Sunday, August 1, 2010

Initial Symptoms

I was asked what was wrong with my plasma cutter before I started fixing it. Pretty much everything worked correctly. The power LED turned on, the fan started, air blew through the torch when the trigger was pressed. The only problem was that there was never an arc nor any cutting.

If you have a plasma cutter that isn't working, chances are there is a burned out component(s) inside. Just open it up, discharge the power as I describe in an earlier post, and look for broken components. Some of the most common ones are: leaking capacitor (oozing out the top), blown diode, burned up resistor, broken chip/mosfet (chunk missing from chip), broken relay.

Monday, June 8, 2009

How to fix a plasma cutter

How to fix a plasma cutter. How to fix a 40 amp plasma cutter, 50 amp plasma cutter, 60 amp plasma cutter.

How to repair a plasma cutter.

This page could potentially help out someone who owns any of the following plasma cutters:

Lotos CUT40 plasma cutter
Lotos CUT40A plasma cutter
Lotos CUT40D plasma cutter
Lotos CUT50 plasma cutter
Lotos CUT50A plasma cutter
Lotos CUT50D plasma cutter

Volt Silverline CUT40 plasma cutter
Volt Silverline CUT40A plasma cutter
Volt Silverline CUT40D plasma cutter
Volt Silverline CUT50 plasma cutter
Volt Silverline CUT50A plasma cutter
Volt Silverline CUT50D plasma cutter

Simadre CUT40 plasma cutter
Simadre CUT40A plasma cutter
Simadre CUT40D plasma cutter
Simadre CUT50 plasma cutter
Simadre CUT50A plasma cutter
Simadre CUT50D plasma cutter

Riland CUT40 plasma cutter
Riland CUT40A plasma cutter
Riland CUT40D plasma cutter
Riland CUT50 plasma cutter
Riland CUT50A plasma cutter
Riland CUT50D plasma cutter

Chicago Electric CUT40 plasma cutter
Chicago Electric CUT40A plasma cutter
Chicago Electric CUT40D plasma cutter
Chicago Electric CUT50 plasma cutter
Chicago Electric CUT50A plasma cutter
Chicago Electric CUT50D plasma cutter

Why? Because when trying to fix my Lotos CUT40 plasma cutter, people sent me resistor values for their plasma cutter's high voltage rectifier circuit and the values were all the same even though the plasma cutters were made by different manufacturers and the models were different.

Continue reading this blog to find out if your defective plasma cutter has the same problem as mine: burned out diode, leading to a burned our resistor.

There are extremely high voltages and currents inside a plasma cutter. I take no responsibility for anyone doing any repair work on their plasma cutter. Before working on any internal components, unplug the power, hold the trigger switch so the capacitors discharge (you will hear them), then hold a screwdriver or other insulated device across one of the four large capacitors on the bottom power board so you see a spark. Good luck!

Sunday, June 7, 2009


Today I spent a little more time trying to figure out why the plasma cutter still isn't working, and more specifically why the new resistor gets so hot.

Well, I found the problem!!!!!! With the multimeter on the diode setting, one of the diodes in the rectifier bridge was reading 0.284V in both directions which isn't correct. The other diodes have a reading of 0.84V in one direction and 0V in the other direction.

The bad diode is a 2000V RU1H fast recovery diode. I couldn't find one in stock anywhere, so I just put three 1000V 1N4007 general purpose diodes in series. I also added a 470,000 ohm shunt resistor across each diode to help protect against transients. There are already disc capacitors across each diode in the bridge, so I didn't worry about adding any more.

As an aside, I noticed that there are two diodes in series in one position of the rectifier bridge, so the designers must have figured their diode choice wasn't big enough. They should have doubled up all four diodes right off the bat.

Original RU1H diode that burned out:

New replacement "diode". Three in series = 3000V:

New diodes installed where burned out diode was.

Thanks again to everyone who helped me find the resistor value!

Sunday, May 31, 2009

Making Progress

I'd like to thank eBay's motorcycleman561 for opening up his plasma cutter and sending me his resistor values. My plasma cutter is a CUT40 and his is a CUT40D, but the resistors seem to be the same!

I ordered a 10W 1kohm resistor (colors Brown, Black, Red, precision Gold). The plasma cutter initially had a resistor of 5W 1kohm, but since it burned out, I'm replacing it with one that can handle double the power...

The results:

The plasma cutter still doesn't work... The resistor gets very hot (270F) when the trigger is held down.

Here's the new resistor:


Wednesday, May 27, 2009

CUT40 Plasma Cutter

I'm creating this blog for two reasons:

1. To hopefully repair my plasma cutter.
2. To help others repair their plasma cutter if it has the same problem as mine.

I have a LOTOS CUT40 plasma cutter, but I'm sure that many other models are very similar, if not identical. Other similar units may include CUT40D, CUT50, CUT50D or Simadre CUT40, etc.

I opened up the plasma cutter and found a burned out resistor on the bottom transformer/rectifier circuit board. The circuit board consists of four large capacitors, a transformer, some rectifier diodes, some resistors and connectors.

I wouldn't be writing this except that before the resistor burned out, it appears to have gotten extremely hot and the resistor color bands burned off, so I can't see what value I need to order. I could make a guess or calculate the value, but I thought that I would ask you if anyone would consider opening up their unit and tell me the color band colors?

I have attached some pictures to show exactly which resistor it is. The resistor is about 1" long, 1/4" thick, gray in color, and it appears that the second band from the left is black.

If anyone has the time to open up their unit and let me know what the colors are on the resistor, I would be extremely grateful.

Thank you for your time, and if anyone needs help repairing their plasma cutter, let me know!


The plasma cutter:

A top view with the case removed:

A side view to show the orientation of the next pictures:

Zoomed in on the location of the rectifier circuit and three resistors including the bottom burned out one:

The burned out resistor:

The color band goes from left to right in the photo below. In-case another unit's resistor is oriented opposite, the color band ends with the color that is slightly separated, ie. gold for the top resistor. The gold band is often silver and shows the precision of the resistor.
Eg: The top resistor has the colors: Brown, Green, Black, Orange, and precision Gold.

The burned out resistor removed:

What's this part for?

If anyone has any idea what the circled electrical part is, I'd love to know. There are two metal bars sticking up, separated by about 1/16", and each bar has a circular copper piece near the end. It appears to be some sort of contact, but I've never seen one before...