Nov 05, 2015, 07:33 PM Registered User Miami Fl Joined Feb 2011 616 Posts Discussion DC inverted to AC voltage It must be that Direct current is alot more powerfull than alternating current but its weird to know that a simple 12v battery can be connected to a power inverter and be able to put out 110v ac current?? How is this explained im still baffled on how 12v can be converted to 110v???
Nov 05, 2015, 07:56 PM
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DC is not more powerful than AC. . Power In = Power out, so it takes 10x the current at 12V and converts it to 1/10th that current at 120V. Do not mistake voltage with energy or power. A lightning bolt and static shock are both thousands of volts, but only one of them has enough energy to kill you.

However, it is generally simpler to make devices and components that work with DC than AC. Two big exceptions are:
-Motors as long as you don't care too much about controlling the speed that they spin at
-Efficiently and simply stepping up or down voltage. Stepping up or down AC voltages is very simply and efficiently done by just using a transformer. Although there are simple ways to step down a DC voltage they are all very inefficient. There are no simple ways to step up a DC voltage.

It is done through very complicated electronics not too unlike the way a brushless ESC works. When you suddenly disconnect current through an inductance (like a coil of wire such as that in an electromagnet) the coil of wire will used the energy stored in the magnetic field of the coil to produce a voltage spike HIGHER than the original voltage being used to drive the current. You can harness this voltage spike in a controlled manner to produce a DC voltage higher than the original voltage used to drive the coil. With this higher DC voltage, you can do some fancy on/off switching to approximate a sine-wave wave form to take an AC voltage.

Sort of like controlling how an engine uses explosions in a controlled manner to make something spin.

This is what this voltage spike looks like in a very high powered system:
 Arc Flash (0 min 9 sec)

The one in the video is AC, but an equally high DC voltage with an equal amount of energy behind it will do the same thing. You don't often see DC voltages that high or with that much energy behind them because all our power generation comes from large spinning turbines which inherently produce AC. To change it into DC it takes additional components and because of the high power, these components are very, very large and very expensive.

We keep it AC for the most part because the AC waveform allows it to spin brushless motors with no drive electronics (no brushes and no drive electronics means simpler, cheaper, and lasts longer) and its also much simpler to step down and up AC voltage using transformers from High voltage transmission lines (transmitting electricity over long distances is more efficient using higher voltages for technical reasons). If we used DC, every time we stepped down the voltage to each house we would need a box of complicated electronics.

Undersea power transmission is one of the few areas where high voltage, high power DC is used because DC provides some benefits in the unique underwater environment which makes it more efficient than AC.
Last edited by DKNguyen; Nov 05, 2015 at 08:20 PM.
Nov 05, 2015, 08:04 PM
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Miami Fl
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Quote:
 Originally Posted by DKNguyen It is done through very complicated electronics not unlike the way a brushless ESC works. It is not more powerful. Power In = Power out, so it takes 10x the current at 12V and converts it to 1/10th that current at 120V. Do not mistake voltage with energy or power. A lightning bolt and static shock are both thousands of volts, but only one of them has enough energy to kill you.
So i guess this will drain the 12v battery alot faster than my lipos do???
 Nov 05, 2015, 08:04 PM Registered User The Northeast Kingdom, Vermont Joined Jun 2004 5,642 Posts The inverter converts the 12Vdc to AC, then the 12V ac can be stepped up to 120V by running it through a simple transformer. A transformer can either step voltage up, or down. The ratio of the number of turns of wire in the primary and secondary windings determine the output voltage. DC cannot be run directly through a transformer to change voltage, which is why Edison's low voltage power distribution at 110/220Vdc was doomed from the get go. Pete
Nov 05, 2015, 08:21 PM
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Quote:
 Originally Posted by flysohigh So i guess this will drain the 12v battery alot faster than my lipos do???
No. It's all about power. How much power are you drawing from a battery? Running a 100W 12VDC motor off of your car battery will last just as long as running a 100W, 120VAC motor from your car battery. Just that the DC motor will run on 1/10th the voltage at 10x the current and the AC motor will run at 10x the voltage on 1/10th the current.

But if you're running something that is 120VAC drawing 10A from your battery though, your battery is going to die 10x faster than if you were running something that is 12VDC at 10A from the motor.

Put another way, it doesn't matter what your inverter or charger is doing with the voltages in between, assuming 100% efficiency: A 12V 5000mAh car battery will charge one 12V 5000mAh LiPo, or two 12V 2500mAh LiPos, or two 6V 10,000mAh LiPos. It doesn't matter if the charger is running straight from the battery or needs an inverter to step it up to 120V for the charger (assuming 100% efficiency of course).
Last edited by DKNguyen; Nov 05, 2015 at 08:28 PM.
Nov 05, 2015, 08:25 PM
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United States, WA, Woodinville
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Quote:
 Originally Posted by pilotpete2 The inverter converts the 12Vdc to AC, then the 12V ac can be stepped up to 120V by running it through a simple transformer. A transformer can either step voltage up, or down. The ratio of the number of turns of wire in the primary and secondary windings determine the output voltage. DC cannot be run directly through a transformer to change voltage, which is why Edison's low voltage power distribution at 110/220Vdc was doomed from the get go. Pete
Modern inverters convert the DC to high frequency AC, then a ferrite transformer to step it up to 170 or 340V square wave, then rectify it back into DC, then chop it into AC at 50 or 60Hz. Cheap inverters produce a "modified sine" wave which is just a square wave with dead space so that the peak and RMS voltage matches that of a sine, while better inverters use a PWM circuit to produce a true sine wave.

Converting DC to AC is actually fairly trivial, all it takes is an oscillator. AC to DC is even simpler requiring only a bridge rectifier. The reason high frequency is used is that as frequency increases, reactance increases so magnetics (transformers, inductors) and capacitors can be smaller, lighter and cheaper. It's the same reason switching power supplies exist, a 1kW switcher you can hold in one hand, a 1kW power supply with a 60Hz iron transformer would be as big as a breadbox and weigh >100 lbs.

A transformer is much like a gearbox, it allows you to exchange voltage for current or vice versa, as a gearbox lets you exchange speed for torque. The total power in and out stays equal, minus whatever losses are incurred in the process, nothing is 100% efficient.

The only difference between AC and DC is that with AC the direction of flow reverses at an interval, 50 or 60 complete cycles each second for the power delivered to your home, depending on what part of the world you live in. In a switching power supply the AC is normally several tens of thousands of cycles per second. One is not more powerful than the other.
Last edited by James_S; Nov 05, 2015 at 08:32 PM.
 Nov 05, 2015, 08:32 PM Registered User Salt Lake City Utah Joined Oct 2009 83 Posts There is no free lunch in nature you can not get more power out than than the input power, the output power will be always less due to resistance of the circuit which is wasted as heat. power=voltage x current, if you keep the power fixed, then if you increase the voltage, the the current drops, the same goes with current ( transformers). Tesla invented induction motor that ran on 110v AC, so when he was asked to design the Niagara Falls power system by westinghouse he choose 110v AC, because of that almost all our appliances in US and many other countries run at high current and subject to wear and failure than European system of 220v AC.
 Nov 05, 2015, 10:22 PM Registered User Miami Fl Joined Feb 2011 616 Posts Wow, lots of good info learned, if I get a parallel board for the phantom 3 batteries how will I be able to charge them at the field? Will it just be get an inverter and connect the charger to it and done? This is using a 12v 35ah AGM battery I currently use for my lipo charging? Just wondering if I will have a way to charge the smart batteries at the field. It would have been easy if DJI would have made a charge lead for their batteries.....
 Nov 05, 2015, 10:45 PM Thinking about flying United States, MN, Eden Prairie Joined Oct 2008 1,885 Posts You could use an inverter to get 110VAC from a DC source but it is not very efficient so you will need a large DC source. If you don’t have access to AC at the field you will need additional batteries. For the cost of a good inverter you could buy a couple more batteries.
Nov 05, 2015, 11:05 PM
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Miami Fl
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Quote:
 Originally Posted by Rhea You could use an inverter to get 110VAC from a DC source but it is not very efficient so you will need a large DC source. If you don’t have access to AC at the field you will need additional batteries. For the cost of a good inverter you could buy a couple more batteries.
Mimi's there a way I can calculate roughly how many charges I can get on a 12v 35ah AGM battery? This will be about charging the DJI phantom 3 batteries using a 1000w power inverter?
Nov 05, 2015, 11:09 PM
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Australia, VIC, Melbourne
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Quote:
 Originally Posted by flysohigh Mimi's there a way I can calculate roughly how many charges I can get on a 12v 35ah AGM battery? This will be about charging the DJI phantom 3 batteries using a 1000w power inverter?
what are you charging, what capacity and cell count? From that we can estimate the number of charges.
 Nov 06, 2015, 04:24 AM Registered User Staffs, UK Joined Nov 2003 11,847 Posts No idea who Mimi is...but it looks like that's a 4S 4500 LiFePO4 battery. So given some inefficiency in the inverter and charger I'd guess at about 80-90Wh per charge. A 12V 35Ah battery can safely deliver around 250Wh before it too needs recharging. So about 2-3 charges is probably your lot. Steve
 Nov 06, 2015, 01:49 PM Registered User United States, WA, Woodinville Joined May 2014 2,864 Posts Most chargers run off DC anyway, so I'm confused as to where the inverter comes in here. I've had a number of different RC chargers and even the few that will run off 120VAC also have a 12VDC input. The AC-powered chargers just have a power supply inside them that converts the line voltage AC down to around 12V DC.
Nov 06, 2015, 01:58 PM
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Quote:
 Originally Posted by fox74 There is no free lunch in nature you can not get more power out than than the input power, the output power will be always less due to resistance of the circuit which is wasted as heat. power=voltage x current, if you keep the power fixed, then if you increase the voltage, the the current drops, the same goes with current ( transformers). Tesla invented induction motor that ran on 110v AC, so when he was asked to design the Niagara Falls power system by westinghouse he choose 110v AC, because of that almost all our appliances in US and many other countries run at high current and subject to wear and failure than European system of 220v AC.
It's a little more complicated than that. There was a lot of parallel development going on at the time and things were evolving separately in different parts of the world. As I recall, Edison was the one who settled on 110V, considering that the maximum that would be safe to use in homes. When AC came along, the same voltage was used, incandescent lamps were the primary load at the time and they don't care whether they run on AC or DC. There was also 25Hz AC used to drive the rotary converters that produced DC for the electric subway systems, 60Hz would have required the converters to spin much too fast for the huge heavy machines that they were. Once these gave way to mercury arc rectifiers and later solid state diodes this was not an issue. DC distribution actually survived for a long time in small pockets of infrastructure. I recall reading that sometime in the 1990s the last DC-fed building in (I think) New York was finally converted to AC.

There are advantages and disadvantages to the 240V standard used in much of the world outside of North America. Incandescent bulbs for example are significantly less efficient at the higher voltage due to thermal losses of the much longer thinner filament required. Look at the lumen rating of a 120V 60W bulb and compare it to a 240V 60W bulb, it's a noticeable difference. Of course incandescent is largely obsolete these days so this is much less of an issue. On the flip side, 240V allows discharge lamps such as fluorescent and HID to operate with a simple choke ballast rather than the less efficient autotransformer ballasts common in 120V land.

Apologies for the long off-topic post, but this stuff has always fascinated me.
Nov 06, 2015, 08:08 PM
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Quote:
 Originally Posted by flysohigh Mimi's there a way I can calculate roughly how many charges I can get on a 12v 35ah AGM battery? This will be about charging the DJI phantom 3 batteries using a 1000w power inverter?
These lead acid battery Amp Hour ratings are based on a 20 hour discharge cycle. Discharging them in two or three hours really affects them, essentially reducing their effective Amp Hour rating, down to a little as 50% of their name plate Amp Hour rating.

So, in your case with a 35 Amp AGM battery, the most you can safely pull out of it is perhaps 10 Amps or so, and even that might be pushing it.

AGM batteries are far better than the "Deep Cycle Batteries" available at Wallyworld and other locations, but the 20 hour discharge cycle rating still applies.

Take a look: