inverter repair

I have inherited a 300w 12v DC to 230vAC inverter that has no output. I was told that the internal fuse was the problem but this is entire and fine.I havent stripped it right down but the visible surface of the board has some smoke marks around what I take to be the base of output transistors but I have gone no further than that.I have little understanding of how these work but can work my way round boards and replace parts etc if I am told what needs doing.

Now,on this pathetically scant bit of info,does it sound like this is worth pursuing ? What are the usual faults with these things and the causes thereof ? And what does one do about it,bar bin the thing and get another ?

Check the price of the components that "look" bad, or have traces of escaped magic smoke. If those parts are cheap enough to you to replace them, (or take a gamble) then you can pursue removing those parts and checking them to see if they are indeed bad.

Typically it is the output side that goes on the inverters so I would say you are on the right track.

Awd,thanks for taking the time to reply to this along with my other query,your effort is much appreciated.

   I worked for some years as a sound engineer and had to cope with many large power amps that let the magic smoke out.When dealing with 20k plus (RMS) of sound in excess of 100dB for 12 hour sessions this tends to happen a lot.Normally when you leave the controls for 1 minute for a call of nature.In a most expensive fashion.The smoke,that is.And none times out of ten it was the output tansistors. Hence the statement that I could find my way around boards.

Having zero experience of these inverter devices (as a seperate unit at least) it is very useful to hear that it is the output side that goes.The one I am looking at appears to have 4 flat square ouput devices marked SAMWIN 0648 and SW5ONO6, the latter presumably the designation,2 on the side I am looking at show heat damage.

   The thing is,what would have caused this? The usual overload situation ? If so it may be worth the fixing. If its another internal fault,probably not worth the time.

   Obviously one cannot give definitive answers on an item one has not seen or had access to but anecdotal knowledge and experience is always useful.

The board is labeled SEEWIN SI 400w V3, 2005.11.25.

The item is case labeled SkyTronic model No.651536

They are probably MOSFETs for each "push=pull" output. They only need to be of a few Amps rating (300W ~360VA/120V = 3A chose 10A or larger.

They probably merely broke down.
Normally FETS are unlikely to damage control circuitry (though I can recall their breakdown modes) whereas transistors are likely to damage their inputs (control circuitry).
But maybe other circuity has gone too (eg, transient absorbing devices, voltage feedback etc).
And faulty control signals may also blow output devices, though that is unlikely.   


The last inverter I bought was 600W (RMS) for ~$60.

The FETs in an inverter are usually on the primary side of the transformer. Usually 50 to 75 amp 50 volt devices. IRF-Z44 IRF-3205 Etc.

Stupid I know but I thought I "blew" one last week, it was the fuse in the cigarette lighter adapter!

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Not surprising considering common inverter loads...


IMO, safety-wise, a "mere 150W" is pushing cig sockets. Even half that (7A) can be risky....
I withdraw the preceding sentences from this block should they in any way infer the duty-of-care responsibility to me!

Times like that I admire Engel and others that use thermal 10A fuses in their cig-plugs. Sure, it promulgates endless posts about what fuse to replace with, or if thermal is necessary (LOL!), but it has certainly prevented more than a few fires....
{And since that is public knowledge, I am off the hook!)

On the subject of fires.....

   the first inverter that I bought was a mini 75w job , from Asda,a uk supermarket owned by Walmart. It was laying in the cubby box for a few months in which time I lost the receipt.When I finally came to use it with a phone charger, as soon as I plugged it in, it began to smoke and rapidly caught fire....NOT what I was expecting at all.

When I returned it to the shop,having checked UK purchase law, I was told that without the receipt they would do nothing. I produced a printout of the relevent law to the poor hapless,blameless lad on the customer service desk....at which point he called his supervisor.The super told me the same,etc etc,I told him that if they could tell me the month that these items were on a sale stand, I could produce the bank statement with the card payment listed,which in UK law is enough proof of purchase.He told me he "couldnt be bothered" as he was too busy.

Well,I wasnt too bothered as it cost less than a tenner and I had no interest in causing trouble for some poor overworked sap on minimum wage but I DID consider that this was a safety issue and asked for the manufacturers name so I could contact them direct (its not on the label) but he wasnt inclined to help with that one.What the hell, I dont want to contact HIS boss and get him in the doodie but I DO want to stop other people having a fire in their car that they may be less prepared for than I am.

Funny thing,it didnt blow the fuse......

Thanks guys. I figured that the FETs were half up,half down like in our old power amps but wasnt sure. I have found a place to buy 10 for under a tenner uk delivered, so that gives me a full new set (4+4) and 2 spares to play with.

IF,a big if, I get this working, I was planning to mount it somewhere with a good airflow and possibly use an extra fan from a PC case. Then hard wire it,fused, to the second battery in my 300tdi discovery. Following the old rule of thumb that cheap electrical goods are usually about a quarter as powerful as claimed (certainly the case with PA power amps) I was hoping that it would give a decent 75w supply to run a basic laptop when away from home.Should it work out I will consider another one to go in my caravan trailer.

I will update the results as they happen.

Had the same problem once with a 13month old Phillips Razor via Tesco. In the end I got a friend to send me over the (half the price) Norelco from the US, same razor, no problems but coming back to your last post, within reason you get what you pay for, it's a shame that in the UK we don't have the same levels of consumer protection, especially on the safety factor you quoted as the US.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Oh yes ! Forgot. Any tips on soldering in FETS, other than heat sink on the leg, that I should know ?

I dont mind my use of the word doodie being edited.... but replacing it with "doodie" ??

Yes, you get what you pay for.........but one DOES expect an item to function briefly before catching fire !

Didn't you know that "doodie" was a name for a part of the Doodie bird's body?
Ah well here's where I get demoted yet again to "standard".


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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Oh, I'm not sure, anyone old enough to remember unbranded car audio from the early 70s?

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

No I was wrong, it was the legless Goollie bird, at least two of you will get that one.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Last chicken in the shop....turkey neck ?

Gollie Gosh!

A plethora of notifications - read last "Last chicken in the shop....turkey neck ?". Oh no - war has broken out...!   

Alas no. Why always expect the worst?


I have several high-current MOSFETs I bought for $2-$3. I figure why not use them with their 80A - 120A ratings instead of relays, else to drive ordinary relays (ie, the high-impedance buffer device for any charge-signal to drive the UIBI battery isolator relay?).
I lack the experience, but from what I understand, essentially a FET is a FET from a signal POV - ie, nano-Amps (if that!) into the Gate 5V above the Source (with a pull-down resistor to S to ensure turn-off when floating!), and the output is chosen to handle the Amperage... at least for simple switching applications like this.


As to warranties on equipment - the owner has an onus of care and - as I found - things like "cleaning" are by the owner, not any vendor's maintenance contract. Hence if a server failed because WE did not clean the filters and remove the dust etc, WE were liable. (That made oft-logic sense, but it was an awakening at the time!)

But I wonder... "laying in the cubby box for a few months..." - if smoking started at initial use, could there have been moisture, contaminants, dust, and the ever-present possum poo, etc.


Alas contaminants in HV AC systems are usually fatal... to the components that is. (Not that fatalities cannot occur!)
So bad storage & exposure may be a contributing factor.....

But inverters are usually well sealed with a meshed cooling fan as the only unsealed area. (But yeah - I've seen possum-wiz cause enough electrical strife!)

Except for larger inverters (say >600W), other than soft-drinks and spiders, I'd suspect moisture. But dust/dirt AND moisture - a bad combination!

Indeed.....moisture and dirt can be fatal to high power outputs.Back in my sound engineer days we had a particular problem caused by what was known (at the time) as "rave juice" and "gak".

Rave juice was the moisture and grease given off by hundreds of sweating ravers,steam and condensation and the like that would condense when we took our ampliers out into the cold truck at the end of the show.

   "Gak" in this instance was the dust,skin paricles and general airborne dirt kicked up by the said ravers and drawn through the amps by the cooling fans.

These two lovely substances would coalesce and settle on the interior (and exterior) surfaces of all the equipment leaving a greasy,moist gritty paste that would cause shorts and untold damage if left. It had to be groken up with alcohol and/or switch cleaner and blown out with air lines between jobs.Which was nice.

Yes,Spark,as you rightly say,storage in the cubby of a truck for months could well lead to a similar situation.However,in this case bearing in mind previous experience the unit had been dried out over the air blowers on hot and then blown dust free with a little in car compressor,for just this reason.

   I dont expect any mileage from the shop as per refunds....the money wasnt much and is long gone.What I DO care about is some young kid having a fire in their car.....all I want to do is warn the manufacturers that there may be some problem.That way my conscience is clean.Ish.

Anyway.....
        I too lack the experience of designing and building electronic devices off my own back but can work to a plan. I am very interested in the use of FETS to control switching relays.....could this be the way to a low amperage switch for the alternator charge light when used to trigger other relays ?

Yes Oldie as you and Mr I. the Dragonslayer (in joke) well know I also can't design a circuit to save my life but I can follow them easily enough.
About 10 years ago we were having a similar conversation about thyristors as a relay substitute, nowadays of course FETs. I think the only reason for t6he continued use of relays is ease of understanding (in theory) for the average mechanic.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

meanwhile,on the subject of "wiz" and electronics......

....many years ago I sold a high end VCR that my parents had passed on to me to a friend on the eve of leaving the country.Upon returning,years later,I saw said mate and was told the VCR worked for about a day and a half before giving up.Being a high end item he sent it for repair.The quote he got back listed damage across all the horizontal boards,seemingly caused by an "acrid corrosive substance" of unknown origin but exceptional destructive power.Oh dear ! What I hadnt considered was that at the time of the VCR use by my parents,my mother was a breeder of persian cats.And these pedigree things had an aversion to using normal toilet facilities....god forbid they would lower their grandiose backsides,outside.So they found the most novel places to go.On questioning the parent on this issue I was told that the then UK grand champion silver tabby would only do her business in two places.....the back of the TV or on the top of the VCR.Given the position and the effect of gravity,the prior would lead to the latter in any case.Thus,the acrid,corrosive substance.In your vernacular,'Sparks, "wiz".

Or the early days of CDs, about 1988, daughter then aged 8 discovered bless her that strawberry jam (jello to our US friends) and CD discs don't go together very well. Or last year spending about 1 hour stripping the Sat Nav disc drive from a Nissan Primera (again, Altima to our US friends) to discover the two £1 coins blocking the transport mech.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Howie, whereabouts do I look for a circuit design to achieve this kind of thing ?

In another thread I was asking more opinions on split charge designs,specifically the use of a relay triggered by the charge light circuit to power a fusebox of supplies to split charge relays and other like stuff. From a previous comment by Oldspark I had developed a concern that the charge light circuit may not react well to powering the coil of a regular 12v split charge relay. And so proposed using a smaller relay as above.

Then I began to worry that the circuit may not even take powering a normal 12v automotive relay of 20a or so rating.'Sparky suggested I use a relay with a very low coil current,such as below 500Ma but I am having trouble finding one.Generally I like to re-use parts from other items where I can but in this case cant even find one new....well,not for sensible money,anyway.

Then   the subject of using a FET to trigger the coil of the first relay came up.I thought that it might be easier on the alternator charge light circuit to use the small current that a FET would need to initiate the next relay. Probably wrong,in my case,but its what I thought........

    ....the alternator concerned is a magnetti marelli version of the lucas A127,a 100a unit as fitted to a LR discovery 300tdi in this case,and used in many other vehicles by other manufacturers.

Howie,with my transatlantic pedant hat on,my US born'n'bred wife,reading over my shoulder informs me that jam would be jell-y   whereas jell-o   would be our jell-y.......

....sorry mate,she made me do it! You know how it is.......

Oldspark is the man, it's beyond my knowledge.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

We're out of synch here but....
I used to "commute" from Ft. Lauderdale to Houston Tx. to work, I always carried a bag of full strength UK sold Tetleys, catering size, I'm also bemused at hoods and bonnets, trunks and boots and wings and fenders; especially on this site.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Sorry Lads, if been missing out...
Rave Juice. In all its flavors. Most destructive.


Shirker - I didn't mean to scare you off with my warnings about over-stressing the charge lamp circuit, but whereas traditional electronic regulators would sink usually well over an Amp (they use to "test" up to 6 dash warning lights; 6x2W = 12V = 1A. 3W globes meant 6x3W = 18W = 1.5A.
But newer models may only supply a small signal if they connect to ECUs etc which then control & power the charge light etc.


I think it was about 6 months ago I wrote that 6 months earlier I had been intending to add a plain old $3 MOSFET as a buffer for the relay. (But why bother with a 30A or 60A relay? - these MOSFETs handle 120A. [Answer - like Howard said - relays are plain, simple, robust & proven. Besides, MOSFETs usually have a higher resistance than contacts (is it ~7 mOhm vs 2mOhm for contacts?), so for those extra milli-volts.... ]     
Anyhow, I still haven't.

FETs are perfect - almost infinite input impedance (ie, only takes nano-Amps; well under 1uA) so they don't load the source.
Yet they can switch (and modulate) many tens of Amps.

They used to be sensitive to static, but modern MOSFETs usually have internal spike quenching diodes so that spikes get shorted to the +ve or -ve (GND) supply. It's a simple noise (spike) limiting technique no different to those reverse-biased quenching diodes for relays - a diode with its Kathode (Line end - as per -|<- = Kathode end.... <--- that way} to #86, and the other (anode) end to #85.

But it's worth being aware that they may be static sensitive. (Oh, the memories of CMOS go sailing past.... No Howard, that's not not a dyslexic UNIX command like CHMOS!)


I can go into battery isolators separately. You may have read my rants about all the bullsh surrounding isolators, though it seems that false claims (like priority charging or charge sensing (ie, battery capacity)) are being cleaned up.

The one advantage with a voltage sensing isolator is it independence from the alternator circuit. (But hence too its downfall - it is trying to determine what the alternator/system is doing. The UIBI gets its info from the alternator itself - no second guessing, no hysteresis or switching delays).   

And if signal independence from the alternator was only to not load its regulator, why not use a FET - even a transistor - and eliminate all that voltage sensing with varying delays and adjustments and "non intelligent" behavior?

I wonder if voltage sensing isolators were developed for marine use {ie, stator systems which have a permanent magnet rotor; hence no rotor regulation and no charge light}, and the auto market misinterpreted?   Nah - someone wanted to make money!


Ooops - I said later... or separately...

Remind me about "priority charging" - that's assuming you don't know why it's double-bovine-poo...
The first cake for how are they sensing battery capacity? (Voltage? Bullsh - it's simply a time delay required for voltage-sensed switching)
The second pattie is for the logic - why limit your output or TOTAL recharge rate? Valid cases rare - ie, where supplemental charging is required to keep the batteries in good health (ie, short trips or undersized alternators).

Got you Peter, I think I had an EDU on my Mits, = Electronic Destruct Unit rather than Electronic Control unit.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.