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500W 12v to 220v Inverter


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hie i was wondering if you could use 3A power diodes in place of the 2A  for back emf protection?

Does the 100R,1/4W resistor on the schematic mean 100 Ohms or you  can use any value of R as long as the ratio between the 100R,1/4W and 0.1R,10W is maintained?

Can i use a 60A Circuit breaker in  place of the 50A power switch or relay

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i am having problems with my square wave generator.What is happening is that at the pins 10 and 11 of the CD4047 there is a constant voltage(I am using 5 Volts d.c to prevent heating of the chip) instead of the original square wave that i was getting initially when i connected the circuit.Then it was oscillating between 5v and 0v.

I have changed the chip 4 times and still there is no change so now i don't know what to do.I tested all the chips and they were fine,together with the LM358 Op Amp and everything is ok but NO SQUARE WAVE!!!! >:(

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Hi Doron,
Something must be wired wrong for the CD4047 to get hot.
It is Cmos which has an extremely low operating current at such a low frequency and its opamp load draws nearly no current.
Didn't you make an oscillator like this one that is on the modified version?


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  • 3 weeks later...

I fixed the problem on the oscillator so evrything is working ok now.
I mounted the 4 2N3055 on one heatsink for each cycle,say when we have 1 cycle so i was curious as to how much heat they are  going to dissipate.you last said you need the specs for the heatsink so i was wondering what you were talking about
What exactly are you talking about when you say "Heat Specs" and can you give an example in relation to the 4 mounted transistors knowing their characteristics?

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The "heat spec's" for a 2N3055 transistor are that it will be at its absolute max temp when it has a PERFECT heatsink that keeps its case at 25 degrees C. Then it can dissipate 115W.
A REAL heatsink also has "heat spec's" that are its thermal resistance. Insulators also insulate heat, so should not be used. 4 2N3055 output transistors plus the 2N3055 driver transistor plus the pre-driver transistor can be mounted on one heatsink per side without insulators, then insulate each heatsink from each other and from the chassis.

With a load of 500W on the inverter, the total heat is estimated at 100W. So each heatsink will dissipate 50W.

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Hi Doron,
A 2N3055 transistor will reach its absolute max internal temp and can dissipate 115W only if it has a PERFECT heatsink that is impossible to have. The heatsink gets hot because it isn't isn't a PERFECT heat radiator. When the heatsink gets hot then you must reduce the power in the transistor. The transistor's case gets hotter because its mounting to the heatsink isn't PERFECT. The chip inside the transistor also gets hotter than its case because its mounting isn't PERFECT.

Another problem is that you will have 6 transistors on each heatsink making it hot.

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  • 9 months later...

i'm trying to build the same project,
seems like i have the same problem - no wave output from cd4047 - just continuous juice....

checked all wiring, but it's fine
what can i do to solve it ?

Did you make the WRONG circuit or my corrected one? My corrected circuit was recently replaced by the wrong one. I don't know why.
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I got several complaints about the circuit you built. It is built like an audio amplifier and does not apply the same rules as all other square wave inverters. Other members have gotten better results by removing some of the power robbing and heat generating parts, making it more like the one which is now posted on our site.

However, if you are not getting a data stream from the CD4047, you have either pulled too much current through it or you have connected it wrong.


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The "500W" inverter schematic now on our site does not have enough output transistors for 500W output. Also its output transistors must be matched.

500W output.
83% efficiency is an extra 100W of heating. Total= 600W.
600W/12V= 50A.
The output transistors must conduct 20A-25A each but their absolute max rating is 15A and they operate poorly at 10A and above.

My corrected inverter had twice as many output transistors so that their current was 10A-12.5A each.
The output transistors used emitter resistors to reduce the current gain of high gain ones for better matching.
It had diodes to arrest voltage spikes.
It had resistors to turn off the transistors.
It had correct values for the frequency determining R and C.

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Hi Photoar,
You made my corrected circuit that has been on the site for nearly 3 years until it was changed recently.
Look at the changed one in POWER CIRCUITS in our PROJECTS section. It is missing important parts and its frequency determining parts values are wrong.

The transformer is a 220VAC to 2x12VAC power transformer rated for 500VA or 600VA. The 2x12VAC windings can also be 24VAC center-tapped.

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Thanks MP !!

can you please attach or tell me the difference between the one i did and the one on your site ?
i found it on this site of course....  ???

The one now on the site is designed just like any other square wave inverter. The one you built is designed just like an audio amplifier with heat generating resistors to ground. Something you would expect to be designed by a stereo technician or audiophile. It consumes way too much energy from the battery. Others have emailed me to let me know they had to make serious changes to get it to work properly for them. It was after these emails that the one you made was removed from the site.

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The one you built is designed just like an audio amplifier with heat generating resistors to ground.
.... It consumes way too much energy from the battery.

With a 500W load, each 0.1 ohm emitter resistor dissipates 10W half the time. Their total power dissipation is only 40W (8% of the load's power). Then the transistors don't need to be matched to have the same current gain. Remove the resistors if you can buy matched transistors.
With less load then there is less power in the equalizing emitter resistors.

Mosfets are much better than old transistors in a simple square-wave inverter. This circuit is old and is used where Mosfets are not available.

Others have emailed me to let me know ...

They didn't e-mail me and I developed the modification. But they e-mail you?
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