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From: John De Armond
Subject: Re: Can I disconnect DC-AC inverter cooling fan to reduce noise???
Message-ID: <>
Date: Thu, 14 Aug 2003 12:55:49 -0400

There are a couple of things you can do.  One is add a Klixon-type thermal
switch to the fan circuit so that the fan only runs where there is enough heat
to require it.  You'll need to find out what generates the most heat and
attach the thermostat to that.  It seems logical that the power FETs would be
the biggest heat source but not always.  In several cheap inverters I've
examined, the bank of filter caps actually gets hotter than the FETs.

The other solution is a proportional fan.  Most of the major muffin fan
companies (EBM, for example) make fans in which a built-in controller varies
the speed in accordance to the demand of an external sensor.  These are not
terribly expensive.

I used the first technique on my 400 watt inverter.

My Vector 1kw inverter the fan does not run until a substantial load is drawn.
It doesn't appear to be temperature-related, only load.  I'm sure that with a
little poking around I could change the calibration of that circuit.  You
might want to check to make sure your inverter doesn't have this type of fan
control before making any changes.


On 14 Aug 2003 08:14:20 -0700, (coyotefred) wrote:

>I have a small, off-the-grid cabin I'd like to add a very basic solar
>electric system to in the near future.  This would be to run only a
>few low-wattage items, specifically 1 or 2 compact flourescent or LED
>light bulbs, a small portable radio/cassette/CD player, and a small
>laptop computer.  I figured a system like a decent 75W panel, a basic
>charge controller, a couple of deep-cycle golf cart batteries and an
>I already have a coleman powermate 750W inverter that I use to run
>some power tools off my truck battery for onsite construction work.
>But it has a very noisy cooling fan, and the whole point of the cabin
>is to get a little peace & quiet.  The fan isn't a problem during the
>day, but at night when things are quiet it is really noticeable if you
>were using it to, say, just power a light.
>So I'm wondering how foolish it would be to disconnect (or add a
>simple switch) the cooling fan.  I know I know it was put there for a
>purpose...and I would imagine I would be shortening the life of the
>thing somewhat by having the components run at higher temps than they
>ordinarily would.  But a somewhat shorter life is a decent tradeoff
>for me to reduce the noise.  And I thought that maybe the low-wattage
>loads I had in mind would be low enough to keep the thing from running
>so hot in the first place.
>But if there are more serious problems with doing this, it obviously
>makes sense to look at other options.  And yes, I have considered
>trying to run straight DC appliances and eliminate the need for an
>inverter, but I've yet to find basic appliances like the
>radio/cassette/CD in DC, or how to run the computer off that, etc.
>'Any thoughts appreciated-

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dc to ac power inverters
Message-ID: <>
Date: Fri, 10 Jan 2003 21:12:27 -0500

On Fri, 10 Jan 2003 12:14:16 -0500, GaryO <> wrote:

>Good information - this may save me some problems.  I recently picked
>up the Vector 350W inverter from Wal-Mart.  I'm planing to install
>this with a transfer relay (Dayton 1A491) to power one branch circuit
>as needed for the TV and a notebook computer.  While I don't care for
>the fan noise, I'm hoping that it will not be audible when mounted
>within a cabinet (plenty of airflow available).

If you install a transfer switch or relay, you MUST switch both the neutral
and the hot leg.  The reason is that when you're not plugged into shore power,
the hot and neutral legs are connected at the generator.  That will prevent
the vector from working.

I don't like the fan noise either.  I opened mine up and RTV'd a small
Klixon-type disc thermostat to the heat sink near the power FETS.  This turns
the fan on and off.  I got a 120 deg F unit.  For my 400 watt Sam's Club
special, running a VCR and TV will not cause the fan to turn on.  Mount the
inverter vertically so that convection air can flow through the unit and a fan
might not be necessary.

Another alternative is a fan control IC.  I have a couple of these laying
around waiting for me to install them in the inverters:

All you need to make this chip work is to put power on the Vdd pin, ground on
the GND pin and the fan on the *FANOUT pin.  The rest can be left
disconnected.  Just RTV the chip to the heat sink, dead bug style, and solder
the wires to the pins sticking up.  You can buy this chip directly from Maxim
(click out of this page to the e-store page for under $2 ea.

>One item I've noted in the manual for this inverter (also observed in
>several other inverter manuals), is the following:
>----------Begin Quote--------------
>CAUTION: Rechargeable Appliances
>Certain rechargeable devices are designed to be recharged by plugging
>them directly into an AC receptacle.  These devices may damage the
>inverter.  Do not use the inverter to recharge items that can be
>plugged directly into an AC receptacle.
>This problem does not occur with the majority of battery-operated
>equipment.  Most of these devices use a separate charger or
>transformer that is plugged into an AC receptacle.  The MAXX350SST is
>easily capable of running most chargers and transformers.
>-----------End Quote--------------
>Unfortunately the manufacturer is not very specific about what the
>actual problem is.  Do you have any insight into this?

Yes.  Some chargers do not contain a transformer and instead rely on a series
capacitor to limit the current going to the battery.  Most all cordless drill
chargers (except Makita) work this way.  If your charger is very light weight,
this is the way it works.

problem is, this scheme is not compatible with the inverter output.  The way
the inverter works, a high frequency DC/DC inverter steps the 12 volts up to
about 145 volts DC which is filtered/stored in capacitors.  Then a set of
power FETs switch this DC, first one polarity and then the other, to the
outlet at a 60 hz rate.  With ordinary loads, this works fine, as they are
either resistive or inductive, neither of which will draw too much current.
But the capacitor instantaneously looks like a short to the FET.  There is a
charged capacitor in the inverter on one side of the fet and a cap that looks
like a short on the other.

If the inverter is well made, the current sense will turn off the FET before
any damage is done.  If it isn't, the blue smoke leaks out of the FETs.  And
very possibly the rectifiers in the battery charger.

I have gotten my DeWalt charger to work on an inverter with the aid of some
external hardware.  If an inductor is included in series between the inverter
and charger it will limit the peak current.  I made up a little box containing
a line cord, an outlet and a 12.6 volt filament transformer from radio shack.
I hooked the secondary (low voltage) winding in series with the hot leg and
left the 120 volt side open (insulate well, as there will be high voltage on
this winding.  This is enough inductance to keep both the inverter and the
charger happy.

You could probably achieve the same effect with a small 120 volt isolation
transformer of the type TV repair techs use to isolate hot TV chassis for
servicing.  But the 12 volt Rat Shack tranny is a LOT cheaper.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dc to ac power inverters
Message-ID: <>
Date: Sat, 11 Jan 2003 14:04:42 -0500

On Fri, 10 Jan 2003 23:03:08 -0500, GaryO <> wrote:

>I am dissapointed with the idle current of this inverter (Vector MAXX
>350W).  The packaging states 0.4A, the booklet states 0.3A in one
>place and 0.4A in another.  I measure 0.56A, which seems excessive for
>a 350W inverter.  :-(

Is that with the fan running?  I bet 100 mils or more is the fan.

hey, ya only get so much for $29 :-)  These cheap inverters do not have any
sort of circuitry that cuts the DC/DC inverter back at light/no load.  The
more expensive ones do.

One thing I know of that consumes a bit of power is the high voltage DC filter
cap bleeder resistor.  This thing is there to make sure the caps are
discharged before anyone can get the case open and get shocked.  Not necessary
to the circuit's functioning.  If you can find the resistor and disconnect it,
the idle current will drop considerably.

Not knowing your specific inverter I can't tell you where it is but it will be
a discrete (not surface mount) resister that is fairly large and will be warm
after some bit of operation.  When you find it, probe it with a voltmeter
(about 300 volts) to make sure it's the right one, then just snip one leg and
bend it back a little.  Then remember that the caps will stay charged for
hours so don't go touching the PCB without proper precautions.


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