Mr. Fixit's Repairing, Upgrading, & Performance Tips
| Power Inverters |
RV's or anywhere 120 VAC is needed but only 12 VDC is available. They can be small enough to fit in a 12V cigarette lighter plug or large as a shoe box. I
400 watt as a back-up for the computer and chargers when the big inverter is in use. As you can see they are convenient, but not without drawbacks.
Power inverter's biggest drawback is the power required to convert 12VDC into 120VAC. For example: My 1,500 watt power inverter requires 144 amperes
(1,728 Watts) at 12VDC just to provide 12.5 amperes (1,500 watts) @ 120VAC! That's no joke. Cables are another factor dealing with power inverters. The
higher the ampere draw and the longer the cables run from the DC source the bigger the wire size must be. Some inverters use 00 wire gauge. The reason
for such large wire size is the result of high resistance of Direct-Current (DC) in copper wires. As the amperage increase, the resistance increases. Larger
wire sizes have less resistance. If the ampere is higher than the rated wire size, the wire heats up and can start a fire. The resistance in a wire can also cause
a drop in voltage causing the inverter to work harder, producing more heat. This can cause thermal runaway. Most modern inverters will shutdown in the event
of thermal runaway. They also have overload protection and Low/high input voltage protection.
The DC source must be able to supply the power the inverter demands. Since my inverter needs 144 amperes to provide 1,500 watts, the DC source must
provide the power at a steady rate. It only takes a matter of minutes that an inverter will drain a single car battery, but you have 2 options. You can either wire
multiple batteries in parallel (all '+' together and all '-' together) or start your vehicle's engine and use the alternator. When using multiple batteries, you are
dividing the load between the batteries, thus the more batteries the longer the inverter can provide power. When using the alternator on the engine, the
alternator must be rated higher than what the inverter requires. The alternator also provides 14.1VDC which is good since inverters use less power at higher
input voltages as long as the input doesn't exceed 15VDC. The alternator should be rated 10%-15% higher than the inverter's power requirement to ensure
safe operations. I'll show later how these figures are calculated.
400 watt as a back-up for the computer and chargers when the big inverter is in use. As you can see they are convenient, but not without drawbacks.
Power inverter's biggest drawback is the power required to convert 12VDC into 120VAC. For example: My 1,500 watt power inverter requires 144 amperes
(1,728 Watts) at 12VDC just to provide 12.5 amperes (1,500 watts) @ 120VAC! That's no joke. Cables are another factor dealing with power inverters. The
higher the ampere draw and the longer the cables run from the DC source the bigger the wire size must be. Some inverters use 00 wire gauge. The reason
for such large wire size is the result of high resistance of Direct-Current (DC) in copper wires. As the amperage increase, the resistance increases. Larger
wire sizes have less resistance. If the ampere is higher than the rated wire size, the wire heats up and can start a fire. The resistance in a wire can also cause
a drop in voltage causing the inverter to work harder, producing more heat. This can cause thermal runaway. Most modern inverters will shutdown in the event
of thermal runaway. They also have overload protection and Low/high input voltage protection.
The DC source must be able to supply the power the inverter demands. Since my inverter needs 144 amperes to provide 1,500 watts, the DC source must
provide the power at a steady rate. It only takes a matter of minutes that an inverter will drain a single car battery, but you have 2 options. You can either wire
multiple batteries in parallel (all '+' together and all '-' together) or start your vehicle's engine and use the alternator. When using multiple batteries, you are
dividing the load between the batteries, thus the more batteries the longer the inverter can provide power. When using the alternator on the engine, the
alternator must be rated higher than what the inverter requires. The alternator also provides 14.1VDC which is good since inverters use less power at higher
input voltages as long as the input doesn't exceed 15VDC. The alternator should be rated 10%-15% higher than the inverter's power requirement to ensure
safe operations. I'll show later how these figures are calculated.
So, how does a power inverter change 12VDC into 120VAC? This is where it gets complicated, so I'll just explain the basics. Power inverters are also known
increase the voltage to 120VDC.The oscillator (OSC) provides smooth on-off pulses at 41KHz the power inverter works from. The pulses are then fed into the
PWM. The PWM turns ON the control element (switch) to allow current to flow. The control element is a Field-Effect Transistor (for example: Metal-Oxide
Semiconductor Field-Effect Transistor (MOSFET)) whose electrical state is either on or off. In the ON state, FET's have very little resistance (0.4 ohm) and the
field collapses inducing the energy back into the inductor. The higher number of turns of wire in the inductor will increase the voltage. A "Catch-Diode" then
directs the flow of energy from the inductor to the capacitor. The capacitor temporarily stores the electrical voltage. A sample of the output voltage is sent back
to the voltage regulator and the cycle repeats. Now, you went from 12VDC to 120VDC. The next step will be to alternate the polarity at 60HZ. A separate
circuit creates a '+' and '-' modified-square wave to apply to different a set of FETs at 60Hz to create 120VAC.
increase the voltage to 120VDC.The oscillator (OSC) provides smooth on-off pulses at 41KHz the power inverter works from. The pulses are then fed into the
PWM. The PWM turns ON the control element (switch) to allow current to flow. The control element is a Field-Effect Transistor (for example: Metal-Oxide
Semiconductor Field-Effect Transistor (MOSFET)) whose electrical state is either on or off. In the ON state, FET's have very little resistance (0.4 ohm) and the
field collapses inducing the energy back into the inductor. The higher number of turns of wire in the inductor will increase the voltage. A "Catch-Diode" then
directs the flow of energy from the inductor to the capacitor. The capacitor temporarily stores the electrical voltage. A sample of the output voltage is sent back
to the voltage regulator and the cycle repeats. Now, you went from 12VDC to 120VDC. The next step will be to alternate the polarity at 60HZ. A separate
circuit creates a '+' and '-' modified-square wave to apply to different a set of FETs at 60Hz to create 120VAC.