How do I make current flow in one direction only?

[Russ]

Hi. I am very new to this, so please excuse any faux pas.
I am trying to connect a pure sine wave 240v inverter into my caravan, but I don't want the current to flow back to the inverter. At the same time I want to limit the current/ wattage output of the inverter.
To limit the output I was going to connect a 5 or 10 w light bulb in series, and to prevent any current flow back to the inverter I thought maybe a diode? Or perhaps I don't need that? Any help would be appreciated.
Thanks

 

[Gryd3]

Hi. I am very new to this, so please excuse any faux pas.
I am trying to connect a pure sine wave 240v inverter into my caravan, but I don't want the current to flow back to the inverter. At the same time I want to limit the current/ wattage output of the inverter.
To limit the output I was going to connect a 5 or 10 w light bulb in series, and to prevent any current flow back to the inverter I thought maybe a diode? Or perhaps I don't need that? Any help would be appreciated.
Thanks

Lets start by clearing a couple misunderstandings.

Current flow is caused by electricity flowing from a higher voltage to a lower voltage... ie, from the + side of a batter through the electronics and to the - side of the battery.
The only way for current to flow backward in this example is if something connected to the electronics produce a higher voltage than the battery, then it will flow from the higher voltage in to the + side of the battery...
This is rarely a concern with electronics except for where 'inductive loads' are concerned... Like power tools... The motors are built in such a way that if you turn them off, they can potentially produce a much higher voltage which will feed back into the system.
Now... the thing is, with AC, current is meant to flow backwards and forwards. That's the entire point of AC. In this situation, large inductive loads should be tuned in such a way with inductors and/or capacitors to reduce the amount of feedback that goes back into the lines... sometimes this can't be helped which is why you may notice lights flicker when you turn your vacuum off.
So, if you want any kind of protection, you will need to put it on the DC side of the inverter... but be warned the diode will cause a small amount of voltage to be lost which may hinder the performance of the inverter. (Probably not... but I grew up around car audio guys who claim it makes a difference... it does, but not any amount to be noticeable or a problem)

So let's move on to power now... What it is and how you can control it.
Power is the product of Voltage and Current. So you give 12V to a light-bulb and it takes 2 Amps. The power here will be 24 Watts.
How do you lower the Power here? Well... you can lower the voltage, or lower the amperage... Notice how I worded that though...
The light-bulb will only take what it needs, based on it's internal resistance. The light-bulb is a pretty dumb device though... so if you put something in-line with it... like another light bulb, only 6V will go to each light-bulb. This will result in the light-bulb only taking 1 Amp. This again, is because the light-bulb is a basic device and the amperage taken is a direct result of it's resistance. The formula for that is here : Voltage = Current * Resistance.
In this example the light-bulb has a resistance of 6Ω, it's set, so in order to change power to it we need to control how much voltage we give it. It will take care of how much current it takes by itself.
There is a little problem though... The Inverter is not a basic component... just like a car amplifier is not a basic component... They have active electronics inside that will constantly change the device's internal resistance to pull more or less amperage as it needs it. If you give it a lower voltage, it will try to take more amperage to compensate. If it want's to put out 60 Watts it's going to try it's hardest... and if the voltage goes too low the electronics may be smart enough to detect a problem and shut down completely.
If you want to limit the output power of the inverter, simply don't plug a really heavy load into it like a microwave or 4-burner cook-top
The alternative would be to buy a smaller inverter that outputs less current. Remember here that the 'Wattage' of an inverter is what it is capable of, not what it will always be running at. Just like your Van, the engine does not always operate at it's peak Horsepower and Torque ratings until you make it.

Additionally, I really don't think you will need a diode for your inverter, but if you want to be cautious it's fine by me... just remember to find one that can handle the load...
A 60 Watt light-bulb will pull over 5 Amps into the inverter from the 12V battery. If you have a 600W inverter... be prepared to find and install a diode larger than 50A.
(I don't know what kind you have... but you should look at what may be required.)
Some extra protection you CAN rely on though would be an undersized fuse... I'm both for and against this idea though... You can size it in such a way that the fuse will pop if the inverter tries to pull too much current, but if anyone other than you looks at the rating on the inverter, they will assume the rated power is available to use and unintentionally pop the fuse. This is an all or nothing approach that would create that limitation you wanted, but would require fuse replacements if someone goes over the limit.


Anyway, give that a read and see if any more questions or ideas pop into your head.

 

[davenn]

all good info from Gryd3 and that covered the DC input side of the inverter

tho the way I read your post ... it sounded like you were talking about the output ( 240VAC) side

The principles are pretty much the same but this comment from you doesn't make sense

but I don't want the current to flow back to the inverter.

AC current flows in both directions, the direction changes with each 1/2 cycle of the sinewave
but you don't need to know that and it isn't a concern for what you are doing


this comment

At the same time I want to limit the current/ wattage output of the inverter.

is covered under the same conditions as for the DC input side that Gryd3 was talking about

The inverter will only supply the current required by the load you plug into it ... fridge, lighting etc etc



Dave

 

[Russ]

Wow. Thank you both. I never expected such a detailed response, and in a way I can understand. I can see I should have given you more information. The reason for all of this is I have a grid connected inverter on the caravan, which is fine when the caravan is connected to the park's power point. (The 1 kW inverter never process more power than what I need, so nothing goes back to the park.)

However when I'm in the bush, I can't use the inverter, and have to run a generator which is expensive. So I thought that if I "created" a grid by using a 300w pure sine wave inverter and a car battery. - could get it to work then. I was worried that I would drain the battery, hence the 5 watt light bulb.

Also I didn't want the 1 kW inverter pushing power into the 300 watt inverter, which is why I thought using a diode would do the trick

 

[Russ]

Sorry I pressed send by mistake.

I know I can buy another inverter, like my 300w, just bigger, 2500w, but I'm a bit strapped for cash and I was hoping to use what I have.

 

[Gryd3]

Wow. Thank you both. I never expected such a detailed response, and in a way I can understand. I can see I should have given you more information. The reason for all of this is I have a grid connected inverter on the caravan, which is fine when the caravan is connected to the park's power point. (The 1 kW inverter never process more power than what I need, so nothing goes back to the park.)

However when I'm in the bush, I can't use the inverter, and have to run a generator which is expensive. So I thought that if I "created" a grid by using a 300w pure sine wave inverter and a car battery. - could get it to work then. I was worried that I would drain the battery, hence the 5 watt light bulb.

Also I didn't want the 1 kW inverter pushing power into the 300 watt inverter, which is why I thought using a diode would do the trick

Haha. I see what you're up to
Very smart, but alas, I don't have much experience to give you any concrete advise.

The grid tied inverter should only be relying on the 60Hz sine wave to synchronise itself too, but as you said above, it never processes more power than it's meant to so nothing goes back to the grid. How can you be certain?
I am under the impression that they are commonly installed and used in conjunction with Solar panels and usually run 1 of two ways... either they cannot provide enough power for the home and the grid supplies the remainder... or they provide too much and the grid absorbed the remainder. I'm sure they occasionally run in a perfect balance, but I don't see it happening realistically. It sounds as though it has an anti-islanding feature that you are trying to bypass to use while you are in the bush...
Realistically, I can see the grid-tied inverter faulting and turning off if you are not drawing close to it's 1kW rating while you have a small inverter 'tricking' it into thinking it's connected to mains... If you are drawing less than 1kW it will try to feed back into the smaller inverter due to the voltage raising... One of two things will happen:
-The Grid-tied inverter will detect this and faul/shut-down
-The smaller inverter will fault, causing the Grid-tied inverter's anti-islanding feature to turn itself off.

Both outcomes result in no more power for you..

As far as preventing the power from feeding back... I can't think of a way unfortunately...
The inverter will output AC, which means the current in the wires will actually flow back and forth. Installing a Diode will limit this and only allow it to flow in one direction which will result in only half a wave-form from the small inverter reaching the grid-tied inverter... The larger inverter may not turn on if this were the case... even if it did, the diode would not protect the smaller inverter...

Think of it this way... if you disconnect a car battery and put a small AA battery in-line with the positive terminal one of two things will happen... The car will either force too much current through the AA forward, or backward depending on which way you connect the AA battery. Both situations will be horrible. So simply stopping excess current from flowing one direction and not the other will not solve your problem here...

Do you have a model number of the grid-tied inverter you are using? If we are lucky, there may be a setting, module, or something we can do to allow it to operate as an 'island'


If anyone else here has any ideas on how to safetly operate a grid-tied non-islanding inverter in the woods without a generator we would love to hear it.

 

[Russ]

Haha. I see what you're up to
Very smart, but alas, I don't have much experience to give you any concrete advise.

The grid tied inverter should only be relying on the 60Hz sine wave to synchronise itself too, but as you said above, it never processes more power than it's meant to so nothing goes back to the grid. How can you be certain?
I am under the impression that they are commonly installed and used in conjunction with Solar panels and usually run 1 of two ways... either they cannot provide enough power for the home and the grid supplies the remainder... or they provide too much and the grid absorbed the remainder. I'm sure they occasionally run in a perfect balance, but I don't see it happening realistically. It sounds as though it has an anti-islanding feature that you are trying to bypass to use while you are in the bush...
Realistically, I can see the grid-tied inverter faulting and turning off if you are not drawing close to it's 1kW rating while you have a small inverter 'tricking' it into thinking it's connected to mains... If you are drawing less than 1kW it will try to feed back into the smaller inverter due to the voltage raising... One of two things will happen:
-The Grid-tied inverter will detect this and faul/shut-down
-The smaller inverter will fault, causing the Grid-tied inverter's anti-islanding feature to turn itself off.

Both outcomes result in no more power for you..

As far as preventing the power from feeding back... I can't think of a way unfortunately...
The inverter will output AC, which means the current in the wires will actually flow back and forth. Installing a Diode will limit this and only allow it to flow in one direction which will result in only half a wave-form from the small inverter reaching the grid-tied inverter... The larger inverter may not turn on if this were the case... even if it did, the diode would not protect the smaller inverter...

Think of it this way... if you disconnect a car battery and put a small AA battery in-line with the positive terminal one of two things will happen... The car will either force too much current through the AA forward, or backward depending on which way you connect the AA battery. Both situations will be horrible. So simply stopping excess current from flowing one direction and not the other will not solve your problem here...

Do you have a model number of the grid-tied inverter you are using? If we are lucky, there may be a setting, module, or something we can do to allow it to operate as an 'island'


If anyone else here has any ideas on how to safetly operate a grid-tied non-islanding inverter in the woods without a generator we would love to hear it.

Thanks mate. It's a pity, I thought there must be way, but no joy. Thanks anyway, I do appreciate your time and effort.

I understand the inverter is set this way to stop it sending power to the utility grid if there's an outage

Oh by the way, the inverter is a Chinese inverter, but it has been painted. Not by me. So there are no markings left. I was told by the guy I bought the caravan off that it was a 1 kW inverter, although a friend of a friend measured the output and said it was incapable of producing more tha 700 watts.

Maybe there's a way to by-pass any extra power, and so not allowing more than 300 watts going to the smaller inverter.

Cheers.

 

[davenn]

Maybe there's a way to by-pass any extra power, and so not allowing more than 300 watts going to the smaller inverter.

It seems you still didn't understand what Gryd3 and I wrote before
Just because an inverter is capable of say 500W doesn't meant that that is what it puts out all the time
its output depends entirely on the load you put across its output

just the same as the 12V battery in your car. Its capable of 800A in bursts for the starter motor
but can still supply maybe 0.5 A ( 500mA) for your car GPS unit that you have stuck to your window
without burning it out ..ng else in series to decrease the power used

do you understand ?


Dave

 

[Gryd3]

It seems you still didn't understand what Gryd3 and I wrote before
Just because an inverter is capable of say 500W doesn't meant that that is what it puts out all the time
its output depends entirely on the load you put across its output

just the same as the 12V battery in your car. Its capable of 800A in bursts for the starter motor
but can still supply maybe 0.5 A ( 500mA) for your car GPS unit that you have stuck to your window
without burning it out ..ng else in series to decrease the power used

do you understand ?


Dave

I think he got that from my other post Dave, and I could be mistaken, but I am under the impression that a 600W grid-tied inverter will run at it's highest possible output depending on it's input. If the home is only using 500W, the excess 100W goes back to the grid, and if the home draws 700W than the additional 100W gets drawn from the grid. That said, the grid-tied inverter will fault and shut-off if the voltage goes too high (ie, when producing more power than is used in situations like when a generator may be used.)
This is different from regular DC-AC inverters that are not meant to be tied to the grid.
Please correct me if I'm wrong, that's just what I have come across thus far in my reading with grid-tied inverters with anti-islanding. (Inverters capable of islanding will not have this behaviour.)

Don't we have a guy on the Forums somewhere that has an abundance of experience with PV, Inverters and Backup batteries? (I can't for the life of me remember the name)