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Pure Sine Wave

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lukeshelton

New Member
I am trying to avoid purchasing a pure sine wave inverter, i cant really afford one, so im wondering what i can do to a modified sine wave inverter to achieve a pure sine wave? I understand that a modified wave output has a square waveform, and generally some harmonic distortion. Is there anything i can do to one of these cheap inverters to round out that wave and quiet the harmonic distortion?
 

Sceadwian

Banned
Why do you need the sine wave in the first place? Most loads don't really care that much. Aside from absolutely massive filter components I'm not sure what else you can do.
 

lukeshelton

New Member
i need a sine wave because of what i am powering. im a photographer, and im trying to power my high power flashes with a car battery. a modified sine wave will burn the circuit board of the lights and ruin them.
 

Mr RB

Well-Known Member
Check the circuit inside your high power flash units... It is most likely a switchmode driver that converts the incoming AC voltage to DC anyway. They almost always have a primary oscillator that runs at a high freq (few kHz) to charge a cap to a few hundred volts, then dump into a flash transformer to power the xenon flash tube.

I would be very surprised if they require a good sinewave input!
 

Diver300

Well-Known Member
Most Helpful Member
If it is a switchmode converter in your flashguns, you can tell because the input voltage range will be wide, such as 100 - 240 V AC. If so, you can run it on DC. Find a cheap inverter, and bypass the chopper part at the end.

To explain, most inverters take the 12V input and run a flyback converter from it. That creates pulses at that charge up a big capacitor to 350 V or so.

To get a modified sinewave, the 350 V DC from the capacitor is chopped by switching between forward, backwards and off to give a modified sinewave.

If you have a switchmode converter in your flashguns, they will probably be perfectly happy with the modified sinewave, or just use the 350 V DC from the capacitor.

(Be careful with the big voltages. It is just as dangerous as mains and the capacitor takes a long time to die when you remove the battery)
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

A power transformer wont clean up the wave, sorry.

You can redesign the thing using a microcontroller so you get better
control of the harmonics, but that would be a bit of a task as you would
have to study the inverter circuit and design something to match.

The way to get the pulsed wave to look like a sine is to add filter components
to the output, like an inductor and a capacitor. The inductor has to
be able to handle the full peak current however, and the capacitor the
full peak voltage. Way back when i designed ac synthesized sine converters
(100 watts to 10,000 watts) we would use relatively large inductors on the
output and ac rated capacitors. Some of the inductors easily weighed in at
over 50 pounds each, and for three phase that meant three of them.

For your app, i dont know what power level you need so it's hard to say
how big the inductor would have to be, but i think you get the idea.
If you only have to work with 200 watts that's not going to be too big,
but at 1000 watts it's going to be bigger. You might tell us how much
wattage you intend to use.
 

Space Varmint

New Member
You wouldn't be talking about high power levels for a camera flash bulb. Get a hold of some ferrite. Maybe a half inch to a one inch diameter toroid. Build you say a pi filter. You would have to play with the values but start with maybe 470uF for the two caps. Put about 30 turns or more of enameled wire on the toroid used for your inductor.

Assuming you have a scope, just play with it until you get something close. Like the other guy said. It doesn't have to be totally pure, harmonic free, but a good pi network will greatly reduce the harmonic energy levels.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
You wouldn't be talking about high power levels for a camera flash bulb. Get a hold of some ferrite. Maybe a half inch to a one inch diameter toroid. Build you say a pi filter. You would have to play with the values but start with maybe 470uF for the two caps. Put about 30 turns or more of enameled wire on the toroid used for your inductor.
Where do you suggest he finds non-electrolytic 470uF capacitors?, and of what use is 30 turns of wire on a toroid? - pretty much a complete nonsense reply!.
 

Sceadwian

Banned
Digikey sells Metalized pollypro caps that high... They're only 250 dollars :eek:
 

Hero999

Banned
Two 1000:mu:F capacitors wired back-to-back will form a 500:mu:F non-polarised capacitor.
 

Sceadwian

Banned
Most caps don't like 100% ripple. Without specialty caps anything you use is going to fail very shortly, and likley catastrophically.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

You know what else *might* work, is to put a resistor in series with the
non sine wave inverter. This might reduce harmonics enough to keep the
flash power supply input safe.
Notice i used the word "might" though...because to build anything for this
unit it would be impossible to predict the final behavior without seeing the
flash circuit first and determining what the input could take.

I've seen 100,000 dollar computer inputs blow out because of inverters
that could not handle the load properly. Caution is definitely the word
and good planning.
 

Sceadwian

Banned
Not too bad an idea, will prevent the massive inrush currents from the sudden on, which I'm asuming is what kills the flash circuit, true AC even with inrush current is less than a square wave by multiple orders of magnitude. Figuring out the resistor to use might be a bit of pain though, get it wrong and they still burn out.
 
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tcmtech

Banned
Most Helpful Member
I've done the inductor and capacitor wave form shaping before on cheap inverters. The actual capacitance I found worked satisfactory is around one or two uf per 100 watts(standard motor run caps). With a inductor sized for the right current capacity. A fair sized toroid (about 1.5 - 2 inch dia) with around 20 -30 turns of wire just heavy enough to carry the inverters maximum output capacity seems to work with the least amount of trouble.
Its not a great sine wave but its closer than that brick it tosses out!
It cleans up that inverter hash you get on TV's and monitors and helps variable speed power tools work more efficiently.
However if the inductance or capacitance gets to high some cheapo inverters get glitchy and wont work right or at all! ;)

If your running sensitive and high dollar electrical gear, why are you trying to get by with a crappy cheap ass inverter? Its just asking for trouble!:D
 

Space Varmint

New Member
Where do you suggest he finds non-electrolytic 470uF capacitors?, and of what use is 30 turns of wire on a toroid? - pretty much a complete nonsense reply!.
Man, I must be totally lost. I thought this was a battery source. I was talking electrolytics. I assumed he was using the output of a DC2DC converter with some sort of square wave output he wanted smoothed to more of a sine wave.
 

Space Varmint

New Member
I've done the inductor and capacitor wave form shaping before on cheap inverters. The actual capacitance I found worked satisfactory is around one or two uf per 100 watts(standard motor run caps). With a inductor sized for the right current capacity. A fair sized toroid (about 1.5 - 2 inch dia) with around 20 -30 turns of wire just heavy enough to carry the inverters maximum output capacity seems to work with the least amount of trouble.
Its not a great sine wave but its closer than that brick it tosses out!
It cleans up that inverter hash you get on TV's and monitors and helps variable speed power tools work more efficiently.
However if the inductance or capacitance gets to high some cheapo inverters get glitchy and wont work right or at all! ;)

If your running sensitive and high dollar electrical gear, why are you trying to get by with a crappy cheap ass inverter? Its just asking for trouble!:D
Right here ^^^^ this is what I was talking about. You can use electrolytic.
 
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