A neighbor gave me a little Coleman 400 watt power inverter to try and
repair and I found 2 of the MOSFETS bad and one of the driver
transistors controlling the upper side of the bridge shorted. Replaced
the bad parts and got it working with a small 25 watt light as a load.

Unfortunately it failed again with same symtoms when installed in his
motor home and connected to a 13 inch TV set. As it failed, the fault
light and beeper came on indicating a fault and the input voltage was
only 10 volts. I assume the fault was due to too much drop in the long
wires. But it was too late to shut it off. The unit continued to
indicate a fault after the load was removed.

Is this one of the usual failure modes where the input is too low to
support the output? I would think the unit should be protected from
overloads and low voltage, but apperently it fails easily when the
battery voltage is too low? Also I don't know if a battery charger was
attached to the battery or what condition the battery was in. The
connection at the unit read 12.3 volts with no load and 10 volts after
it failed. i don't know how much current was being drawn after it
failed.

This thing is only worth about $40 and I see them on sale at Harbor
Freight for $19.99 so it's probably not worth spending too much time
on. If he buys another unit, what brand would you recommend for long
life and ruggedness? Something more expensive perhaps?

-Bill

"BperryB" wrote in message
ups.com...
A neighbor gave me a little Coleman 400 watt power inverter to try and
repair and I found 2 of the MOSFETS bad and one of the driver
transistors controlling the upper side of the bridge shorted. Replaced
the bad parts and got it working with a small 25 watt light as a load.

Unfortunately it failed again with same symtoms when installed in his
motor home and connected to a 13 inch TV set. As it failed, the fault
light and beeper came on indicating a fault and the input voltage was
only 10 volts. I assume the fault was due to too much drop in the long
wires. But it was too late to shut it off. The unit continued to
indicate a fault after the load was removed.

Is this one of the usual failure modes where the input is too low to
support the output? I would think the unit should be protected from
overloads and low voltage, but apperently it fails easily when the
battery voltage is too low? Also I don't know if a battery charger was
attached to the battery or what condition the battery was in. The
connection at the unit read 12.3 volts with no load and 10 volts after
it failed. i don't know how much current was being drawn after it
failed.

This thing is only worth about $40 and I see them on sale at Harbor
Freight for $19.99 so it's probably not worth spending too much time
on. If he buys another unit, what brand would you recommend for long
life and ruggedness? Something more expensive perhaps?

Do not waste any more time (unless you enjoy troubleshooting). Tell him to
replace it and select one with better specs (if the size and cost are not a
problem).

Most likely some other parts that you did not change are also defective.
These are not worth servicing. I would suggest to buy a much better quality
inverter if you want it to last. You will get what you pay for.

--

JANA
_____


"BperryB" wrote in message
ups.com...
A neighbor gave me a little Coleman 400 watt power inverter to try and
repair and I found 2 of the MOSFETS bad and one of the driver
transistors controlling the upper side of the bridge shorted. Replaced
the bad parts and got it working with a small 25 watt light as a load.

Unfortunately it failed again with same symtoms when installed in his
motor home and connected to a 13 inch TV set. As it failed, the fault
light and beeper came on indicating a fault and the input voltage was
only 10 volts. I assume the fault was due to too much drop in the long
wires. But it was too late to shut it off. The unit continued to
indicate a fault after the load was removed.

Is this one of the usual failure modes where the input is too low to
support the output? I would think the unit should be protected from
overloads and low voltage, but apperently it fails easily when the
battery voltage is too low? Also I don't know if a battery charger was
attached to the battery or what condition the battery was in. The
connection at the unit read 12.3 volts with no load and 10 volts after
it failed. i don't know how much current was being drawn after it
failed.

This thing is only worth about $40 and I see them on sale at Harbor
Freight for $19.99 so it's probably not worth spending too much time
on. If he buys another unit, what brand would you recommend for long
life and ruggedness? Something more expensive perhaps?

-Bill

JANA wrote:
Most likely some other parts that you did not change are also defective.
These are not worth servicing. I would suggest to buy a much better quality
inverter if you want it to last. You will get what you pay for.



While I would agree about buying a better one, that doesn't mean these
aren't worth servicing if you can do it yourself at home. They're not
very complicated and parts are not expensive. It can be worthwhile to
fix to keep it out of the landfill.

James Sweet wrote:
JANA wrote:
Most likely some other parts that you did not change are also defective.
These are not worth servicing. I would suggest to buy a much better quality
inverter if you want it to last. You will get what you pay for.



While I would agree about buying a better one, that doesn't mean these
aren't worth servicing if you can do it yourself at home. They're not
very complicated and parts are not expensive. It can be worthwhile to
fix to keep it out of the landfill.

Well, I'd like to fix it, but I don't have the schematic diagram, and
there are 14 small transistors in there and a whole bunch of small
diodes. The 140 volt DC supply appears to be working but the modified
sine waveforms on the gates of the FETs are not correct. One is
correct, and another looks like a square wave with a very slight drop
that should be zero, and another two are low amplitude. I ohmed out a
few transistors but they all look good. Hard to figure out without a
schematic.

I went shopping at Harbor Freight and found a 1000 watt unit for $100
and a 700 watt for $50 and a 400 watt for $19.95.

-Bill

It shouldnt fail IMMEDIATELY with a low input voltage-- low input
voltage is going to lead to higher than usual currents, and if the
design is any good... well, maybe that's the problem.

Also a TV is not a happy load-- lots of peak current draw at the top
of each cycle.

Probably should shop for a higher-quality inverter, say one with a
one-year guarantee.

Harbor Freight stuff is cheap, which often translates to "marginal
quality".

Ancient_Hacker wrote:
It shouldnt fail IMMEDIATELY with a low input voltage-- low input
voltage is going to lead to higher than usual currents, and if the
design is any good... well, maybe that's the problem.

Also a TV is not a happy load-- lots of peak current draw at the top
of each cycle.

Probably should shop for a higher-quality inverter, say one with a
one-year guarantee.

Harbor Freight stuff is cheap, which often translates to "marginal
quality".

Well, I think the Harbor Freight design is good (except for the circuit
board), it's just the workmanship that needs improvement. Looking at
the circuit board, the traces are only separated by less than 1/64 inch
and there are many little solder drops laying in the groves between the
traces. I tried to pick them all out but missed a few. Also the
component leads are bent over so they almost short to the adjacient
traces. But I bet the design is good if properly built. They must have
tested this in the lab and subjected it to all the possible problems,
input voltage low, high, shorted output, over temperature, etc.

You can probably buy 5 units, and one will keep working for a long
time.

-Bill