# 250 watt grid tie inverter build

#### Nigel Goodwin

##### Super Moderator
well unless for large setups that practically invalidates this discussion, it will probalby be more expensive to get a 250 W GTI validated than buying one.
I would imagine it would be very many times the cost, UK approval for anything is an EXTREMELY expensive process.

#### tcmtech

##### Banned
All valid points, but again how many things can you find on the internet that you are not suposed to have yet can build yourself? How many things are on THIS WEBSITE that could kill the user or someone else if not properly built or used?

ALSO there are many companies building and selling plug and play units already. YOU do not need a permit from the power company to use one of them!

If your worried about saftey DONT BUILD ONE! If you do build one or buy one the power company will not know you have one unless you tell them! If you start producing enough power to make your monthly electric bill go negative then yes they will probibly want to know why.

Otherwise how do they know what you are doing?
If you go on an extended vaction and shut the power off to your place they dont come an investigate you because your electric bill is zero KWH that month.
If your electric bill drops a fare amount, as far as they know you just got more efficient in how you use electricity!

I know a few people that did a very critical look at their power usage and were able to cut their electric bill by almost half in one months time just by being more diligent and efficient in what they were using. The power company did not send engineers out to see why.

IN my country Grid tieing is legal. As long as you have the right saftey precautions dont worry so much. If your that worried DONT build one!

#### bbhs

##### New Member
I agree I have no problem in using one to reduce my usage and would only consider the approved unit if I felt that I had alot of spare power to export for credit to the grid.
Depending on the spare capacity and the payback from the supplier would determine if it was worth while investing in a G83 unit.

#### Nigel Goodwin

##### Super Moderator
All valid points, but again how many things can you find on the internet that you are not suposed to have yet can build yourself? How many things are on THIS WEBSITE that could kill the user or someone else if not properly built or used?
Silly point - almost anything can be dangerous if used wrongly - but connecting un-approved items to other peoples equipment is obviously NOT a good idea.

ALSO there are many companies building and selling plug and play units already. YOU do not need a permit from the power company to use one of them!
You certainly need permission, and as already mentioned the unit has to have passed certain standards for you to be allowed to do so - the UK is far more strict about such things than the USA.

#### tcmtech

##### Banned
Control system circuit design and theory.

This GTI build up Thread is intended for residants of the united states only! Should you decide to build and use such a device as this you may be breaking laws and can face possible fines, and or jail and prison time for it. Should you chose to build such a device as this and intend to use it for saving energy in your home or dwelling you are still breaking the law. Beware you are considerd a pirate Grid tie operation, which is considered illegal in many countries!

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THE CONTROL SYSTEM.

How simple or elaborate you get here depends entirely upon you. I do strongly recommend at least a basic control system that has both input and output voltage monitoring.
Ideally you will want the whole GTI to be off until a minimum input voltage level is reached. There is a input voltage point that will be the sweet spot were the GTI goes from power use to a power return. Every unit will have its own break even level due to the variations in components and the overall efficiency of the design.
That’s why the adjustable control setup is recommended.
What I use is a multi level control. As the input voltage rises the control system comes on and starts running the main power transformer. As it gets higher it will reach the break even point and only then will it connect to the grid. The grid voltage and frequency are also monitored. It the correct line voltage is not available or if the line frequency is not right it won’t connect.
This may sound complicated but it really is not. There are only two main types of comparator circuits needed and one frequency to voltage converter which is optional.

SINGLE STAGE VOLTAGE COMPARITOR.

This one is a basic voltage comparator. It just sends an on/off signal based on the voltage set point. If the input voltage is below the set point its output is off. If its input voltage is above the set point the output is on.
This circuit is used for the stage change functions and for the basic line voltage and part of the frequency monitoring circuit.
I have chosen the LM324 quad op amp for its simplicity and easiness to find.
(See Spec sheet below)
(Any other voltage comparator can be substituted if you wish. Just makes sure it has suitable voltage and output current capacity ratings.)

TWO STAGE WINDOW COMPARITOR.

The second comparator is a window or dead band type. It has to go above a high level set point before its output will change from off to on. But it then can drop below that level without turning the output off. It uses a second lower level comparator that controls the second set point to reset the output to off. Once it has turned the output off it can not turn on until it passes the high level set point again.
This dead band allows for monitoring voltages that tend to fluctuate during operation. Being able to set independent high and low points will keep it from oscillating during load changes or power fluctuations.

FREQUENCY TO VOLTAGE CONVERTER.

This circuit is used only on the line side of the GTI. Its only function is to watch the AC line frequency to make sure it’s in the right range. Typically using a plus or minus 10% limit is acceptable.
The circuit is based on an LM2907 frequency to voltage converter IC.
(See Spec sheet below)
(Any other frequency to voltage circuit or IC will also work here: just make sure it has the correct voltage and output characteristics to make it compatible with the rest of the circuit)

OUTPUT STATE CHANGE DELAY TIMER.

These are just for added precaution. They go on the output of each monitoring comparator. They are purely a second back up to further prevent unnecessary on/off cycling do to input power dips or spikes and line side dips and spikes. This is purely and optional function.
This is a simple 555 based timer circuit. It has an adjustable time delay of .5 to 5 seconds.
(See Spec sheet below)
(Any other type of time delay circuit or IC will work here also; just make sure it has the proper voltage and output current capacity)

If its input state changes from off to on, its output state won’t change unless the input has stayed on until the time delay has been reached. Only then will its actual output turn on.
Then it does the reverse of that after the input signal turns off. The input state must stay off for the time it has been set to or it won’t reset the output to off.

This is purely a glitch filter to keep the control system from false triggering and turning the GTI on or off or changing stages when it was not actually necessary.
Again it’s purely optional, but still recommended.

CIRCUIT LAYOUT.

Each of these circuits can be made individually or preferably on a one circuit board.
By combining several comparator circuits on one board you can use single IC’s like the LM324 to do multiple comparator functions. Also if you are going to be using several output glitch filter timers you may want to integrate them into LM556 dual timers or other multi timer ICs as well

Basic circuit diagrams and IC spec sheets are below.

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#### tcmtech

##### Banned
Well when you guys get tired of being kicked around by the corporate utilitys come on over!

We dont have the same rules you do I know that. Over here anything from the customers side of the meter box is ours. And what we do with it or to it is our business.
We do have to use safe practices to ensure what we do does not cause harm to anyone or anything up line from us. But here we are alowed to backfeed up to 50kw an hour legally.

Sorry your utility law system does not work that way.

I guess I will have to add this disclamer:

This GTI build up Thread is intended for residants of the united states only! Should you decide to build and use such a device as this you may be breaking laws and can face possible fines, and or jail and prison time for it. Should you chose to build such a device as this and intend to use it for saving energy in your home or dwelling you are still breaking the law. Beware you are considerd a pirate Grid tie operation, which is considered illegal in many countries!

#### Thunderchild

##### New Member
well its pretty plain on here who is dreaming and who knows the practicalities of things, the situation we are talking about requires much more than theoretical considerations.

this thread reminds me of a joke I was told the other night:

whats the difference between the maths of a physicist, a mathematician and an engineer ?
To a mathematician 1+1 = 2
To a pysicist 1+1 = something between 1.5 ad 2.5
To an engineer 1+1 = erm.... about 9

#### tcmtech

##### Banned
All I can say is check out as many web sites as you can related to companies that sell Grid tie inverter sytems in the United states and read them carefully.

Please find me some honest liturature about hook up rules and post it. Check out eBay and see if you can find one Seller that says you need permission to hook up there product in the United states.

I may very well may be wrong on some things but until it is honestly proven that I am wrong I have no other alternative but to believe what I have read and understand is right.

If you need further information check out the National Electrical Code rules Related to;

Interconnected Electric Power Production sources.
ARTICAL 705

THIS IS WHAT I HAVE TO COMPLY WITH IN THE UNITED STATES!
and I do!

Everywhere else in the world has to follow the code rules and laws that relate to their own country.

#### tcmtech

##### Banned
whats the difference between the maths of a physicist, a mathematician and an engineer ?
To a mathematician 1+1 = 2
To a pysicist 1+1 = something between 1.5 ad 2.5
To an engineer 1+1 = erm.... about 9
Hahaha

My last time around at college for my electrical engineering degree showed me otherwise!

The math classes I had to take tried to tell me 1+1 = 9

Calculus was the dumbest thing I have ever seen that was even remotely related to mathematics.

My Calculus instructor spent a week on how claculus based formulas can change any number to be equal to any other number.

One day in the middle of class I asked him; Can you use this in real life situations?
He said; Definitly!
I then asked; Are you doing anything after class?
He said; No, Why?
I said; Then will you come with me and prove that what your are saying is true and can be used in real life?
He said confidently; Sure! Where would you like me to prove it?
I said with a big grin; Lets go to my bank and you can prove that your equations are real and do work in real life situations by making the $40 in my checking acount equal to$40 million. AND I will give you $38 million of it in cash if you do it! The whole class cracked up! He then said; Thats not how it works. These formulas are only good for when you have mathematical problem and the answer you keep getting does not match what you need to make the equation work. I said; But how I want to live does not add up with what my checking account says. The number I need in my finacial formula does not match what I have in my checking account. The class though my argument was good. It did meet all the requirements of what he was saying, and it did fit in the formula on the board perfectly! But after much debate I still only had$40 in the checking acount the next day.

#### Thunderchild

##### New Member
Everywhere else in the world has to follow the code rules and laws that relate to their own country.

#### Thunderchild

##### New Member
Hahaha

My last time around at college for my electrical engineering degree showed me otherwise!

The math classes I had to take tried to tell me 1+1 = 9

Calculus was the dumbest thing I have ever seen that was even remotely related to mathematics.

My Calculus instructor spent a week on how claculus based formulas can change any number to be equal to any other number.

One day in the middle of class I asked him; Can you use this in real life situations?
He said; Definitly!
I then asked; Are you doing anything after class?
He said; No, Why?
I said; Then will you come with me and prove that what your are saying is true and can be used in real life?
He said confidently; Sure! Where would you like me to prove it?
I said with a big grin; Lets go to my bank and you can prove that your equations are real and do work in real life situations by making the $40 in my checking acount equal to$40 million. AND I will give you $38 million of it in cash if you do it! The whole class cracked up! He then said; Thats not how it works. These formulas are only good for when you have mathematical problem and the answer you keep getting does not match what you need to make the equation work. I said; But how I want to live does not add up with what my checking account says. The number I need in my finacial formula does not match what I have in my checking account. The class though my argument was good. It did meet all the requirements of what he was saying, and it did fit in the formula on the board perfectly! But after much debate I still only had$40 in the checking acount the next day.
funny counter story but you erm... missed the point. I'll make it simpler.

I'm going to control a 2 A current with a mosfet (or other transistor) now i don't go and actually use a 2 A transistor do I ? same rules apply in any design and situation evaluation, perticularly where human nature is even a minor factor !

#### tcmtech

##### Banned
Still not following. I am seeing an apples and oranges comparison. I need a better pont of reference. Did I completely miss something?

#### tcmtech

##### Banned
Hi Tcmtech, I would like to know if it is necessary to solder the solar cell tab wire the full width of the cell, or just solder it on the top edge ?
Sorry I am not much of a solar panel guy. Someone else will have to answer that question. The few panels I have just use a spring loaded connector to grab onto them. (ancient and cheap)

I am more of a wind generated power person.
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hate to spoil the DIY spirit but where can one buy a reasonablt priced GTI ?
Ebay sells some in that 250 watt range for around $300 or so. But I think they are designed more for the American and Canadian power systems and run at 120 volts AC 60 cycles. At least for the smaller units anyway. That way they are just plug in and go. I am not sure what they have relating to the other parts of the worlds electrical systems. There has to be stuff outhtere some place but I am not aware of what they have available or what is leagal for differernt countries. #### Thunderchild ##### New Member Still not following. I am seeing an apples and oranges comparison. I need a better pont of reference. Did I completely miss something? when designing anything or planning a specification (in this case that which determines the application of GTI's) you always leave room for allowances because the unforseen will always happen especailly when its not a one off instalation. Last edited: #### tcmtech ##### Banned Most Helpful Member Ah... Much better! I understand completely! Margen of error. I did cover that in the first post here relating to and recomending the minimum 4:1 switch device current rating Vs the average sytem running Current, and the minimum 2:1 switch device voltage rating Vs the peak system voltage. THE MAIN H-BRIDGE DEVICES. You will need to find some Mosfets or IGBT's capable of at least four times your amperage that you want to run at. SO if we are working with a 20 amp design I would recommend something with at least an 80 amp capacity. Also they should have a voltage rating of at least two times the maximum voltage peak voltage your main power transformer puts out. If it’s a 12 volt battery charger most of them use a center tapped transformer and can actually put out 24 volts with a full wave bridge rectifier set up. (The one I have will put out 28 AC volts open circuit. That gives me a peak of nearly 40 volts. 28 * 1.414 = 39.6 volts) Knowing that, I would recommend a minimum voltage rating of 80 volts on your Mosfets. Still higher is better. If you’re using IGBT's most of them have a 200 - 1200 volt rating. So there are no real voltage concerns with them on this low of voltage circuit. You can go with multiple units or one big unit for each leg. It’s entirely up to you how you want to approach this part. I would recommend going on EBay and getting some large IGBT or Mosfet devices if you don’t have any yet. The devices you use for the H-bridge do all of the work so over building never hurts here! (I have some commercial take outs that are 600 volt 100 amp half bridges so I will use them.) You get a B+ for participation but a D- for not reading and or understanding the text book! But thanks for giving me a good reason to look closely at what I wrote. I do most of these longer posts as a cut and paiste from Office Word and once in while I do miss a paragraph when I am moving the text over. When I get to the part about the actual circuit layout I will also cover some thermal protection tips. But ultimately no matter what I print up, if the person building it cuts corners, uses undersized or inadiquate parts, or runs it way past what the design is built for there is nothing I can do about it! #### blueroomelectronics ##### Well-Known Member I wonder how much you actually get per kWh when you sell power back to the power company. After all when you buy it you also have to pay taxes & delivery charges. #### killivolt ##### Well-Known Member I wonder how much you actually get per kWh when you sell power back to the power company. After all when you buy it you also have to pay taxes & delivery charges. Ha, I'll bet if they had it there way we would have to pay taxes and delivery fee's just to sell it back to them. Utility Company's are like big tobacco or Big Oil. They charge we pay. Even if someone were to stand out a little and they were to take notice they won't lobby against it. Over all it reduces wear and tear on their equipment. Resulting in a Maintenance Reduction. If everyone were to attempt to do it. Suddenly there would be so much regulation ( After the Lobbyist got through with it.) No one would be able to afford it. kv #### tcmtech ##### Banned Most Helpful Member What I have been told as of lately is that my local power comany pays equal cost per KWh returned. BUT I have also been told that may not be true all over the united states. Right now I pay around 9 cents per KWh. Plus a fixed$29 month conection service fee. IF I were to sell back to them I would get 9 cents per KWh or I could just bank my retuned KWh's and reuse them later at no aditional cost. My local utility is pushing very hard to encourage its customers to invest in alternative enegry and energy saving devices.

But not all power utilities are like that as I undrstand it.
I have heard that now that its legal nationwide to sell back they do have to buy it from you. BUT they only have to pay you the going rate of what they pay for it at super bulk rate. Which as I have understood can be anywhere from 1.5 cents per KWh to around 5 cents per KWh depending on the location and demand in the country.

Plus If they want to they can require you to have a second meter on your service connection to read what is going back and then charge you for that second meter.

SO if your getting 1.5 cents and spending $30 amonth for a second meter on top of the$30 a month you pay for the first meter you would have to make around 4000 KWh returned just to break even.
And you still have to make what you normaly use too!

But if you are lucky and have ulility company like what I have then you only need to make enough at 9 cents per KWh to pay for the one meter that runs both ways. Or about 334 KWh returned to break even.

In the first example I would have to make about 5500 KWh a month to break even including my own monthy use.
However in the second example I only need to make 1834 KWh a month to break even.

And my numbers may be off a bit on actual meter costs and utility bulk rates. I just used the numbers I have been seeing in articals I have read.
And the rules are changing all the time so this may not be exactly acurate in every detail.

I dont know what Canada is like for any of this though.
But its worth checking out!

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#### Val Gretchev

##### Member
Forum Supporter
Ontario Prices and Rules for Renewable Energy

Here are a couple of documents released recently in Ontario:

Price Schedule.

Fit Rules.

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