DIYbanter - View Single Post

mike[_22_]

On 10/12/2017 12:27 PM, wrote:
I have a 300W, 12VDC to 120VAC inverter. It's worked fine in my car for
years, to power my laptop computer, a medical device I need when I am
"on the road", and even some small power tools. Recently it started
acting weird. Sometimes it wont turn on, (LED is lit). Or it keeps
cycling on and off. Often, I have to plug it into the cig lighter in my
car repeated times to get it to work.

I tore it apart the other day and after removing it from the aluminum
case, I carefully checked for loose solder joints, using a magnifying
glass, wiggling parts, looking for any burnt or other obvious bad parts.
Everything looked ok.

With it out of the case, I plugged it in, and found it worked perfectly.
I used it numerous times out of the case, and it worked each and every
time. However, I was careful not to load it too hard, because the case
is used as a heatsink, and outside the case, the transistors are not
heat sinked, and I could feel them getting warm, but not hot, becuase
the load was low.

After several days of working perfectly, I put it back in the case, and
immediately it failed to work. I was careful to make sure the power cord
was not applying pressure to the circuit board, and everything else was
correctly mounted into the case.

One thing I noted is that the board slides into slots on that molded
alum case, and there are solder joints extremely close to that edge.

My thinking is that even though it worked fine for years, one of them
solder joints is touching the case and grounding out. But why it did not
do that in the past makes little sense. If I could, I'd put some
electrical tape along that edge, but it slides into a groove with no
spare room.

Since I need this badly, for medical needs, I decided to just buy a new
one . I got a 400W, and thought that would work fine. However, that one
has a built in buzzer which is loud and extremely annoying. As soon as I
plug it in, it "screams" at me until it's "settled". Besides annoying, I
often use these inverters when I go camping, with a portable 12V
battery, and having this think scream in the middle of the night is not
acceptable. Needless to say, I'm returning it for a refund.

So, until I am able to find one without that annoying buzzer, I need to
use my old one. All I can think is to leave it out of the original case,
mount some heat sinks to the transistors and put it into a
non-conductive case, made of plastic or wood.

I am posting this because I wonder if anyone knows what else could cause
it to fail to work inside the case, yet work perfectly outside. The
amount of work and cost of another case is almost not worth all the
trouble to re-case it, but it seems that most of the new ones have those
annoying buzzers. Why they have that, is beyond me, but I refuse to have
to cope with that noise.

The transistors are likely insulated from the case. Check for insulator
failure.
Painting the board to insulate it from the slots in the case is unlikely
to last for long. Use something more robust like Kapton tape.