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Would this be possible, I would like to build a low voltage inverter that would go 0 to 24v ac.
I was thinking if I made a variable dc power supply circuit then invert it to AC voltage. Basically My question is if I invert DC to AC would I have to use a step up or step down transformer, and what would my lowest voltage start at and how many watts could I pull, and how pure of a sine wave could I expect.

What and how big is load, THEN we design to needed Voltage and Wattage.

I was thinking about 5 amps at 24 volts tops

Adjustable voltage AC out? Hummm?

You must have particular load in mind. For non adjustable I would have suggested an off-the-shelf 120VAC invertor and a step down transformer.

why not a pure sine wave inverter, and a step down transformer, and then a small Variac on the end ?
bout as simple as i can think of..

24 volts is the max voltage, most likely I would be be using this at 3-16volts. Really don't want to go the Variac rout.

24V and 5A is 150W. Ignoring losses, a 3VDC source would need to supply 150W also and that amounts to 50A peak source current. with losses it may approach 60A. a 12V car battery would fare better with plenty of source current for a while with a modest 13A load.

You basically need a variable DC SMPS design for the front end if you want a range of input voltages then that goes into a chopper and filter.

Maybe you can use a Car stereo amplifier of 150W power and attach a small variable sine generator to the input. The low current variable sine wave generator is a standard instrument with proven designs you can incorporate. Its not quite as flexible as 3V to 16V but may meet your needs. Cool thing is if you need complex test waveforms, you can record them in WAV files and play them back as audio. Just make sure the amp you choose can handle the impedance range of the load you want to use, most should handle higher than 4 ohms just fine but a 1 or 2 ohm load may pose a problem. A newer D class amp might be a fun DIY and are much more efficient. With an analog amp, 150W of output power may take 250W of DC power or more. As long as you don't sustain the highest power you should not get hot and kill your battery too fast.

[quote Ignoring losses, a 3VDC source would need to supply 150W also and that amounts to 50A peak source current. with losses it may approach 60A. a 12V car battery would fare better with plenty of source current for a while with a modest 13A load.][/quote]

I think the OP means he would be using the output of the AC supply mostly between 3 and 16 volts. I haven't seen anything mentioned about operating the supply on DC power. Do agree with using a audio amp. A 60 hz sine wave generator into a high power amp could supply 24 volts into a 4 ohm load. That's 144 watts at 6 amps.

Steve G

What I was thinking of doing was using a variable dc power supply to control my voltage .

To get in the ballpark, I did a Google image search on "Inverter schematic". I scanned for simpler schematics that could work with variable input voltage. Here is one that's pretty close except I'm sure it can't do 5A. Probably keep everything and stuff another pair of 10A transistors after C4 and beef up L1 and T1 for 5A or replace with Q1,Q2 with darlington pairs and adjust R3 for appropriate base drive current.

http://www.sentex.ca/~mec1995/circ/555dcac.gif

Can we assume you want fixed frequency 60 Hz?
Is your variable DC supply positive and negative or just positive? what are it's specs, it probably needs to be 10A or at least a little more than 5A

These specs are kind of ideal, you may want to back off on some

Input voltage 3~16VDC (3V is pretty low and as previously explained will cause huge source currents, it also makes it hard to turn on FET devices fully)
Output Voltage 0~24VAC (16VDC to 24VAC makes this a more difficult design than if it were the other way around, now you need a DC-DC boost supply, an easier design would have the output constrained by a volt or so less than the source voltage)
Output Current 5A
Power 150W (that's actually quite a lot, by specifying 5A we have to assume that at all voltages but if you specify power, then the max current changes with voltage and is more like how things actually work)
Frequency 60Hz +/-??? (this ??? is a design driver, it will restrict your choices, precision costs in terms of complexity so does stability as you vary voltage)
Load: Stable from open to shorted (shorted load stability is difficult, open is easy, what are you driving, that would help)
Distortion: Pure sine or modified sine, square wave?????

1moldmaker wrote:Would this be possible, I would like to build a low voltage inverter that would go 0 to 24v ac.
I was thinking if I made a variable dc power supply circuit then invert it to AC voltage. Basically My question is if I invert DC to AC would I have to use a step up or step down transformer, and what would my lowest voltage start at and how many watts could I pull, and how pure of a sine wave could I expect.

My take on it is it would be best to go from the set DC voltage directly to the AC that you can adjust the voltage of, especially if the AC doesn't have to be electrically isolated from the input DC. I think the base waveforms could be of a high frequency and then modulated with 60hz to get that on the output.

Am asking myself if a 120VAC to 24VAC at 5 A transformer with an E I core can be made to vary by adjusting the core gap with the help of a screw.
Or perhaps another torus shape core.
Is it for a hot wire cutting tool ?

Whoa...I guess I'm a bit lost here!

The O.P. only wants a varying AC voltage so where does the DC come from? What is the ultimate use for this scheme?

If it is battery power that is available then that will call for an inverter circuit.
On the other hand if it is an DC converted from the AC, then a simple transformer is the easiest. The primary voltage can easily be changed with a variac on the primary, or a motor speed control like for a ceiling fan. A room dimmer can also be pressed into this service but sometimes the transformer makes noise or may get a bit warmer than expected.

Converting from AC to DC then to AC again seems rather senseless and inefficient unless you need a different frequency or waveform for your application than the input AC.

Note:
A room dimmer and a fan control are quite similar except that a fan control has a minimum speed setting, and the dimmer can go to zero.
That is to keep the fan motor running at low voltage so it does not stop and draw higher current because of the locked rotor condition.

Len

I think going with a variac is the best option. Like this one.

http://www.automationtechnologiesinc.co ... red-variac

Steve G

Be aware that a variac DOES NOT ISOLATE the input from the output and as such can become a shock hazard.

Since you only need 24v at 5 amps that is only 96 watts! Why such a large unit?
You still should use a transformer to isolate and drop the voltage to 24, and at less than 100 watts ....try a room light dimmer!