Inverters take direct current (DC) from a battery and convert it to alternating current (AC) power electronically. They use switches to rapidly open and close, generating a square wave that is fed into a transformer to produce AC. Inverters can produce different waveforms like square, modified sine, or true sine. Smaller inverters are typically square wave or modified sine wave, while larger inverters produce true sine waves.

1. Inverters
Inverters take dc from a
battery (etc) and
convert it electronically
to ac power.
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3. Block diagram of and inverter
AC Out
DC In Switches Transformer Rectifier Filter DC Out
DC to AC
Output is sampled to
adjust switching for
voltage regulation
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4. Inverter OPERATION
• Inverters are classified by their ac output
waveform.
– Square wave
– Modified sine wave
– True sine wave
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5. Inverter operation
Sine Wave
Square Wave
Modified Square Wave
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6. Inverter operation
• Most smaller inverters are either square wave
(cheap) or modified square wave.
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7. Inverter operation
By closing and opening the switch rapidly, a square wave supply to
the transformer is achieved. This used to be done mechanically with
a “vibrator”. (Yeah! I know…)
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8. Inverter operation
The switches close in turn, producing a pseudo-AC in the primary.
Not that rectification is centre-tapped, full wave, with an L filter.
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9. Inverter operation
Here the rectifier is a full wave, bridge, with L filter.
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10. Inverter operation
A single winding is used here on the primary, with the switches
closing is sequence to provide AC to the primary.
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11. Vibrator
Vibrator
Here is what one looks like.
Inside are vibrating mechanical
contact points that oscillate and
generate a low voltage AC
signal from the DC voltage
applied to it from the battery.
The AC signal then feeds a
transformer where the voltage
is increased.
AC Out
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13. Square waves
• Cheap
• Very poor regulation
• Produce large amounts of RFI
• Very rough AC
• Large amounts of harmonics
• Often used for small appliances eg; fluro lights
Very few but the very cheapest inverters any more are square
wave. A square wave inverter will run simple things like tools
with universal motors with no problem - but not much else.
These are seldom seen any more except in small, very cheap
or very old inverters.
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