Explain How Car DC Inverters Work - Fiberglass RV
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Old 07-22-2014, 01:14 PM   #1
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Explain How Car DC Inverters Work

Here's a sample product: I changed name of actual product to XXXXX
Quote:
XXXXXX 200w power inverter car dc 12v to 110v ac inverter dc adapter with 3 AC outlets + 4 4.2A USB charging ports
I am showing my ignorance, but if it is a 200 w power inverter, how does it put out more than max 2 amps? (200/115 = 2)

The reason I am asking is I am trying to determine what I can run off an inverter plugged into one of my dc circuits. I recharged my laptop using a 75 w inverter and it seemed to work fine.

But if I have a 12v circuit with a 15 a fuse, aren't I limited to 180 w? Would the 200 w inverter blow my circuit?

Should I run cables directly from battery to inverter and what does this buy me? I guess more amps?

I would prefer not to run the additional cables if I don't have to. Right now, the only things that have to have ac are laptop and possibly a printer. I can run both of those with my 75 w inverter (1 at a time).

But let's say I wanted to run the microwave. In that case, would I need at least a 750 w inverter connected directly to battery?

Thanks. I'm going to get the hang of this yet.
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Old 07-22-2014, 01:22 PM   #2
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Inverter wattage is how much power the inverter can deliver - each load plugged into it draws a certain number of watts. If your plugged in demand totals more than 200W your inverter can't deliver that.

If your inverter tries to pull more than 15 amps from the 12V line you will blow that fuse.

If you want more wattage you will need a larger inverter and bigger wires to the battery.

Think of the inverter as a supply you can draw from, not something that pushes energy out.
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Old 07-22-2014, 01:28 PM   #3
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That microwave will suck the life out of the battery very quick.

It takes the DC and they run it through a multiviberator that outputs a square wave at the correct cycle then through a transformer to get it to the required voltage (cheep one). then if you spend enough money for it they will add filtering circuitry to give you a pure AC sign wave and so on. The more you spend, the bigger unit you can get. Most will come with a direct connection to the battery as the shorter the connection to the DC the less loss. Others come with a digerati lighter plug that you just plug in. To go all out, you would install multiple batteries, controller and solar panels, etc and it gets very expensive very quick.

You can use your small inverter and plug into your battery receptacle in the camper or You can purchase a battery jump pack at the where house clubs that have jumper cables, air compressor and inverter built in and that is something to think about for the PC and Printer.
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Old 07-22-2014, 01:55 PM   #4
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Originally Posted by tractors1 View Post
Inverter wattage is how much power the inverter can deliver - each load plugged into it draws a certain number of watts. If your plugged in demand totals more than 200W your inverter can't deliver that.
I think this addresses my 1st question, so let me be more specific. The inverter is 200 w, so at 110 v, that's a little less than 2 amps (P(W) = I(A)◊ V(V)).. So how can it support 3 ac outlets and 4 4.2a usb ports? Where are all the amps coming from?
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Old 07-22-2014, 02:27 PM   #5
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What you plug into the 4 outlets determines the draw so "what it can support" is not much draw. More you pull the more it pulls from the battery. But you are correct 2 amps is about it. And you will be drawing 20+ amps from the battery. Which can comfortably handle that for less than 3 hrs. on a 100 amp hr. battery.

So you can charge a couple phones and your laptop. Or run a 110 light or maybe a fan.... But I'm thinking not going to run a coffee pot.

Might find this table of use. Usage Chart: How Many Watts Do You Need?

200 watts is fine for running a laptop, charging a camera, or running a hand held DVD player but next to useless for most appliances. As someone already pointed out you end up needing dual batteries and solar to support things such as appliances.
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Old 07-22-2014, 02:35 PM   #6
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Now, when you charge your laptop from the inverter the laptop power supply takes the AC and turns it back to DC. If your laptop operates off of 12VDC, skip the inverter altogether and charge it directly from your 12VDC supply. They sell cigarette plug devices that plug to the laptop to do just that.
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Old 07-22-2014, 02:46 PM   #7
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Now, when you charge your laptop from the inverter the laptop power supply takes the AC and turns it back to DC. If your laptop operates off of 12VDC, skip the inverter altogether and charge it directly from your 12VDC supply. They sell cigarette plug devices that plug to the laptop to do just that.
I've got that part down. It just seems to me that these devices magically create amps. I understand the amps come from the battery, but the devices seem to pull more amps than the power rating they have allows.
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Old 07-22-2014, 02:54 PM   #8
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You are assuming that each outlet has it's own rating of 200 watts, that's not correct. It's a total of 200 watts from all three outlets combined, and that's a peak power rating. If you look at the specifications it will probably say something like 150 watts continuous. And the USB outlets aren't a part of the inverter at all, they are a separate 12VDC to 5VDC reducing circuit, but do add to the total DC load.



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Old 07-22-2014, 03:58 PM   #9
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I've got that part down. It just seems to me that these devices magically create amps. I understand the amps come from the battery, but the devices seem to pull more amps than the power rating they have allows.
Huck, when you get this figured out, key me in would ya?

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Old 07-22-2014, 04:10 PM   #10
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Huck, when you get this figured out, key me in would ya?

Frank
Everybody else knows. They just won't tell us!
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Old 07-22-2014, 04:11 PM   #11
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everybody else knows. They just won't tell us!
lol!
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Old 07-22-2014, 05:10 PM   #12
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This stuff hurts my head. While investigating running my CPAP off a battery and inverter I found contradictory information on the subject. The machine says 120V and 2.5A on the bottom. So, that would indicate 300 watts right? But, upon further investigation I discovered that the CPAP manufacturer recommended a 150 watt inverter. I ordered the inverter (150W) that they recommended and what do you know, my CPAP runs just fine on the half-sized inverter.
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Old 07-22-2014, 05:31 PM   #13
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I think one only needs to convert watts to amps on the demand side if amps are not provided. Some devices might say 110 volt @500 watts a different device might specify volts and amps or even possibly all three. But most of the time I only see 2 values given.

I think while the device may be built to work with a 110 volt input that does not mean internally it uses 110 volt current. Simplest example I can think of is your computer. Plugs into 110 source but everything inside is either a 12 volt or 5 volt circuit.

On the other hand something such as a blow dryer that is 1000 watts is using that 110 volt internally and then the formula would yield the correct amperage draw.

This might also explain the CPAP machine, internally it may not be using 110 volt. It just needs 110 volt input to have the correct voltage output on the components using the power. Watts required are based on what uses the power.

And this hurts my head and my whole face.
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Old 07-22-2014, 05:35 PM   #14
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I have decided to run some AC through a transformer and convert some of the volts to amps.
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Old 07-22-2014, 06:33 PM   #15
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Everybody else knows. They just won't tell us!
A good place to start figuring it out is to read through the whole 12 Volt Side of Life website. The 12 Volt Side of Life - Part 2 has lots of info on inverters and what power usage you can expect as well as what the draw is on some typical appliances.
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Old 07-22-2014, 07:21 PM   #16
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We have a 1200 watt Inverter in our Scamp but never use it.

Instead we typically use a 150 watt cigarette lighter Inverter to run our TV and Sat dish when boondocking. They draw very little power.

We have installed three 12 volt outlets for Inverters in our Scamp, one under the dinette, one near the couch and one above the stove counter.

We also carry onw in the car for charging the laptop.

Our son's Cpap machine runs on AC or 12 volt DC. In his Scamp we installed a 12 VDC socket for his Cpap. He has 2 12 volt batteries and an 80 watt solar panel.
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Old 07-28-2014, 01:17 PM   #17
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...how can it support...4.2a usb ports? Where are all the amps coming from?
Easy, a USB port is 5vdc, not 110vac or 12vdc. Thus it can have 4.2a available at a 5vdc USB port.
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Old 07-29-2014, 10:12 AM   #18
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I am probably only going to confuse things more, but here is my best explanation.

Volts and Amps are almost meaningless on their own. Watts is power. Power is work, or what you want to make happen. Your Cpap machine is doing work, it requires a certain amount of power to do this. If they recommend a 150 watt inverter, the device likely only consumes ~100 watts. If you have a 300 watt inverter, it will work fine. It just wont be working as hard as it could.

Watts are a function of current, (amps) and voltage, (volts). 10 amps at 100 volts is 1000 watts, (10A x 100V = 1000W). This is always true in DC applications. It is also mostly true with AC, but AC can get a bit complicated when the current and voltage are not in phase. This happens with coils, and capacitors, but lets ignore that.

If your Cpap requires 120 watts to run, then at 120 VAC, it will draw 1 amp. Your inverter can almost be considered a transformer. At the 12 volt side it will draw 10 amps to make the same 120 watts, (12VDC x 10A = 120W). This ignores the power consumed by the inefficiency of the inverter.

The 5VDC is likely provided by DC to DC converter in the inverter. You can think of this as a transformer as well. DC to DC converters used to be expensive, but not so much anymore. This one is $2.27, free delivery:
P4PM DC12V Step Down to 5V 3A 15W Converter Car LED Display Power Supply Module | eBay
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Old 07-29-2014, 10:42 AM   #19
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Originally Posted by David Tilston View Post
If they recommend a 150 watt inverter, the device likely only consumes ~100 watts. If you have a 300 watt inverter, it will work fine. It just wont be working as hard.

Yes but, just doing the math a diligent person would not know that the machine would run fine (and not let out the magic smoke) on an inverter half the size indicated. And, the 300 inverter had a cooling fan and was oversized (and therefore electrically wasteful) for the job. The smaller inverter is not only more closely matched to the load and more efficient but does not have a cooling fan drawing energy. When you are boondocking on batteries you have to watch every watt!
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