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Using less than 12 dollars worth of parts, get more power / fix your broken power inverter

Step 1: Diagnosing the Problem

This instructable is a guide for repairing/increasing the output power of a simple dc-AC power converter (this instructable address the boost dc-dc converter based power inverter). For the record, a power inverter converts ~ 12V dc--> ~120 AC (normally non-sinusoidal). to increase the power output, the amount of output current the device can source is increased, whereas its output voltage remains the same.

Below is an MS Paint rendition of the first stage of a boot dc-dc. the boost stage amplifies the current from a lower voltage to a higher voltage, all in a DC environment. Read about dc-dc inverters on wikipedia.dc-dc converter (boost)

The boost topology can be made smaller and lighter than huge transformer systems, like an APC or UPS power supply.

At the heart of this system is two N channel MOSFETS. (your inverters configuration could have anywhere from 1 to 4 based upon design)

For those who dont know what a mosfet is, its simple enough to say its a kinda like a really fast switch.
The characteristics of this switch are what determines how much current the dc side of the device can source.

Step 2: Digging Deeper

Below displays the power inverter used in this modification. it was a cheap ~200 W inverter and i had blown it apart on an electric bike project electric bike project. So obviously i needed more power and a working inverter.

inside the inverter are the some of the DC-DC converter's main fet's. They are normally clamped to the heatsink.

The first step is to identify them, so grab your spectacle and find a part number. MOSFET'S have many characteristics aside from current and voltage rating. RDS on for example, is the resistance while the fet is enabled. this value should be as low as possible. the higher the RDS on, the more heat is dissapated at the junction, and the more chance for overheating. The gate threshold range is also important. your replacement fet should have about the same range.

Finally, make sure the pinout of your fet is the same and in the same package (EX. mine is a TO-220) dont order the wrong package size! for your replacement fet's

Step 3: Making the Modification

Using a solder sucker / copper braid / a brillo pad (whatever works for you) remove the existing mosfets for the DC-DC converter. Try not to destroy the pads on the board!

carefully reinstall you new fet's. re-solder back in and reattach to any heatsink.

at this point (when everything is disassembled) its a good idea to give it a trial run.

1. make sure the surface your working on is free of conductive debris.
2. attempt to power up the inverter. (use a current limited source like a power supply/ wall wart if possible, not a car battery)
3. carefully check the output voltage (set meter to vac and check the output side)
4. if the output is 100VAC, try a simple 100W lamp.

5. note, i take no responsibility if you blow up you laptop/ house/ solar-system by following this instructable. prolly not a good idea to plug in an expensive laptop, but a blender / lamp / electric pencil sharpener / your 386 can take a beating and are less finicky to odd power supplies.

About This Instructable

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Bio: Im a Student at RPI in Troy NY. The best part of college is most definatley 'Spelunking' or dumpster diving for parts in the elctrical ... More »
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