The first thing I need to point out is that inverters generate 240V AC (alternating current), just like the electricity at home IT IS DANGEROUS, and it is very easy to make a mistake with it, thus it is critically important that fixed inverters are ONLY wired in by professional licenced electricians.
Even the small inverters that plug into a 12v socket or just clip on to your battery with crocodile clips still generate 240V AC, so please be careful and follow the instructions when you use one.
What is an inverter ?
Inverter is a term that appears in many places, and to understand what it is we need to understand another electrical concept Direct Current (DC) and Alternating Current (AC).
Your 12V system is a direct current system, where one terminal of the battery has too many electrons, and the other one is short of electrons. When you connect something between the terminals (example turn a light on in the caravan) electrons flow through whatever you have connected to get from one terminal to the other. This is an example of Direct Current (DC) where the current (electrons) only ever flow one way.
Your 240V system at home is an AC (Alternating Current) system, where the current flows one way for a while, then changes direction and goes in the opposite direction for the same amount of time. In our 240V domestic AC the current travels in one direction, and back the other way 50 times a second.
Now the term inverter refers to a device which can take DC and turn it into AC, and in the application we are talking about here they are used to run a piece of 240AC equipment from a 12V DC battery.
How can you use and inverter?
If you want to run a small plug in AC device like say a laptop charger, you can get a small inverter (more on size later) that plugs into your 12V socket, or clips onto your battery. It will have a 240V socket on the end. These are simple, and easy to use, you just need to make sure you have the right size, and type.
On the other end of the scale are the large inverters designed to be permanently wired in, and to feed power through the wiring of the RV. These MUST be wired in by a licenced electrician, and require the safety switch on the RV to be changed to a special type of safety switch that works with inverters.
Lets assume you are looking at one of the inverters that plugs in to a 12V socket, or clips onto your battery terminal, what do you need to know before you can select one to use?
How big does it need to be ?
It needs to be big enough to supply enough power to run whatever you want to run, power is measured in watts, and the device or devices you want to run will typically have the power use listed on the label of the device. The label will either state a number of watts, and or a number of amps that the device will consume. You need to make sure that the inverter you select has enough power to supply the wattage of the device, with a bit to spare. I would add about 50% just to make sure it is big enough.
How many Watts does my device use?
Now my device only lists volts and amps, there is no mention of watts?
That is OK, because you can easily calculate the number of watts by multiplying the volts times the amps to give watts. Lets look at a few examples:
If you are reading a charger label (example laptop charger), it may say something like Input Voltage 240V at 1.7 A, this equates to 408 (240 * 1.7) watts.
Alternatively your charger may say input voltage 110V 240V (indicates it has been designed to work in countries with different domestic supply voltage standards), but not much good to us, thus we need to look at the output specifications where it is likely to say something like Output 19V @ 26A, This tells us the output from the charger is 494 watts. We need to assume that there is some loss in the charger (nothing is 100% efficient), thus we should add say 25%, and allow for at least 600W
You could power either of these examples from a 600W PURE SINE WAVE (PSW) Inverter.
What is this pure sine wave bit? I hear you ask.
Because the current in alternating current changes direction fifty times a second, this means the voltage across the device needs to reverse fifty times a second (this is what makes the current reverse). In our domestic supply at home it reverses gradually and smoothly so if you were able to look at the voltage on an oscilloscope (device that lets you see the voltage over time on a screen as a pattern) you would see a series of nice rounded waves, this is called a sine wave.
The early or low spec inverters produce an output where the voltage changes suddenly from one direction to the other (square wave), or had a ragged wave that changed in steps called a modified sine wave. If an inverter is a Pure Sine Wave inverter it will clearly state that it is a Pure Sine Wave inverter. Electronic devices can be damaged if they are used with a square wave, or a modified sine wave inverter. I would recommend that you only consider Pure Sine Wave inverters.
That is probably enough information to take in in one article.
Very informative thank you Plendo and thanks for going to the trouble to write this info up for us who's talents lay on other areas
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When the power of Love becomes greater than the love of power the World will see peace ! 24ft Trailblazer 5th wheeler n 05 Patrol ute and Black Series Dominator camper trailer ( for the rough stuff)
A rectifier converts AC to DC.
An inverter converts DC to AC.
A transformer changes voltage (up or down).
So converting 12V DC to 240V AC is actually an inverter / transformer. Converting 240V AC to 5V DC (usb charger) is a rectifier / transformer.
My electric train set when I was a kid had a big rectifier, a transformer and then the speed controller. It was on a piece of board about the size of two bits of A4 paper end to end and was really heavy. Today rectifier / transformers are incorporated into the plug (phone charger) and some are in the device (shaver).
A transformer passes an AC current through a set of wire windings this in turn induces (causes to happen) a current in another nearby set of windings which produces an AC current with the same frequency. The voltage in the second coil is controlled by the ratio between the number of coils in first and second set of windings. Note this is a vastly simplified explanation as transformers are a science all by themselves.
Important things to remember Transformers do not work with DC, and they are a heavy, but incredibly reliable way to modify voltage and current in an AC circuit.
Where transformers and rectification used to be used in the power supplies for many devices, they have now been replaced by either solid state voltage regulators or "switch mode" power supplies, both of which are lighter and more stable.
Further to what Plendo has stated, a rectifier will produce DC from AC BUT some may be pulsing or lumpy DC and not suitable for anything else other than battery chargers (half wave rectification).
Im fascinated by modern technology, as Plendo has said transformers were big heavy suckers in days gone. The old valve amplifiers had very large transformers (would you believe 240 volts to 600 volts) and equally very large inductors called chokes to smooth the DC. But they produced great sound, far superior to digital noise we hear now days. Yes Im an old fart that loves good, well produced music, im also a musician (no not drums lol).
Cheers
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Ex RAAF, now retired. EX Electrician/Teacher.
Homebase is Murray Bridge Tourist Park (in a cabin). New Horse.. 2020 Ford Everest Titanium, Jayco swan for touring.
