Component Selection. Input Side.
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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I will start this build out with the basic two transformer design and we will build on it from there.
THE MAIN POWER TRANSFORMER.
I have a 20 amp battery charger transformer too so I am going to build this one right along with you. The one I have is out of a generic 20 amp charge 75 amp boost unit with 6 and 12 volt capabilities. Its physical size puts it in the 250- 300 VA range so it should be able to run at 250 watts all day with out being over worked. I intend to make this one into a 3 stage system also since the transformer is capable of it. You will see how later.
However I do expect there will be some slight variations in the actual numbers in everyone’s design due to the vast differences of transformers and components out there.
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.)
THE CAPACITOR.
The capacitor is a standard electrolytic 250 volt 100 uf. The reason for it is to absorb the small switching spikes that occur every time the H-bridge devices reverse the cycle. I have found that around 5 uf per running amp works well. To big and they are wasting energy. To small and the switching spikes can get high enough to blow through the switching devices.
The high voltage rating is just because that part of the circuit goes from 0 to maximum volts every half cycle when there is no load. That tends to heat up lower voltage capacitors. swinging from 0 to 40 volts every half cycle is too much ripple for a 50 volt capacitor. But its only a fraction of a 250 volts units working range.
THE CONTROL TRANSFORMERS.
To control this GTI I am going with the basic transformer drive system. What you will need a total of 4 isolated 12 volt AC sources to run the H-bridge. If you can find one transformer with that many independent outputs, Great! If not you may have to run 4 individual ones. Or two 12 volt single outputs and one 24 volt center tapped output.
(What I have are two matching 7 VA units with 120:240 primaries and two isolated 12 volt AC secondary windings on each.)
THE STAGE CONTROL DEVICES.
Being my Power transformer has a center tap I am going to use it for one of the three stages. To switch it from one stage to the other I am going with a pair of SCR's that I have. I would recommend going with the same minimum ratings as the H-bridge devices. No less than 80 amps and 80 volts.
(What I have are take offs that are rated at 150 amps and 800 volts. Overkill again but the price was right!)
The basic input and H-bridge layout is below.