Electronics Forums > Homemade DC to AC inverter questions

# Homemade DC to AC inverter questions

HC
Guest
Posts: n/a

 02-29-2008, 10:57 PM
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
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):
http://www.uoguelph.ca/~antoon/circ/555dcac.html (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:
http://www.uoguelph.ca/~antoon/circ/741-ps.html 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).

--HC

Bob
Guest
Posts: n/a

 03-01-2008, 12:06 AM

> 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
> 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.

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).

Bob

Bob Monsen
Guest
Posts: n/a

 03-01-2008, 01:06 AM
"HC" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> 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
> 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.
>

AC is simply a current that occasionally changes directions. So, imagine

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...

> 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):
> http://www.uoguelph.ca/~antoon/circ/555dcac.html (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:
> http://www.uoguelph.ca/~antoon/circ/741-ps.html 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).
>
>
> --HC

JeffM
Guest
Posts: n/a

 03-01-2008, 02:21 AM
HC wrote:
>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
>

If you think the utility is going to allow you to connect to their
grid
using uncertified gear, you are deluded.

HC
Guest
Posts: n/a

 03-01-2008, 02:30 AM
On Feb 29, 6:06 pm, Bob <(E-Mail Removed)> wrote:
> > 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.

>
> 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).
>
> Bob

(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.

--HC

HC
Guest
Posts: n/a

 03-01-2008, 02:33 AM
On Feb 29, 7:06 pm, "Bob Monsen" <(E-Mail Removed)> wrote:
> "HC" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed)...
>
>
>
> > 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.

>
> AC is simply a current that occasionally changes directions. So, imagine
>
> 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...
>
> > 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):
> >http://www.uoguelph.ca/~antoon/circ/...c.html(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:
> >http://www.uoguelph.ca/~antoon/circ/741-ps.htmlwhich 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).

>
> > Thanks for your help.

>
> > --HC

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.

What you've said helps, thanks again.

--HC

HC
Guest
Posts: n/a

 03-01-2008, 02:41 AM
On Feb 29, 8:21 pm, JeffM <(E-Mail Removed)> wrote:
> HC wrote:
> >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

>
> If you think the utility is going to allow you to connect to their
> grid
> using uncertified gear, you are deluded.

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?

:-)

--HC

Paul E. Schoen
Guest
Posts: n/a

 03-01-2008, 03:31 AM

"HC" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Feb 29, 8:21 pm, JeffM <(E-Mail Removed)> wrote:
>> HC wrote:
>> >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

>>
>> If you think the utility is going to allow you to connect to their
>> grid
>> using uncertified gear, you are deluded.

>
> 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
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.

Paul

HC
Guest
Posts: n/a

 03-01-2008, 07:09 AM
On Feb 29, 9:31 pm, "Paul E. Schoen" <(E-Mail Removed)> wrote:
> "HC" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed)...
>
>
>
> > On Feb 29, 8:21 pm, JeffM <(E-Mail Removed)> wrote:
> >> HC wrote:
> >> >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

>
> >> If you think the utility is going to allow you to connect to their
> >> grid
> >> using uncertified gear, you are deluded.

>
> > 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
> 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.
>
> Paul

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.

Thanks for everyone's time.

--HC

none_of_the_above
Guest
Posts: n/a

 03-13-2008, 07:46 PM
On Feb 29, 11:57 pm, HC <(E-Mail Removed)> wrote:
> 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
> 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):http://www.uoguelph.ca/~antoon/circ/...c.html(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:http://www.uoguelph.ca/~antoon/circ/741-ps.htmlwhich 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).
>
>
> --HC

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)
http://www.diyaudio.com/forums/attac...amp=1174836346
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 .
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).
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 .

Be carefull and good luck

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