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Sulaiman

Mon Aug 13 2007, 11:59AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The waveforms are better but still poor (and a little confusing)
Is the earth of the 'scope connected to 0 Vdc ?

If it works reliably then the main consideration is - How efficient is it?

Efficiency = (Power Out) / (power In)

If you get less than 0.75 that's not good

If you get more than 0.9 that's very good

If you get more than 1.0 check your measurements

P.S. What is the time-per-division on the 'scope OR what frequency is it oscillating at?

Munkey

Mon Aug 13 2007, 12:57PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

Yep the scope is on 0V
Well I don't know if it is that reliable, I don't want to power it for more than 1min in case something blows up, Oh yes on the subject of that I blew up another 2 regulators by stupidity, the appropriate name for this should be "Inverter of the dead component"

Efficacy... I think is quite good, surprisingly now it draws 3A (36W) and can light the 40W light bulb fully (I think) and no heat produced from the 3055s not surprised because they are mounted on a hestsink the size of a hippo. Huh? that does not add up, I put in 36W and get out 40W.
Ok so let's use your way. So 40/36=1.11111111111111111111111 and that will probably carry on until time ends.
It is not putting out 40W then, So how do I measure the output power? 202x??=W

Edit: the scope is on 2.5ms at 0.5V

SpyrosF

Mon Aug 13 2007, 01:41PM

Registered Member #352 Joined: Tue Mar 28 2006, 08:39AM
Location: Greece
Posts: 14

Hello
If the scope is set at 2.5ms then the waveform probably has ac ripple from your power supply.
If this happens the calculations are wrong (3A (36W))

Munkey

Mon Aug 13 2007, 02:21PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

But the waveform only goes weird when an indicative load is connected. Although that is very possible as my high current PSU does not have any smoothing.

draws 3A at 12V 3x12=36?

Marko

Mon Aug 13 2007, 02:46PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

How much trouble can you guys generate from something so simple.

2N3055 is not going to always drop 0.7V, not even close. Why it seems that *no one* here actually looked at the datasheet?

Datasheet gives saturation voltage of 1V at 4A Ic/400mA Ib and 3V for 10A/3.3A IB.

So in common emitter mode you need to always provide like several amps of base current.

To provide these 3,3amps from second figure, one needs to drop like 10V at least on a transistor or base resistor, resulting in 33W of dissipated power just from base drive.

You could limit the massive base current by using the transistors in emitter follower configuration, but efficiency is still going to suck at higher powers.

'True' way how proffesionals drive bipolar transistors is proportional current drive, which doesn't make much sense in this circuit.

PS. I apologize for this, I'm really looking severe here. The schematic is OK, it's my mistake.
I could have really been nicer to someone like bacon, I tought better of this post.

Munkey

Mon Aug 13 2007, 03:29PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

Marko wrote ...

In bacon's schematic, there is nothing that actually limits the current, those pre-drive transistors practically create a short through the base.

Darlington pair.

I never knew someones problem could annoy you so much. This is not "simple", to me.

SpyrosF

Mon Aug 13 2007, 04:34PM

Registered Member #352 Joined: Tue Mar 28 2006, 08:39AM
Location: Greece
Posts: 14

draws 3A at 12V 3x12=36?

With all this ripple you can't be sure its 12V. Check again with the scope
using a car battery.

Sulaiman

Mon Aug 13 2007, 05:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Bacon,
Don't worry about Marko - he often gets very negative - don't take it personally.

Since you clearly stated
"I want to try to make this with none of these SMPS controllers, PLL chips, PWM devices.."
and you already have a near working inverter I'll try and help you to continue.

First, to clear up Marko's "contribution"
Since you want 10A in the primary
and you have two transistors forced to share the current fairly equally by the 0.1 Ohm resistors
each transistor will pass less than 5A, so you can ignore the 10A/3.3A as it's irrelevant.
As for the 1V at 4A I guess he means guaranteed maximum,
the graphs will show you that it's more like 0.3V to 0.6V depending on which manufacturers data sheet you use.
The bases will require up to 0.45A each at 4.5A collector current each, so that's 0.9A maximum required.
The two bases are driven by a transistor with it's collector connected to the collectors of the 2N3055
so almost all of the base drive current for the 2N3055s goes through the load.
This means that for 10A total, the 2N3055 collectors actually carry just over 4.5A each.
You didn't give a part number for the transistor that drives the 2N3055s
but the current gain will be better than a 2N3055 at 4.5A ... I'll assume current gain = 30
which would require about 30 mA base current .... about 0.36W base drive power (30mA x 12V)
... not 33W as Marko thought, so don't worry about that.
The 2.2K resistor from the collector of the 'pre-drive' transistors to +Vdc will protect the bases
so don't worry about that either.

OK, to continue, what do you mean by "my high current PSU does not have any smoothing."
If you mean there is no large electrolytic capacitor after the rectifier, then I'm surprised how well the circuit works!
You definitely need a smoothing/reservoir capacitor. Several thousand uF.
If you just meant that there's no regulator - that's ok.
Using a 12V car battery as suggested by SpyrosF is an excellent idea, just to eliminate one potential problem.

Did you change the connections for the diodes? (you should see no difference but do it anyway)

Have you tried 0.1 uF (or thereabouts) from the 2N3055 collectors to 0V?
If that doesn't get rid of the ringing try it from the collectors to the base of the transistor that drives the 2N3055s.

Try the above and let us know how it goes ... good luck

Munkey

Mon Aug 13 2007, 05:35PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

I tried it on the car batery and the waveform looks like this:http://i96.photobucket.com/albums/l178/ binary10101/SCOPE.jpg Yay! I'm not sure why it goes up at the end and slightly curved when it comes down. I am surprised that an un-smoothed power supply can change that at the base of the transistor. Anyway I have changed the schematic a little so in half and hours time Ill edit my post with the new one.

Munkey

Mon Aug 13 2007, 05:36PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

I tried it on the car batery and the waveform looks like this:

Yay! I'm not sure why it goes up at the end and slightly curved when it comes down. I am surprised that an un-smoothed power supply can change that at the base of the transistor. Anyway I have changed the schematic a little so in half and hours time Ill edit my post with the new one.

Edit: here is the new schematic

Sorry about the double post, I don't know how I even did it!

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