A theoretical question concerning DC/AC Inverter

[Carlsp]

Hello all. I am new to the world of electronics and was wondering if this is possible. What happens if you take a DC/AC Inverter, 12v DC to 120v AC, 300 watt 4 amp, and provide it with a variable DC input. Say, for example, what happens to the output if you only apply 6v DC input to the inverter? Does the output reduce to about 60v AC (or less because of energy loss to like heat)? Could you control the AC output by changing the input voltage? Or is this not possible? What about current and how is it affected? Or am I way off base with this line of thought. I know that you can use a variac to control and AC output but was wondering if the same could be achieved by varying the input voltage to an inverter. I was thinking of using by DC benchtop power supply as DC source voltage. Thank you to any responses to this.

[Photon939]

Usually the inverters I've poked around inside tend to work in two stages (the small high frequency switching types)

A DC/DC boost converter first boosts the battery voltage to somewhere between RMS and peak voltage for the specified output, then an H bridge type chopper circuit converts the HV DC into AC. So if you were to undervolt the input the boost converter would work harder to maintain the HV supply and draw more current and vice versa. They are usually at least semi-regulated.

Also pretty much any inverter will have a supervisory circuit that monitors input voltage and load to protect against over-draining batteries and shutting down on overload. So likely if you fed 6vdc to a 12v inverter you would get nothing on the output.

[Carlsp]

Ah, thanks. That make sense. That does help me to understand inverters some more. I just thought/assumed that they converted the DC input to AC first, then stepped up the voltage to the 120v output. Thank you for clearing it up for me, much appreciated. Just thinking of ways to create a low cost way to vary the AC output from the wall without using a variac as I cannot afford one right now. Also, for bench testing a AC circuit, should there be an isolation transformer between my source (the wall outlet) and the DUT for protecting things like my test equipment? Thanks in advance.

[Photon939]

If you watch on ebay you can often find good deals on variacs. It helps to search the more technical term "variable autotransformer". I scored an old Powerstat 15A 120v variac for $35.

And as far as isolation goes, take a read on the several threads here on the forum about using isolation transformers for test equipment, it may not be ideal for your situation.

[David Hess]

An inverter can be designed to operate open loop so the AC output voltage follows the DC input voltage.  Saturable transformer (Royer) inverters operate this way.  Or the DC input can be stepped up to 170 volts for a 120 volt modified sine output and then chopped though a low frequency transformer.  Then the output voltage follows the stepped up 170 volt DC supply.

[Zero999]

Ah, thanks. That make sense. That does help me to understand inverters some more. I just thought/assumed that they converted the DC input to AC first, then stepped up the voltage to the 120v output. Thank you for clearing it up for me, much appreciated. Just thinking of ways to create a low cost way to vary the AC output from the wall without using a variac as I cannot afford one right now. Also, for bench testing a AC circuit, should there be an isolation transformer between my source (the wall outlet) and the DUT for protecting things like my test equipment? Thanks in advance.

Old inverters use to convert DC to AC, before passing through a transformer to boost the voltage. Indeed lots of hobbyist inverter projects still work that way because it's very simple and easy to do.

Modern inverters use a DC:DC converter to boost the voltage first, before converting it to AC with an h-bridge. The DC:DC converter works by converting the DC voltage to AC, passing it through a transformer and rectifying it back to DC again. This might seem odd, but there's a reason for this: the DC:DC converter's transformer works at a much higher frequency than the mains, so it can be much smaller, more efficient and cheaper than a mains transformer. An inverter which uses a DC:DC converter will be much smaller, lighter weight and cheaper, than one which converts the DC to AC and stepping it up using a larger, heavy mains frequency transformer.