What is an inverter?

[Terry]

"Inverter" to me means 12 VDC in, 120VAC out. Is that not the case?

Alan posted this query in another thread which is now locked, but I feel it deserves a reply:

I assume the name inverter comes about because it converts voltages up rather than down but the input and output can be anything that the user wants.
For example, for use in a large vehicle - a bus or lorry, say - the input would be 24V and, on this side of the pond, the output would be 240V, of course.

In vintage car radios, an inverter was provided by a vibrator and transformer arrangement but the outputs would be what the radio required, not a replica mains supply - and in some of these the input voltage would have been 6V, to suit the vehicle in which it would be used.

If an inverter is being designed as a stand alone unit, to provide lots of power as part of an uninterruptable power supply, for example, a 12 or 24V battery would be enormous so it is common, in my experience, to see such units powered by a number of 12V lead acid batteries connected in series. This makes the battery size more manageable and much easier to handle both in initial construction and later routine replacement.

[Mark Hennessy]

This raises an interesting point about terminology. In that other thread, NM was referring to an "inverter", but it sounded like it also contained a battery charger as well. So perhaps it's more accurate to describe it as a UPS, given that a UPS consists of charger, batteries and inverter? (Obviously, batteries can be internal or external). I've no experience in the marine electrics world, so don't know if it's common usage to call the box an inverter. I can say that in the outside broadcast vehicles that I've seen, the charger and inverters are usually separate boxes, and referred to by their separate names.

All this reminds me of a similar thing. What do you call "the box that does your broad-band"? Chances are, you call it a router, right? Certainly, most people do in my experience.

But that box contains many things, including:

• An ADSL (or VDSL) modem
  
• A NAT router/firewall
  
• A network switch, usually at least 6 ports (4 of which are available to the user)
  
• A wireless access point
  
• A DHCP server
  
• A web server
  
• Perhaps a print server or a media server?
  

You get the idea. So why is it called a router?

Back to inverters, I remember being impressed with the APC way of implementing a cheap UPS. When you take one apart, there is only one transformer in there. When the mains is present, the transformer steps down the AC to charge the batteries. When the mains fails, a brace of relays change over, and now the transformer is acting for the inverter, stepping up an AC waveform that is generated by MOS-FETs connected to the batteries. Neat. Obviously the change-over isn't seamless, but switched-mode power supplies usually have a fairly generous "hold-up time" to cover the gap. Slightly OT, but as time goes by in the PC world, the power supply will eventually become a single 12V output, with all power conversion done on the motherboard, and that'll open up some interesting options for cheaper, more efficient UPS systems. Right now, most of the other lines are in (or nearly in) the "legacy" category - for some motherboards, it's only the PCI slots that need some of the rails. And drives, of course.

There are many ways to implement an inverter. Most of the cheaper ones use a modified square wave approach because that's simple and relatively efficient. Absolutely fine for anything that has a switched-mode supply (the vast majority of those would happily run on DC), but not so good for anything else. There are techniques to improve them, but the sine wave inverter is the preferred option. These are a lot more complex. The problem is the linear nature of the waveform, which implies heat loss, plus the magnitude of the required voltage.

To get around the efficiency problem, you can use switching techniques. A class D amplifier can give 90% or more efficiency. Then you can stick the sine wave into a 50Hz transformer and step it up to 230V as required.

But 50Hz transformers are heavy, so it's tempting to do away with them. There are many options:

• Take the DC, feed it into a boost converter to make a high DC rail. Use that to power a class D amplifier that is able to work at the high voltages. Take AC output direct from amplifier, via some filtering.
  
• Make a class D amplifier with a HF transformer in the output stage that does some stepping up.
  
• A hybrid of all options considered so far.
  

Factors that drive the topology choice will include available semiconductors. E.g., high voltage MOS-FETs don't tend to switch on as fully as low voltage examples, so will have conduction losses. Obviously, that's changing as time goes by, but at a fixed point in time, you might choose to turn 24V into 170VDC with a boost converter, then apply that to the class-D stage, then have a 2:1 stepup on the ouput (in either the HF or 50Hz domains). For example...

Don't forget the need for overload protection, over-temperature shutdown, protection against reactive loads on the output, and possible "load dump" from a vehicle battery system. The more you look into these, the more you realise just how well engineered they have to be. There are relatively few firms making them (ignoring the really cheap ones). These are the ones I'm familiar with: http://www.antares.co.uk/pure-sine-wave-inverter.html - funnily enough, the more complex diagram on that page looks exactly like one that came with one of our vehicles (surely a coincidence )

Interesting topic

[Terry]

This raises an interesting point about terminology ... All this reminds me of a similar thing. What do you call "the box that does your broad-band"? Chances are, you call it a router, right? Certainly, most people do in my experience.

But that box contains many things ...

You left out the option of a DOCSIS modem for CATV, Mark!

Don't forget that originally the modem only had one output. If you wanted more than one feed you would connect that to a separate router/switch which might or might not also contain a wireless access point, etc.

Then manufacturers started bundling everything into a single box. To the average non-technical user, the only thing that had multiple outputs on the back was a router, so the name stuck for the entire unit.

[ppppenguin]

I tend to use the term modem/router for the device that has a phone line (or Virgin F connector), some ethernet ports and wifi. It's an ugly construction but sometimes needed to distinguish from a pure modem such at the HG612 modem supplied by Openreach for many FTTC customers. Or a pure router (or is it a hub or a switch) that I use to provide extra ethernet ports.

Of course in the USA we could also confuse it with this: http://www.airmadagascar.info/wp-content...401831.gif

[Terry]

You don't need to go to the States to get one of those, Jeffrey!

Here's a nice selection ...

Although the spelling is the same it is the pronunciation that varies - you would call your unit a rooter but the woodworker calls his a rowter ...

Funny old language, isn't it?

I wonder if the same distinction exists across the pond ...?

[ppppenguin]

In case anyone here is unfamiliar with US usage, in the US both sorts are called ROWTER, hence my comment. In many parts of the US it's closer to ROWDER.

[Mark Hennessy]

Similar in Australia as well...

When doing PCB layouts, they might use the "auto-rowte" option

Because I watch a fair few of the EEVBlog videos, these differences no longer leap out at me in quite the same way. Perhaps I'm being assimilated

[Alan Douglas]

Back in the old days before electronics, a numbered highway here was a "root" but in parts of the southwest it was "rout." So it annoys me to use "rowter" for the electronic device, although the woodworking tool was always called that, and that's been so even when it was a hand plane (used for making flat-bottomed recesses for hinges and the like).

"Inverter" has always meant something that takes DC and provides AC, as opposed to a "converter" that takes DC and supplies DC at a different voltage. These were rotating machines.

And strictly speaking, a "generator" puts out DC while an "alternator" supplies AC. However everyone talks about their "emergency generators" for household power, or "portable generators" to run tools a on a jobsite, so one just has to go with the flow.

[Nick]

They shouldn't be called modems anyway - tecnically they are "NTE"s - network termination equipment (or NTU -unit) and are digital encoders/decoders.

A modem is a "modulator/demodulator" and harks back to the old analogue days.

Mark: you forgot the DNS server

They are all really just SBCs running embedded Linux with some layered applications...

[Nowhere-Man]

Below is a photo taken from the instruction manual of my inverter. It will explain what I was explaining earlier when I went a wee bit off-topic talking about boat electrics. Seeing as a few of you are interested in inverters, I'll now explain more carefully how all of this was set-up.

You can see in the pic that the inverter is shown plugged into mains supply. There is a fused cable at the back (via a small recess where the 15 amp fuse lies.)

You will see two cables that connect to the batteries. All of this is showing a simple battery charger at the moment. This is how it is being used at the moment as it's been taken out of the actual boat circuit for now.

I should add that the DC cables that go to the batteries need to be considered carefully. Charging current is set at 10 Amps. That isn't a great deal of current. However I used 100 Amp cable here due to the fact if we switch the unit from charger to inverter use, larger currents may be drawn. More of that later.

The AC electrician who wired my boat did something I don't really agree with. What he did was take the plug supply you can see in the diagram and connect these AC leads (live, neutral and ground) to the shoreline supply via the shoreline female and male plugs and sockets. That means, when you take your 15 Amp shoreline socket and plug in external power into your boat from the boatyard meter, the charger unit is switched on whether you like it or not. That means it's always charging the batteries and the fan is always going round (and nothing lasts forever).


Anyway, that is all history. I took the unit out and fixed the fuse. I fitted a plug the same way as in the diagram above and I use it as a battery charger. One of the batteries was of no use. I ought to point out that I found if a battery is half-way good, voltage does tend to rise quickly. However, the charge that was finally held was only 12.4 volts so my batteries aren't that great (they're old).

Should you wish to use this as an inverter, simply remove the mains supply and switch the unit on. At the front you will see output sockets. If you plugged a DVD player or TV into this, it would power the device just drawing from the batteries. If you plugged in something that's heavy on Kilowatts, you would get high current through those battery cables so it's wise to fit 100 amp cable.

These units can be expensive and mine is modified sine wave.