Inverter Talk.

You didn’t know you needed one.

Off grid amateurs often forego having an inverter as part of their power capability, most likely because DC powered radio gear doesn’t need one. An inverter also adds expense, and in the case of portable operations, adds weight and space. But inverters bring a lot to the table and add capability to your off grid power plant. Let’s go over the types of AC inverters and bring some clarity to this often overlooked & unappreciated device.

What is an inverter and how does it work?

Briefly, a power inverter is a device that changes direct current (DC) to alternating current (AC). Inverters allow the operation of common 120 volt AC devices from DC or battery power. Obviously, this is very practical and useful to off grid amateurs.

Early inverters worked via analog transistorized oscillators feeding a transformer. The oscillators would take the DC input and alternate it at sixty cycles per second, then the transformer bumps the voltage up to 120 or 240 or how ever many AC volts are needed.

These early inverters were heavy and inefficient. Furthermore, most of them did not produce a “real” AC output. Rather, they created a positive-negative alternating DC square wave. True sine wave inverters, to the extent that they existed, were prohibitively expensive. A compromise design is the “modified square wave” (also referred to as “modified sine wave”)  inverter. These inverters produce a voltage that goes positive to negative in stepped increments that roughly mimic an actual sine wave.

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Square wave inverters come with serious limitations. Electric motors and transformers in particular do not run well on square wave power because they depend on a true 360 degree sine cycle. If you attempt to run a motor on square wave power, you may notice that it runs slower, hotter, and makes more noise than it would otherwise. Many manufactures of critical electronics, such as medical equipment, explicitly state in product documentation that square wave inverters should never be used to operate their devices.

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The technology advances.

Engineers developed the insulated gate bipolar transistor in the early 1980s. IGBTs, are, in essence, switches. Really, really fast switches! This new technology made inverters more efficient and simpler to build. Today, inverters use a network of metal oxide semiconductor field effect transistors (MOSFET), which are descendants of IGBTs. A microprocessor and a series of logic gates control the MOSFETs to create a true sine output.  As with most technology, the cost of these devices came down over time. Do you have a switching power supply in your ham shack? Well, a modern inverter is basically a switching power supply running in reverse.

Real life applications.

Now that we’ve had a quick, half-page overview of inverter history and theory of operation, the big question remains: What’s available now, and which kind should you have?

Square wave inverters.. Square wave inverters are obsolete technology. While they are almost impossible to find as new models, used units pop up quite often on eBay and at ham radio swap meets. There is no good reason to use these old inverters unless A) you know for sure it will work for your application, and B) the deal is too good to pass up. Seriously, when you see one for sale, just keep walking even if if the price is really good.

Modified square (or modified sine) wave. These are probably the most popular style of inverter. New and used modified square wave inverters are widely available and are a great “sweet spot” between price and performance. They will work for nearly any application a ham may have. Electric motors, transformers, and similar reactive loads should operate adequately. Essential medical devices may or may not operate safely; consult the product manufacturer for your specific device.

Sine wave. Sometimes referred to as “pure sine wave,” “full sine wave,” or “true sine wave,” no matter what catchy marketing term one sticks on it, they are at the top of the inverter pyramid. With the advancement of technology, the price has come down to levels the average off grid amateur can afford and the output should be cleaner than what you get from the local power company.

Other than price, there are no real downsides to sine wave inverters. For the low-end units the cost difference is not that much between sine wave and modified sine wave. At the mid-to-high end,  sine wave versions can be significantly more expensive than their lesser counterparts.

Cheap vs. expensive.

As with most things, you get what you pay for. Avoid places like Walmart and home improvement stores. While you may occasionally find a solid unit there, ninety nine percent of the time they are selling junk. That $89.00 inverter may seem like a great deal, but don’t take the bait!

For a good, modified sine wave inverter with enough capacity to run most devices expect to pay at minimum a few hundred dollars; considerably more for a pure sine wave version. It’s worthwhile to shop sales because there are deals out there. At the time of this writing, Renogy has a really nice 2000 watt sine wave inverter on sale for $30.00 less than an off brand model from Harbor Freight.

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Pro tip: Your inverter should be produced by a company that specializes in inverters and power products. For example, Amazon sells DieHard brand inverters. Car enthusiasts may recognize DieHard as a very high quality line of automotive batteries that used to be sold exclusively at Sears. Well, today Sears barely exists and DieHard was auctioned off and watered down to being just an average product sold everywhere. The point is, why buy an inverter from a company that offers them as a sideline to their traditional market segment? It’s like going to the dentist for a pair of glasses. These inverters are very likely not designed & built by DieHard but rather rebranded cheap junk made by some no-name third party.

Samlex, Exceltech, and Renogy specialize in inverters and related power products. There are other good brands but these three are the ones I’ve used and can speak to their integrity. You’ll pay more, but a well chosen inverter will chug along for decades without failing.

So what should you buy?

Forget square wave inverters, even used ones offered for a crazy-good price. If money is a primary concern, go for a modified sine wave inverter. If money is not a primary concern or your equipment must have a stable & accurate AC power source, go for a sine wave inverter. No matter what you buy, buy it from a company that actually makes inverters. Steer well away from brands whose primary focus is not inverters.