Inverter Smoke

Hi there,

Recently i borrowed someones 12v inverter for use for soldering something on an automobile. The inverter is a Vector Maxx type 400 watt unit, they are colored red.

What happened was after connecting to the battery via the supplied clips, then turning the unit on, then after about 5 seconds smoke literally starts pouring out of the fan area as the fan blows the smoke out of the enclosure. It looks like the unit is totally blown, but that's not the case.
Connecting the soldering gun to the output 120vac connector and holding the trigger 'on' to get the gun to heat up, then turning on the inverter with the power switch, the gun works fine. So the inverter works just fine when it has a load. With no load, smoke comes out. It looks like the smoke from a resistor but i cant take the unit apart and examine it because it is not my unit.

Now many switching power supplies (this is also a switcher basically) need a load to work properly so i can estimate that this inverter needs a load too, but because it is a decent model i would have thought they would have built in some protection to keep it from smoking with no load, such as a auto shut down like PC power supplies have when there is no load.

So you think this unit is faulty or could it be that they overlooked something during the design phase? I think i've read about this in the past on the web but cant remember the web site.

Also, a schematic of this unit or a similar unit would be nice so i can examine it in detail and understand the problem better.

This https://www.electro-tech-online.com/custompdfs/2012/05/VEC02420MANUAL.pdf may be a manual. Inverters shouldn't smoke with no load.

You did say supplied clips which basically rules out poor connections or inadequate wire gauge. The protections seem to have it covered although I don't think I saw a quiescent current spec.

Hi,

Hey thanks KeepIt it helps to at least look at the manual. From what i have read there, it appears that some sort of fluke must have happened that caused the smoke because they make no mention of any minimum load. Maybe something fell into the fan port and temporarily burned when it was turned on. I am getting a report now that the smoke has stopped so i guess it was not something permanent.

Any more information would be nice too if it ever shows up somewhere, like a schematic. I'd like to see what they are doing. Apparently they use a DC to DC converter followed by an H bridge. The DC to DC converter gets the 12v boosted up to line levels, then the H bridge provides the dual polarity needed for symmetrical AC. The pseudo sine output is just a pulse without any attempt to really mimic a sine wave. They are calling this a 'modified' sine wave (chuckle).

Modified sine is better than a square wave which would be a pulse. In this case, you could have a "few" different height pulses. It also means when you measure with a TRMS meter, you'll get 120 VAC.

You can barely get a schematic for a toaster these days.

Hi,

Well actually what they mean is a single pulse, where the pulse does not take up the whole half cycle time period like a square wave does. So it's a delay, a positive pulse, a 2x delay, a negative pulse, a 2x delay, a positive pulse, etc., etc. There's no amplitude difference because that's harder to obtain, and the only pulse width variation is to regulate RMS output voltage.

Here's a better description of a "modified sine wave": **broken link removed** I earned something too.

Hi,


Yes but there are many ways of doing it and the cheap ones dont bother to do that much. The cheap ones only put out one pulse every half cycle and sometimes vary the pulse width to regulate the output. The better ones will have multiple pulses that vary in width.
The vector one mentioned here puts out only one pulse every half cycle.

The smoke is probably an undersized snubber resistor.