Hey, all, I want to build a homemade inverter to convert a DC source into AC. Ultimately I would like to try to build a grid-tie inverter (240 VAC in the US), but in the meantime I hope to get a better understanding of AC and just simply build a small unit (where I won't care what the input voltage is or the output voltage is; probably 6-12 VDC input, and similar output but as AC) with that purpose in mind.
I'm sorry this is so long; it covers an area I do not yet understand well and I want to explain what I don't know (as well as I can) and what I've tried.
I'm frustrated at my lack of understanding of the basics of AC and I am trying to gain that understanding through hands-on work (reading books on electronics (Understanding Basic Electronics is finished and I'm about 1/4 through Basic Electronics Theory) hasn't answered my questions).. I get that AC switches directions, but I built an H- bridge to switch a 6 VDC source and it has no negative signal portion (according to my oscilloscope) but when I hook up the 'scope to a 12 VAC wall-wart it DOES have negative signal portions; both have two wires, both are supposed to be alternating but one is zero-line and above, one has negative and positive signal portions. There's gotta be something basic I'm not grasping.
So, in an attempt to learn this and to build the inverter I ultimately want I have built this (Thanks to Tony van Roon for his site):
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(without the transformer; I just wanted to see the signal output) and it gives me AC with negative signal portions. Since it runs the output through a capacitor, would it be correct to think that ALL the current that flows out of this device will have to pass "through" the capacitor (I mean, it wouldn't really go through it but it would run into it then out of it cyclically, and the current output would seem to be limited by it)? If that is correct, would that mean that the maximum current output of this type of inverter would be limited by three items: VDC supply, TIP41 (or whichever power transistor is used) AND the 2700 uF capacitor? The part there that is a big question for me is the possible current limiting of the capacitor (I can figure out what the current capacity of my supply and the transistor are, that's easy; I've never seen a capacitor marked for it's current carrying capability). I mean, ultimately, I will need to know how much current this setup can produce.
I tried the H-bridge, but while I get a pulsed output, it's all zero- line and above, never a negative output. I thought about using an H- bridge with 4 power transistors and feeding one side a sine-wave maybe from a function generator, and the other side a similar signal but 180 degrees out of phase. I would think that would swing the voltage across the bridge in a wave similar to the wave of the function generator. But, since a square-wave H-bridge didn't yield a negative portion of the signal I'm pretty sure that doing something like making it swing slowly from one polarity to another won't do it, either.
I found a schematic for a dual-voltage supply on that site:
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which might be useful for my purposes; to maybe use the zero volt output as the reference, and then try to swing an output from +12VDC to -12VDC. I could feed the input transformer the square-wave "pulsed" DC I can generate with an H-bridge. I would still need a way to gently switch the output from + to - but maybe I could put a totem pole of complementary power transistors on it and feed it a sine wave (the "unified" base). That would give me the gentle swing from from + to - (maybe), and the 0 volt line would be the "neutral". Ultimately I could feed that output into a transformer to step it up to the line voltage I'll finally go for. In theory, I could parallel several voltage regulators to increase current capacity, and several power transistors to get the power output I want (5-10 amps). I don't have a center-tapped transformer (yet) to play with so I can't try this (yet).
Voltage is relative. Voltage is the electric field between two points.
An H bridge has two output connections. Whent he H bridge switch the voltage between the two output connections changes polarity. If you consider one direction to be positive then the other direction is negative.
If you measure the voltage of one of the H-bridge output relative to the zero volts connection of the h-brige then the result is zero volts or six volts. If you measure the output relative to the h-bridge positive supply then the h-bridge outputs are either zero volts or minus six volts.
You may be getting confused because the zero volts connection of your scope is connected to mains earth and the zero volt output of your power supply is also connect to earth. If this is the case when you connect the ground of your scope lead to an h-bridge output you are shorting it out through the earth connection. Measure with a multimeter across your h-bridge output. You will see the voltage reverse when the h-bridge switch s(assuming you havnt blown the transistors by shorting the output with your scope ground).
AC is simply a current that occasionally changes directions. So, imagine your "H-Bridge":
V1 ---- RRRRR ------ V2
When V1 is 0, V2 is 6V. So, the current flows right to left. However, when V1 is 6V, and V2 is 0V, the current flows left to right.
As a consequence, the load RRRRR is seeing AC.
Your oscilloscope is measuring voltage, relative to some ground. However, since I = V/R, then the current is proportional to the V across the resistor.
So, if your oscilloscope has a 'subtract channel' feature, put one probe at V1, and one at V2, then subtract the channels. You'll then the voltage that the resistor sees across it. That voltage will first be positive, then negative, then positive, etc...
Hey, Bob, thank you for your reply. What I had was a control circuit (555 timer and 4013) driving the gates on the MOSFET's, running them on 5 VDC. The "load" across the H-bridge was driven by a wall-wart that put out 6 VDC. I interconnected the ground from the proto-board and the wall-wart, so there was a common ground. I connected the probe of my oscilloscope to one totem-pole pair output and the ground lead to the other totem-pole pair output. What I got was a square- wave signal that was always zero or above and flickered back and forth (time, left to right) on the display. If I'm understanding what you're saying about the voltage being relative, what I needed was to have a voltage point that was halfway between the +6 and ground to attach the oscilloscope ground lead to. That would be a reference point that was 3 volts less than the +6 and 3 volts above the ground. Further, if I'm getting any of this right, then it's always only a matter of reference whether a signal has a positive and negative component or is always in the 0 to positive or 0 to negative range.
If that's right, how could the oscilloscope show me a sine wave with both positive and negative components off the 12 VAC wall-wart when there are only two wires, which, if I understand AC transformers, should only be one piece of wire wrapped many times around a common iron-core (common, in this case, to the winding from the supply voltage of 120 VAC)? There would not seem to be a "reference" line, yet the oscilloscope did show me positive and negative portions of the waveform.
I feel like I have a brick for a brain. Thanks for your help.
Thank you for the reply, Bob. I will check it to see if it does (I've only been using the most basic of its features, so I'm not sure; I'll check the docs). For clarity, and maybe this is a problem, I have been checking the voltage without a load in place, just wanting to the wave form.
First, we're in a rural area and it's a community of "good ol' boys"; I graduated our local HS with some of the people who now work for my local utility; rules get very gray when you know, and have history with, the people who enforce them. Second, I care to try to do things right and if I ever did actually hook something up that I built I would take great pains to ensure it was right first and had a boat- load of safety features and auto-disconnects to prevent islanding (big worry) or noise and interference. And finally, I'm going to do it regardless of what they allow, if I think I've got a functioning device that I believe is safe, just say I did it, just to learn how to do it, and then maybe I'll go from there to see about getting the thing (or things) certified.
I have to start some place; today I have an idea and I'm exploring it. I'll worry about which GPS nav system I'll need to sail the Titanic AFTER I raise it, ya'mean?
The biggest safety issue for the utility company is that your device will be a dynamic load that can and will generate voltage and reverse current flow, so they need to be aware of that if they need to service the lines. Also, if there is a fault in the distribution system, and a protective device opens, your system will try to power the entire load, so you will need a way to sense overcurrents and disconnect quickly. There are also lots of transients and momentary brownouts that could wreak havoc on a solid state inverter.
A safer way to do this might be to connect a suitably sized motor to the incoming service, and connect its shaft to another motor which you can control with your system. If you advance the speed of your motor just over its normal speed, it will make the motor on the utility side into a generator. This gives you safe isolation from the utility power, and if their power is disconnected, your system will no longer generate power into the grid (if you are using an ordinary induction motor).
This should work fine if you have 3 phase service. For single phase, a PSC motor might be OK, but you might need to remove the capacitor, at least when you start generating. But maybe not.
Hey, Paul. I really do care to get it right and to not hurt anybody. I will better understand the precautions that will be needed when I understand better what the device does and how it does it (i.e., when I've built it). Even then I would only hook the thing up for a few minutes to test it and then I'll work it through the right channels to do it by the book. And before I'd test it for "a few minutes" I would compare its output to the line voltage to verify phase and voltage and waveform and so forth (running it connected to my power supply, but not to the utility).
But before I can do any of that, I need to understand AC a *%#$# lot better than I do now. :-( My lack of understanding is quite irritating. And I still come back to my posted example of the H- bridge, the AC wall-wart, and the readings I took from them both. I get AC with positive and negative swings from the AC wall-wart; there is no "reference" line, just two lines from the wall-wart. However, when I place the oscilloscope across the leads from the H-bridge IN PLACE OF the load (there is no load in place) I get a waveform with NO negative swing, just a left-right (time) jitter squarewave.
If, instead of measuring the wall-wart, I had been measuring the line voltage I could maybe understand those results; one line would be neutral, the other would be AC (one line would be connected to the center tap on my transformer from the utility, fed by a single phase of power from them (I have verified by talking to the utility company, and posts to the great folks in sci.physics, how this works in the US), the other to the "phase" which would alternate from 120 V to 120 V below (+120, -120) from the center tap. But this is 120 VAC that has been run through a transformer so I would think of it as a big U for a visual; as the line current moves one direction the transformed side moves one direction, and then they both switch. But since the transformed side has no center tap, how can it have one line be "reference" and one be phase?
Years ago, when I was a kid and absorbing whatever I could get my hands on, I learned a little about electricity. Someone told me that current flows from positive to negative. Okay, gotcha. They told me that electrons move. Okay, gotcha. And I knew electrons were negative. So, I always wondered, since current flows from positive to negative, and since electrons move, not protons; did that mean that the positive terminal of the battery is actually where the electrons come from? Much frustration later, and too many animated discussions with people to mention, I finally found something in a book somewhere that made it all come into focus. A ridiculously simple distinction that nobody had thought significant enough to mention to me; there is conventional current flow and there is electron flow and they are opposite. Pow. Yes, the electrons move, but before anybody knew what an electron was they thought current flowed from the positively charged areas to the negatively charged areas, so they considered current to come from the positive. Only later, when science advanced, did we learn that, in fact, the electrons move, so the real current comes from the negatively charged area. So, we have two ways of viewing what happens; conventional current flow and electron flow. I think we all take that as self-evident now, but when I started to learn this stuff that jammed me up; nobody told me that so I was left postulating about silly stuff like maybe batteries were labeled + when that terminal really gave off the - electrons.
My point is that I think with the AC stuff there is something really basic that I do not know yet. I expect I'll "get" this sometime and look back on this and think, "boy, what a doofus I was". But right now I don't know what the "Pow" is. I take a reading from a device that is supposed to reverse the polarity of two wires, from positive to ground, and get a 0-6V squarewave. I take a reading from a wall- wart that puts AC out on only two wires (so I would think it just reverses the polarity there, too, since there is really only one wire that runs to the transformer, around the core, then back) and I get a nice sinewave.
Tomorrow I will set my H-bridge back up and do some more testing. Tonight I'm going to keep reading online to see what I can find.
Hello there . Last year i been building an inverter 50 Hz 230 volt . Although i didn't use the 555 it was a so called modified sinus inverter . As squarewave generator i used the sg3525 ,cheap 50n06 mosfets & normal ringcore 11 to 230 volt transformer (center tapped) The schematic looks like this . (its not my schema its something i quickly looked up on the web)
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This is the standard push pull stage . By switching one mosfet you make the current flow up after a short break the next mosfet makes it flow down . The end result is that you have an ac squarewave come out of the secondairy side . Now about your schema . Its not a halfbridge ,Halfbridge configuaration has capacitors to devide the powersupply in half . (what it is i cant tell you exactly) the totempole output they made on the 555 connects it to the plus on high output to minus with low (minus Vce ofcourse). About the capacitor ,yes it does limit your maximum power. In halfbridges you use it to prevent staircase saturation by a dc offset ,in that case a small capacitor is there to block the dc . In this case the capacitor is way to big for that purpose but the way its connected makes me think of the quasi resonant powersupply's . Sorry that i can't shed more light on the schema with the 555 . If you want a homebrew inverter i would like to advise the sg3525 . Its cheap easy and easy adjustable to high powers . Last year i builded it for 500 watt's but i know its build up to several KW's .
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