Hey TCM,

It looks freakin sweet! Question for you: On those transformer windings used to drive the gates, what's the real delay you get between the AC zero-crossing and the time that the mosfet turns on?

On some units, I saw a big pedestal here where the grid current went to 0. Nothing dangerous, but it affects the THD score.

 

Your Right about the center tap. It is purely optional. If you are going to run as a two stage turning off the triggering circuit to the SCR (X1) and turning on the SCR (X2) that goes to the CT will give you a half voltage input in reference to the H-bridge.

This works well for getting a scavenging mode with a widely varying power source.

For instance if your GTI was set up to run on a 12 volt system it would use the X1 input. That would give you the full running voltage going to the h-bridge.
But if your power system is capable of still putting out power but below the bottom of the 12 volt range input, (5 - 8 volts) thats where the CT comes in. It allows the GTI to run at half voltage and still feed back some power.

With a Permanent magnet type generator on a wind power system there are many times when the wind has enough power to run the generator at a speed just below the high range inputs lowest voltage limit. Being able to drop the GTI's input voltage requirements will allow it to pickup that lower voltage and still usable but limited power available.
Being the capacitor C1 is connected across the full H-bridge it will still clip the switching spikes created during the half bridge (low range)mode and return that spike power to the system.

With solar panels I am not sure if that is really a necessary function or not. If your transformer does not have a CT just skip that part of the circuit. It is purely optional!

If you remember your SCR design theory they are only able to conduct in one direction just like a diode. But that conduction ability can be turned on or off.
In off mode an SCR does not conduct in either direction. In on mode they can conduct in only the normal diode direction.
They block the higher voltage present in the H-bridge part of the circuit from getting fed back to the DC power source. Thats why it does not actually cause a short circuit.
As far as the actual SCR turn off part, the H-bridge has a zero crossing point that is deliberately where all switching devices are off. Without that current draw the C1 value is the only thing that allows the SCR to have any current being drawn through it. But the inductive kick back from the power transformer that occurs at every turn off point for that zero cross dead band creates a big enough voltage spike to effectively over come the incoming EMF and try to reverse the SCR's conduction state thusly turning it off every half cycle. Thats why the SCR triggering circuits have to be continually on in order to keep the SCR's actually conducting.

I hope that clears it up some. But that is a great question!

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

Another good question!
The simplest way I can explain it is that due to the ratios of where your switching devices are turned on VS the actual sine wave instantaneous voltage you will get a sort of crude PWM effect. That will give you a fairly smooth and reasonably linear current draw increase as the input voltage climbs.
BUT when your input voltage exceeds the peak voltage of the transformer plus the inductance and IR losses associated with it it goes more exponential in its voltage input VS current draw ratio.

But ultimately due to the very simple design there is no real current or power limiting built in with this design.

For example, if you were using a 12 AC transformer your actual startup point that its input power is greater than its own running power would probably be around 6-8 volts. But this number will vary greatly depending upon the actual circuit design and where the switching devices are turning on in reference to the sine wave.

At say the 6 volt point your system uses 12 watts idle. That is, it will draw 2 amps just to break even on the conversion process. Zero actual returned wattage. When it goes above that point is when you actually will want the grid connection to happen.
As the input voltage climbs your amp draw will follow proportionally. however when you reach the peak voltage of your transformer (about 17 volts) your now only limited by the transformers internal losses. At that point is where your current will go up exponentially fast! If you were drawing say 15 amps at the 17 volt point, by 18 volts you may see 20+. By 19 volts it could be 30+.
That is were this circuit does have weak point if the transformer and switching devices are not sized to match the peak available power of the source.

A good transformer will take a 2X power surges for a short time with no damage, but ultimately you would want to have a temperature sensing system on the transformer and the switching device heat sinks.
Should it get over heated you would want to have it shut down the GTI and save itself.
However you would at that point be wise to have a secondary load dump system in place that can take that power and do something with it.
At least in the case of a wind generator based power source.
I dont think solar panels have a problem with going open circuit.

But if you sized your GTI's power capacity right it should under all reasonable circumstances be able to handle the power source even with some good over powering surges!

Its when you build for your power sources average power output and not maximum power output, or if you add on additional power generation without resizing the GTI to keep up with it you will start having the potential for problems.

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

Where did my response to electronrancher go?

I am not making this up! I know I have had this happen before too!
I post something and its accepted. I go out of that thread and come back and its there. But I come back later and its gone!

Is there a hosting glitch that erases last posts some times?

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

Well here is the rewrite. I see I am going to have to start copying all of my longer posts onto word again just so when this happens I have a cut and paste back ups.

The best way I can explain it is that the phase control effect of the control transformer and the actual minimum turn on voltage of the switching devices are what actually give you your delay.

Here is how it works.
Assume you are using a 12 volt AC transformer for your power transformer, a 10 volt AC transformer for your control transformer, and the switching devices have a minimum turn on voltage of 3 volts.

As the sine wave rises from the zero crossing point the switching devices are off. Once the control transformer reaches 3 volts at the gate of the switching devices they turn on. That gives you a turn on point of 3 / 14.4 = 20.8% of the peak sine wave. Or about 3.5 volts on the power transformer.
If you raise the control transformer AC voltage to say 15 volts you would drop the switching points even further. 3 / 21.2 = 14.1% of peak voltage.
Or about 2.4 volts on the power transformer.
If you look at it from the phase angle reference having a +- 2.4 volt window on each side of the zero cross point is fairly small.
That window is what allows the slight phase lag between the actual line voltage reference and the actual switching device state changes to be covered.

If you start getting to low on the sine wave the actual switching current the switching devices will get very large. Plus the switching into and out of a near dead short creates line noise and lots of heat in the switching devices while very little usable power is being returned back to the AC lines.

Given this type of circuits simplicity the most effective way to keep it working smoothly and efficiently is to try and design the switching points to occur at around or above 15 - 20% of the peak AC voltage of the power transformer.

Being the switching devices are voltage controlled a simple voltage divider on each gate circuit would allow for some adjustably to fine tune the switching points for that particular circuits design if it was felt that it was needed.
By raising the switching points higher or lower in reference to the AC line voltage it can produce a good PWM effect for actual load current control too.

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

And yea sorry about the small and some what blurry print. It was the best I could come up with for converting the schematic from SIMetrix to a format that this website will take.
Anyway we could get electro-tech to set up the download function to take EMF files directly?
I dont care for the blurring either.

Or is ther a better way of converting the EMF format to a clearer style?

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

bbhs I can read it now (nothing changed on my end) I'm guessing TCM uploaded a new pic.

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I tried a different approach to the format conversion on my end. And reposted the schematic. But I didn't see any change on the posting though.
I guess it must have worked after all!

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

The thumbnail didn't change, when I click the image, I can read it.

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I just reposted the schematic notes in the actual post text.
Just in case the blurry text Nazi's come around!

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

It wasn't blurry tcm =) I don't mind blurr, I seriously couldn't read it =)

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Thanks for the explanation, tcm - I get it now.

 

With the text notes gone, the schematic scaled better. The schematic is easy to understand. Thanks!

Earlier schematic wasn't exactly blurry, but symbols and letters were about two pixels high. We could blow it up, but all we saw was enlarged two pixel characters. All fixed now.

 

Just trying to please everyone!
First people want full and complete schematics and then I give them one and after format changes it translates as being too small to read!

It looked fine on my duel 30" HD monitors!

I will try to keep the schematics larger and easier to read even if that means having to break them up at times.

Does the independent schematics followed by block diagrams work Okay for every one?

__________________
"Issue a general safety warning. Then look the other way and allow stupidity the chance to eliminate itself." -- tcmtech
"Those who can, Will. Those who can't, will achieve positions of power over those who can and then promptly stop them." -- tcmtech
"Your impossibility may just be my day to day routine." -- tcmtech

 

Have you actually built it?

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com/