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Munkey

Sun Aug 12 2007, 02:58PM

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

Hi everyone,
Im a little confused about my inverter circuit here:

I have made it on breadboard and it is acting very weird. The output voltage with a load is too high for the multimeter to read... The waveform at the base of the 3055s is mad! it looks nothing like a square wave it has no shape at all, just a jumble of green lines... Also I am not sure if it fully saturating the 3055s (in parallel at right of schematic), the voltage at their base is about 0.5V Surely that is too low for it to turn on... This is the first time ever I have used a op-amp so it was a guess where to put the + and - marks on it. Are there any suggestions on how to make it better?

My first attempt was to use the UCC37321/22 driver chips on transistors- it worked perfectly! But then I lent on the voltage adj knob on my PSU turning it up to full and blowing up every chip. the thing that really anoys me is that I have done this twice. Its almost like it does not want to be made!

Thanks,

Marko

Sun Aug 12 2007, 03:21PM

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

Please make the world favor and don't use bipolar transistors for those applications.

Just take look at this.

Sulaiman

Sun Aug 12 2007, 04:10PM

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

There are many subtle ways to have such a curcuit not work properly
Can you put values for the components, supply voltage (12V assumed) and
what current do you want in the primary, and what is the transformer?

For a start,
1) I agree with the KISS (Keep It Simple, Stupid) philosophy, but let's try to make your circuit work anyway
2) a TTL ic can sink 16mA but source only 400uA, LSTTL can only sink 8mA and source 400uA
So in general, if you want to drive something try to use the sink mode (Low output voltage active) (Later)
3) Temporarily replace the output transformer with two resistors to +V,
e.g. 100 Ohm 2 to 5 Watts OR 1kOhm 1/2W
now check the circuit operation, especially the voltage at the 2N3055 bases.

If you can't get any improvement/see the problem, ask again here

Munkey

Sun Aug 12 2007, 06:39PM

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

Marko wrote ...

Please make the world favor and don't use bipolar transistors for those applications.

Just take look at this.

Ahhhhhh I spent Â£8.00 on the 3055s! And yesterday I was going to get another 2, Just to add to the insult!!
Ok I know now that FETs are good, but I want to try to make this with none of these SMPS controllers, PLL chips, PWM devices... They drive me nuts because they are so specialized and expensive.

Sulaiman wrote ...

There are many subtle ways to have such a curcuit not work properly
Can you put values for the components, supply voltage (12V assumed) and
what current do you want in the primary, and what is the transformer?

2) a TTL ic can sink 16mA but source only 400uA, LSTTL can only sink 8mA and source 400uA
So in general, if you want to drive something try to use the sink mode (Low output voltage active) (Later)

Well the current I would like in the transformer primary would be max 10A and min around 6A

Sink mode, ISTTL?

And here is the scope waveform with the 1K resistive load

I am not sure why it is not turning fully off, the bottom line is not on the 0 center line.

And here is the diagram with all the marked values

Marko

Sun Aug 12 2007, 06:57PM

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

Mosfet's aren't 'good', they are *only way* to get something as such to work. IRFZ44 can be got for about same price as 2N3055.

Trying to use 2N3055's this way is just silly. They consume large amount of power to remain in saturation, need complex drive circuit, and still drop like 3V at 10 amps forward current resulting in poor efficiencies.
Those are transistors for use in linear mode, not switches.

SG3525 is neither expensive or specialized. Some versions of 555 are more expensive than it.

Other than that, you can dig out TL494 out of old computer power supply, it is also good.

Reason why should you use SMPS controller is because you want 50% duty cycle to keep RMS/peak voltage ratio same as with sine wave, which is complex to do with logic.

Then you use a transformer with primary voltage equal to about 0.7 of your supply voltage.

For a 12V battery (wich is usually 13-14V) 9 or 8V transformer is good match.

Munkey

Sun Aug 12 2007, 08:11PM

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

Well I have seen many inverter circuits using 3055s or are they crap?

So what about this: I buy a load of logic level FETs then use the flip-flop and 555 as they are now
OR
Use the 555 and the UCC37322/21 to drive a load of normal FETs and just feed the 555 straight into the UCCs
OR
Use a Darlington to drive the FETs straight from the Flip-flop
OR Swipe everything that I have already made onto the floor in a stress

Everytime it is a major redesign from a simple problem. Grrrrr *turns around from typing this with arm stretched out pushing everything on the floor*

Sulaiman

Sun Aug 12 2007, 10:35PM

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

Things I would do;
1) Find out why the waveform shown is not a nice 50:50 squarewave
The waveform shown, even if the voltages are correct, would cause the transformer to saturate,
which would 'blow' the 2N3055s.
Check the waveform at the Q and /Q outputs -they MUST be
exactly the inverse of each other AND exactly 50% duty.
If not then disconnect the op-amps and find the problem.

2) You have wired the op-amps as inverting switches
Although this mode is possible I don't like such huge differential voltages at the inputs,
Try connecting the op-amp output to the (-) i/p and the Q and /Q to the (+) i/p, as a unity voltage gain buffer
i.e. change pins 2&3 and 5&6

3) I would not use the op-amps at all
Connect each of the Q and /Q outputs to +5V via a Resistor of 330 to 2200 Ohms (lower is better)
Connect the bases of the driver transistors directly to the Q and /Q outputs.
Now with resistors instead of the transformer check the waveforms - should be ok, re-connect the transformer.

4) the diodes are in a strange/useless place, connect them from 0V to the 2N3055 collectors instead.

I agree with Marko that MOSFETs are generally better,
but it's better to get your circuit working to learn some of the easily encountered problems.
In this type of circuit a 2N3055 is quite good as the frequency is quite low
and at 5A per transistor the collector-emitter voltage drop is about 0.75V, quite ok
Wiring the transistor that drives the 2N3055s with it's collector common to the 2N3055 collectors will result in
1.5V to 2.0V minimum which is a lot worse, and in this application there is no need to prevent the 2N3055s saturating
so later you could try connecting the collectors of the transistors that drive the 2N3055s to +12V instead.
A 10 Ohm 10W resistor between the collector and 12V would give protection during fault conditions but is not required.

Munkey

Mon Aug 13 2007, 08:56AM

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

Well I got it to kinda work by removing the pull-down resistors at the bases of the 3055s, now it draws about 5A and can light almost fully light a 40W bulb. The waveform looks like this:

top and bottom are for transistor set #1 and #2. Hmmmmm... A little messed up, Im not happy with that so I am going to do what you said. I always disliked op-amps, now I have a reason why!

Excuse my stupidity but what is a unity voltage gain buffer and what do you mean by "transformer to saturate"
Yes I could look these up in google, but it is 99.999% that I will not understand, its better to talk to a "human"

Thanks,

Sulaiman

Mon Aug 13 2007, 10:18AM

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

Transformer saturation due to 'flux walking'

Your circuit puts about 12V across one half of the transformer for half of a cycle
and about 12V across the other winding for the other half-cycle

The main current in the primary of the transformer is the output current x (secondary turns/primary turns)
Usually written as Ip x Np = Is x Ns ... no problems here

Because the transformer primary has inductance, energy will be stored in the magnetic field in the transformer core
This is due to 'magnetizing current' which is I = (integral of volts x time) / Inductance.

On the next half cycle an exactly opposite magnetizing current is created,
so over many cycles the magnetizing current, although changing polarity each half-cycle, is constant.

If the time OR voltage applied to one half of the primary is a little different than the other
then cycle-by-cycle the magnetizing current will build up, one little step per cycle (hence 'walking')
eventually this current will be so large that the core cannot accomodate the magnetic force an it saturates.

In practice various circuit resistances and slight saturation prevent disaster, but maximum output will be reduced.

To eliminate this effect (volts x time) should be the same for both halves/directions of the primary.

When the inverting input of an op-amp is connected to it's output
the voltage at the output will be (almost) exactly the same as the input voltage at the non-inverting input.
So the voltage gain is 1, and it's called a unity gain buffer.
This configuration is usualy used to monitor the voltage across a high-impedance source.
Output voltage = Input voltage,
input current = very small, (op-amp input leakage current)
output current = very large (op-amp output current capability)

An 'emitter-follower' is an ac unity gain buffer also (almost)

P.S. Those waveforms are crappy!
Try a 0.1uF capacitor from each transformer/transistor junction to ground (two total)

Munkey

Mon Aug 13 2007, 11:42AM

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

I'm totally lost

On the positive side I have removed the forever cursed op-amp and it works better, but the waveform for the is a total mess here is it:

And is it sort-of working-ish! giving about 202V lighting the bulb at the bottom of the pic, it is drawing 4A from the battery charger (at the back) Is this acceptable? or still in its primitive stages...

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