important that when you are going to challenge a design, that you build the circuit, find the faults and then post your findings. Otherwise you only cause more confusion.

Please note that I have locked this topic. This thread is not useful to the building of a project.
Experience shows that this type of commenting or ridicule does not help anyone.

MP

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Guest Kasamiko

It's been years that i'm contemplating to build this circuit..saw it from the other site. But there is a big warning that ONLY 68uF TANTALUM capacitors works for it..That ordinary Electrolytic will explode!!
Base on your analysis is this true?? ??? ???

Regards

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MP,
Thank-you for unlocking.

Kasamiko,
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.
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 shown backwards.
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" electrolytic capacitors to explode.

But those expensive tantalums also explode (shooting bits of molten metal- VERY DANGEROUS) and there are debates about whether it is because of the high currents, or due to their reversed polarity, in the forum about this project on the author's web-site.

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To all,
Shall we re-engineer this dangerous (expoding capacitors) project and make it work (the author's web-site forum has many complaints about low output)?
Well, the author recommends "more powerful" transistors, but many have tried 600V, 30A transistors and other ways, attempting to make it work, but nobody has suceeded.

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audioguru, I looked at the author's forum and it does not look much different than threads on this forum. There were those who claim the inverter works and several who have problems with it. It could be due to instability of the design, the knowledge level of the project builders, mistakes in the schematic, or any combination of these.

Before we will be able to improve on this, someone has to decide they want to work with it on the bench. We need verification that it will fail and then to decide how to improve it.

We might even want to make contact with Aaron Cake to involve him in this.

MP

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Ante,
I agree that a power circuit will have a variation in its performance if incorrect wiring techniques are used, such as moving a computer's power-supply into the next room without considering the the voltage-drop of the long wires.
A power circuit will also have varying results if the circuit-designer does not consider the sample-to-sample huge variation of a transistor's current-gain. The circuit should operate well with ANY guaranteed sample. But let us not discuss "yield" nor luck.
Circuit performance and safety also depend on assembly details:
The instructions for this project do not mention heat-sinks.

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Ante, You make a very good point. One that is not observed very often. When dealing with power inverters, a lot depends upon the transformer at the output. In Aaron Cakes original article, he spoke about using a microwave transformer and re-winding it. Anyone who went to Radio Shack to get a small off the shelf transformer would have problems in such a case, if you need more VA.

audioguru, once again, I want to remind, this thread has been unlocked so that a way to make the project work can be discussed. Not to keep pounding on the project or point out why it might not work.

MP

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MP and Ante,
You made very good points about the transformer.
Although the author recommended an over-rated 15A transformer (15A X 24V = 360W), he did mention re-winding a powerful microwave transformer.
Maybe it is the transformer that produces the power, not the transistor circuit.

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Ante,
I was agreeing with you when I mentioned computer wiring, since you mentioned wiring then I knew that you were also considering PCB trace sizes:


I know that many factors must be looked at, the size and length of wires


In this project do you believe that the capacitors are shown with the correct polarity?
Do you have any comments about its high capacitor currents?
What are your comments about many amps of current going into avalanching base-emitter junctions?

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audioguru,

About the circuit; I have seen a very similar circuit many years ago. It was a circuit I once repaired that where made for the purpose of running your electric razor in the car. I remember the transistors were AD149 and the caps where un polarized type like the ones you use for speaker cross over filters, maybe they had un polarized caps for a reason? They perhaps figured out that the polarity is changing in the circuit and that standard electrolytics is not the way to go?

Ante ::)

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In the interests of safety (exploding capacitors) and quality (previously posted), this dangerous and defective project should be removed from our site. Let us not have anyone injured.
We already have a good re-engineered inverter (Low-cost, 500W).

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I have not yet heard that this re-engineered device works. Has someone built it? ...and who on this site has had the inverter that we are discussing here blow caps or come close to being injured?

I would like to hear some input. Other than yours. You seem to be the only person on this forum condemning this project. No one has the experience of building it.
In regards to posts about projects, I have noticed that those who have a good experience building a project, usually do not post. It is usually those who cannot make a project work or those who are asking for help that post. Therefore, if you are going to base your opinion on how many posts are about problems with a project, you are not going to get a good overall view of how people are doing with a project.

MP

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Jonathan,
Even better, replace your 555 and gate drivers with a 4047 oscillator/divider/direct-and-inverted-outputs chip that directly drives the gates. Its digitally-divided outputs are exactly 50-50 symmetrical. An output of 300W from one pair of MOSFETs would be easy.

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