Last March 2004 I bought a cheap Speedway Plasma Cutter on Ebay. I've
used it once. Last week I tried to use it again and it appears to be dead.
Yes I know I did a dumb thing buy it.
Anyway, I'm a consulting engineer and have been reverse engineering
electronic stuff since I was 9 years old. I will not be beaten by cheap
chinese designs. I reverse engineered the main electronics board and
95% of the control paddle board. I say 95% as I can not read the
numbers on the five integrated circuits. If anything blows I suspect it
won't be the paddle board electronics but the power electronics.
My question is... this plasma cutter is being sold under other brand
names now. Does it make sense to make up a CD with the schematics and
"theory of Ops" and sell it for $15 to cover costs? Are there enough of
these P.O.S.'s in the field so as to make this CD useful to others?
I appreciate your inputs.
best regards all
Simon Shabtai Evan
Shabtai Evan wrote:
How many success stories have you had with this kind of circuit? My
experience with them is that beginners tend to focus their attention on
the power circuit. To view an authority who agrees with this point of
view, please consult Sam Goldwasser's excellent and comprehensive
electronics repair FAQ. He has an extremely valuable list of case
I hate it when people try to judge other's expertise in a field from a
single line in their post, but what else is one to do? Usenet does
have its limitations. But, I have seen some pretty high powered
discussions in which everyone missed the "sleeper" in their midst, just
because he was slow to answer just one critical time. My apologies in
advance if you are that sleeper.
Yes, there probably are. Somebody says you will sell 3 of them, and I
think that's incorrect. I have reverse engineered much more esoteric
things than cheap plasma cutters, and have ended up selling ten times
that many schematics. On the other hand, one of my reverse engineering
jobs did not sell a single copy, but it drew a lot of inquiries. The
big determining factor is not really the market, but what is the
alternative. In the case in which I sold zero copies, the device was
easily replaced by a superior and less expensive, well-known and
well-documented alternative. Only antique art lovers would be
Some comments (and I would appreciate your thoughts on these):
1. The legal grounds for reverse engineering are eroding. I once had
to deal with an angry company and their idiot lawyer. Unfortunately,
he was not such an idiot that it was unnecessary for me to consult a
lawyer myself. Make sure that you are ready to defend yourself in case
the unthinkable happens.
2. Look into other companies which perform this service, such as
Bomarc (or something like that).
3. You posted a circuit diagram in pdf, binary encoded. Try not to do
this. It does not show up on Google Groups. Use a URL to your own www
site. Get a free one, such as Yahoo, if you must.
4. Your circuit diagram contains a serious error. It is missing the
DC bus return lead on the negative side. This error is sufficient to
cause the more sensitive among us to stop paying attention.
5. You have not fixed the unit yet. Another beginner type comment:
"a schematic is necessary and sufficient for the repair of the unit".
Not consistent with the experts. Again, consult Sam Goldwasser's
6. Even if you (luckily) repair the unit once, this may not get you
the credibility you desire. The publication should contain more than a
schmatic. It should contain a parts layout, guide to operations,
troubleshooting flowchart, and, for difficult circuits, waveforms or
simulations. In addition, it is helpful to include some photos and
mechanical disassembly instructions. Yes, yes, mine has these
sections, but so does any other pro-style guide that you may find at
the library. Check out Howard W. Sams in your local library. Many
libraries have them on microfilm. Consider these as a template. Are
you familiar with this publication?
7. Many of the potential purchasers of your product are not students
or repairmen. They are hobbyists who are looking for a set of plans.
This is especially true on E-bay or the various electronic discussion
groups. Your document should address this market, even if not
If the above comments haven't scared you away or given you the wrong
impression, I can give you some more directed assistance, perhaps on
It was a pleasure reading your post. Please see my comments within your
Good input. thank you
The circuit was posted in pdf to degrade image quality until validate
circuit is released. If ever.
You are correct. This is a cleanup version of the raw schematic.
I did lose that connection in this version. I hope the 'sensitive among
us' aren't to upset. 8-)
Actually I have fixed the unit. The unit was entering its overcorrect
mode and shutting down. As it was designed to do. The problem was
trace through the power inverter circuit to the torch itself. It was
shorted out. In the cutter's last use, on completion of the cut, the
torch's internal electrode welded itself to the torch cup. Not a strong
weld, but a weld none the less, causing the short. Break the weld, sand
electrode and cup lightly, and the unit is operational again.
The torch has an electrical and a mechanical inter-lock. Normally, when
not in operation, the torch mechanically holds the internal electrode
against the torch cup. When air pressure is applied to the torch the
electrode is physically pushed away from the cup, thereby allowing an
arc path through the gas to occur. It appears the purpose of pushing
the electrode into the torch cup is to discharge the inverter
capacitors on the high voltage side of the transformer thereby
preventing the user from inadvertently touching 770 VDC if the cup is
A complete schematic is not always needed to repair a piece of
electronic gear, especially when one has been the electronics industry
for so many decades (am I that old?). But, gee it's nice to have.
Eric, I'm not looking for credibility. I'm just trying to help others,
like me, in a fun hobby. I don't need to make sales for money. I have
reached for and caught the American dream. In my career, may others
have been generous, to me, with their time and knowledge and that has
aided me greatly. I'm just trying to give back.
Eric, I truly enjoyed your reply. And it was excellent. Thank you for
taking the time to respond to my post. It is people like yourself that
make this a most useful and powerful newsgroup.
Simon Shabtai Evan.