# zener diode question

Discussion in 'General Electronics Chat' started by zzz, Jan 20, 2005.

1. ### zzz Thread Starter New Member

Dec 1, 2004
6
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:blink: I was wondering? The newer high voltage inverters that take 50-250 volts DC
and spit out a clean sine wave 120 AC, do they use a big zener diode like a regulator would to breakdown to a steady 120 DC? And how do these pure sine units convert to AC from there? Anybody?

2. ### n9xv Senior Member

Jan 18, 2005
329
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An typical 12/13.8-VDC power inverter takes the DC, chops it up into a square wave, refines that into a good sine wave and buffers/amplifies the current ability and then applies that to a coil/transformer to step it up to the typical 120/240-VAC.
I'am curious of where you would get 50-250-VDC from? It would actually be easier to deal with the higher DC voltage in a power inverter as far as chopping it and applying to a transformer. A zener diode would'nt be a very efficient way to accomplish that. Although you could use a massive array of a serries/parallel configuration to get there. With cost being a factor I doubt thats the case.

3. ### kell New Member

Jan 20, 2005
4
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Sine alternating current of 120 volts consists of a voltage that varies from -170 volts to 0 volts to +170 volts, and back again, from moment to moment.

You can't make the voltage vary like that using a linear circuit, like a big rheostat or a classic linear voltage regulator with a sine wave signal varying the output voltage, because such circuits accomplish their voltage regulation by taking a high power input and burning up any voltage above what they are supposed to be putting out. It would waste lots and lots of power and require very bulky heat sinks.

Instead, modern pure sine wave inverters, and power supplies like dc to dc converters, and ordinary switch mode power supplies (like in your computer), use transistors that switch on and off very fast. The output voltage, in effect, becomes an average of the amount of time the transistor is on versus off. And because the transistor is always either turned on all the way or turned completely off, it doesn't waste power. The frequency is many tens of kiloherz. So an inverter has no problem varying the ouput voltage at a frequency of 60 Hz. There is plenty of time in that 120th of a second for it to switch on and off many times so that the average it puts out can match the required voltage at that moment in time, whether it is 5 volts or 150.

By the way, most inverters are not sine wave. The mass-market inverters like you would buy for your car put out a "square" wave, turning the power on and off at 60 Hz. In such a "modified sine wave inverter," as they are called, the voltage and duration of the pulses are tailored so that the total power output over time matches that of a real sine wave.