Pure Sine Wave Inverter

[securespark]

Number 2 son blew up the inverter yesterday by connecting it RP.
Thinking of replacing it with a pure sine wave model but I wonder if there is a model with protection against RP connection?

Anybody bought such a beast?

 

[Spark123]

Can you stick a diode in series with the supply?

 

[Echo the husky]

What sort of wattage are we talking here? 12V supply, 36V etc?

 

[Deryck Tintagel]

Can you stick a diode in series with the supply?

or a suitable bridge rectifier - no need to worry s it will work either way round. Downside is that you'll have two diode drops in circuit

 

[ericmark]

Can you stick a diode in series with the supply?

or a suitable bridge rectifier - no need to worry s it will work either way round. Downside is that you'll have two diode drops in circuit

If anything like the one I got for my son looking at 300 amp I had enough problems finding an isolator never mind a diode to work with that current. However each FET had it's own fuse. There was a 16A blade fuse to each FET what caused the problem with his was the fuse is used to balance the load on each FET and there was no indicator to tell you when the fuse had blown, so as each one went the load on the remaining went up and slowly but surely it gave out expensive blue smoke, his was the simulated sine wave type 3 kVA with 6 kVA peak.

 

[bernardgreen]

never mind a diode to work with that current

Polarity protection can be achieved with a contactor ( though 300 amp is a big one ) between input socket and invertor. The coil of the contactor is fed from the DC supply via a couple of diodes in series ( in case one fails short circuit ). If the supply is reverse polarity the contactor remains open and no supply reaches the invertor.

 

[ericmark]

never mind a diode to work with that current

Polarity protection can be achieved with a contactor ( though 300 amp is a big one ) between input socket and invertor. The coil of the contactor is fed from the DC supply via a couple of diodes in series ( in case one fails short circuit ). If the supply is reverse polarity the contactor remains open and no supply reaches the invertor.

At 300A one does not simply clip it onto a car battery, it is hard wired, so just as easy to get the protector circuit wrong way around as the inverter. I think a big stick is a better tool? Unless son it bigger than you of course.

 

[ban-all-sheds]

If #2 son is financially independent, he can buy the replacement.

 

[securespark]

Ban sadly not!
Echo 12V sub 1kW inverter.

 

[davelx]

Ban sadly not!
Echo 12V sub 1kW inverter.

So around 100A at 80% efficiency. A silicon diode with that rating will have a forward voltage drop (Vf) of around 1.5V and will dissipate 150W at that current - needs a very big diode and big heatsink/fan! Using a Schottky diode is better as Vf is about 0.7V at that current, so only 70W to waste/get rid of!

Using a bridge rectifier is worse - double the dissipation as you have two Vf's

 

[ericmark]

So as I expected not really an option of using a diode. In hind sight and hind sight is easy, we should have converted the narrow boat to 24 volt it would have halved the cables sizes, and the inverter would have run far better, in the Falklands our battery back-up for office UPS was 48 volt a far better voltage to use.

I converted my van to 12 - 0 - 12 volt so I could jump start wagons but still retain 12 volt for van. The cranes I worked on had built into the alternator a three phase transformer to give both 12 and 24 volt outputs Delco Remy alternator brushless. But it depends what it is used for, in the caravan moving to 24 volt would be easy, but in a car not really an option.

 

[endecotp]

A "crowbar" is the traditional way to do this - you put the diode in parallel with the inverter, after a fuse. Connect it in reverse polarity and the fuse blows. Some care is obviously required to make sure the diode doesn't blow up before the fuse blows, and that the fuse doesn't blow due to normal inrush current.

An alternative is to put a power mosfet in series, aranged so that it is off when polarity is wrong. It will have a much lower voltage drop than a diode. Do a google image search for "reverse polarity protection" to see some circuits. E.g. http://www.instructables.com/id/Reverse-polarity-protection-for-your-circuit-with/

 

[Jackrae]

On the basis that every invertor requires a "switch" in its supply line, the diode + contactor suggested by Bernard is the obvious and simplest way to go. This method is also advocated by 4QD. As to "so just as easy to get the protector circuit wrong way around as the inverter" isn't really worthy of comment if you take care in the build. It becomes irrelevant once it is built correctly; after which reverse connection will prevent the contactor from closing.

 

[ericmark]

On the basis that every invertor requires a "switch" in its supply line, the diode + contactor suggested by Bernard is the obvious and simplest way to go. This method is also advocated by 4QD. As to "so just as easy to get the protector circuit wrong way around as the inverter" isn't really worthy of comment if you take care in the build. It becomes irrelevant once it is built correctly; after which reverse connection will prevent the contactor from closing.

At least the one I had there was no switch. I wanted a method to disconnect quickly and I used welding cable bayonet connectors as I could only find battery isolators rated at 100A so it seemed the easy way.

The current is massive and the terminals small we used two 16mm² in parallel at 50mm² put too much strain on the terminals.

OK the one in question is far smaller only 1kVA instead of 3kVA but still drawing very high current. Using fork lift stuff one could use diodes and relays but most fork lifts are at least 24 volt. Finding a 12 volt contactor small enough to go with the inverter yet able to take 100 amp is rather a tall order. Gordon Equipments Limited, (Durite) likely have relays big enough

but as you can see they are not small. And at
£156.25 likely costs nearly the same as the whole inverter.

 

[Jackrae]

here's one at 1/3 of the price via googling "300A 12v dc contactor"
http://www.12voltplanet.co.uk/12v-300a-high-performance-heavy-duty-relay.html