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TRANSFORMERS
* Carbons Sent to: Esondrmn-at-aol-dot-com
> From: Esondrmn-at-aol-dot-com
> Subject: Transformers
ES> Is there a good source for high voltage transformers?
ES> I can only find 15,000v 30ma used neon sign transformers
ES> for about $35.00. One guy says he can get me a 15,000v 60ma
ES> transformer for $75.00 - used of course. Is this a good deal?
A brand new 15 60 runs about $120.00 retail. 15 30s run around $75.00
new. I guess it is time for another re-post:
-----------------------------------------------------------
Date: 11-05-93
From: Richard Quick
To: All
Subj: 10KVA Tesla Coil
------------------------------------------------------------
If you are interested in making a high voltage, high current,
power supply, I can tell you how to do it for free....
First call the local neon shop(s) and tell them that your working
with Tesla coils. Ask them to hold all of their failed xfrmrs so
you can pick them up. Make sure you talk to the boss or foreman,
and tell them that you want dead units. I have never had a shop
turn me down for free cores. They are happy to get rid of them.
There are two types of failed neon xfrmrs: warranty units, and
old junk. The local shop must return units that fail within the
two year warranty period back to the distributor for credit. Old
junk (older than two years) you can pick up for free right from
the local shop, but I also scavenge from the distributor. Ask
where the failed warranty units go.
If you can locate the distributor who sells wholesale, and
handles failed warranty units, you have found a gold mine of high
voltage xfrmrs. The distributor removes the PLATE from the xfrmr
for return to the manufacturer, and throws the unit away. The
manufacturer credits the distributor for the plate, as the
shipping is too expensive. The cores go to the dumpster.
After locating your source of failed units, be selective. Try to
bring home the high current units. Ratings commonly used are 9
kv, 12 kv, & 15 kv, with common current ratings of 30 & 60 ma.
Once in awhile you will come across a 120 ma unit. I grab all
of the high current units (60 ma+) I can get in these voltages.
First test your units. Use wire with a 15 kv rating or better.
This wire can be obtained where you pick up the transformers.
If you ask they will usually cut you off a few feet for free.
I prefer using the solid polyethylene core from RG-213 coax, as
it will withstand the voltage with gobs of extra safety margin.
Draw an arc from the HV bushing to the case, one at a time.
About 50% of the "failed" units I pick up are just fine and need
nothing other than a clean up. There is nothing wrong with them.
Often shops get these units from signs they have dismantled, and
they just toss them into the junk pile with the rest. The other
50% are bad. Either one, or both, of the HV windings have broken
down. These units can frequently be repaired.
Remove all hardware, and insulators if possible. Take a hammer
and a chisel and remove the cases by splitting them down the
corners. Break off any stubborn insulators, but try to preserve
the lead wire. You are left with a block of tar. Set the unit
outside when it is very cold, and let it freeze solid overnite;
or place in a freezer or deep freeze until frozen solid. The next
morning, short the high voltage lead wires with a clip lead, and
connect 110 volts across the primary. Since the cores on these
transformers are shunted, they may be shorted without harm or
blowing fuses. Let the unit cook for 15-30 minutes.
Disconnect your leads, and with the chisel and hammer, chip a
groove around the block. You want to score a groove lengthwise
that will allow the block to cleave in two. Then starting from
one end of the block, chip until you hit the core, then do the
same with the other end. Pry and chip the tar away from the core
until the xfrmr is free. The core may then be disassembled, and
the windings removed and examined. Kerosene and a stiff brush
will clean up the windings and core of any remaining tar.
The "cold-cook" method is fast, clean, and works very well. Since
the tar is frozen it chips away cleanly. The "cooking" softens up
the tar around the core allowing it to release. The only other
ways I know to free the cores are long soaks in solvent such as
kero or gas, (the nasty waste does make a good crack filler),
or melting out the tar with external heat from a fire or oven.
Most units fail when the high voltage breaks down the tar insul-
ation. The resulting carbon track shorts the winding. Simply
removing the tar brings them back to life. Other times the coils
break down internally. In this case I discard the winding after
disassembling the core, and replace it with a good winding from
another unit of the same model with the same type failure.
While the core is apart, you can beef up the current output by
removing a few of the shunting plates between windings. Never
take out more than 2 or 3 of these plates per side, as the
additional power output will burn out the secondaries. Generally
I get about 70-75 ma out of 60 ma units after I have finished.
Rebuilt units need a little protection from the high voltage
secondary outputs. The first thing I do is solder on a new lead
wire to the high voltage windings. The HV secondaries are wound
with very fine magnet wire, in the 30 ma units the wire is not
much thicker than a coarse hair. Once a good solder connection
is made, bed the connection and the first 1/2 inch or so of lead
wire to the top of the HV winding with hot glue or clear epoxy.
The lead wire need not be anything special, any thin insulated
stranded wire may be used. Heavy wire increases the chances of a
failed connection due to mechanical stress. When setting the unit
up to fire you simply have to route it on insulators.
The windings themselves are wedged against the core to prevent
vibration. I have seen wood, bakelite, and plastic wedges used
commercially. What I like to do is to soften up some 30 mil
polyethylene plastic sheet in boiling water, and heat the core in
a warm oven. I wrap dry softened plastic around the core and
gently force the windings down on it. Once cooled, the windings
have some insulation from the core, and they will not vibrate.
The base wire from the HV windings must be grounded to the core.
Use the original grounding point if possible, if not you may
split the core apart slightly with a thin blade and insert the
wire into the gap before you clamp the core back up. If required
you may splice on a small piece of wire for added length.
Neon sign transformers that have been rebuilt may be fired dry.
The tar used to pot the cores for neon use does not really
insulate well against the RF and kickback from the Tesla Tank.
The units last longer when they are freed of the tar potting. The
only other choice is to sink rebuilt units in mineral or xfrmr
oil which is a very good RF insulator. I choose to fire them
"dry"; it works, and there is no mess.
Neons may be run in parallel to deliver the current required to
fire medium sized coils, and I have run up to 4000 watts with
banked neon power supplies. The general practice is to run these
banks off of 240 volt feeds controlled through a variac. Neons
with matched outputs are run in pairs in these banks. The
primaries are paired up in series, and the secondaries are all
paralleled to the HV buss. Phasing is important here, and each
transformer must be checked as it is added to the bank to ensure
it is in phase with the other units. If an xfrmr draws an arc
from a lead wire brought to the HV buss, the primary or secondary
connections must be reversed.
Neons typically have an efficiency of about 50%, in that they
draw twice as much power as they put out. This problem can be
resolved with the use of power factor correction (pfc) capaci-
tance across the line. The pfc capacitors used are the same as
for alternating current motors. The voltage rating should be at
least twice the line current used, and I like a 4x voltage margin
for long life. The formula used to determine ballpark pfc is as
follows:
9
10^
C = Corrected kVA ------ 2
2(pi)fe^
This should read C = Corrected kVA times (10 to the ninth power)
over, (2 pi times f times e squared)
C = required capacitance in microfarads
f = frequency of applied voltage
e = applied voltage
CORRECTED KVA is determined by dividing the volt*amps (watts)
output of the neon sign xfrmr by 1000
Using a pair of rebuilt 12 kv, 60 ma neons, with 2 shunting
plates removed from the core next to each HV winding, and power
factor correction capacitance, you can get a nice 1.5 KVA Tesla
power supply with over 90% efficiency. Total cost: $5.00 for the
pfc capacitors, and a few hours of time.
I have unpotted dozens of neon transformers from many different
manufacturers. I have tried to make this as informative as
possible, and have checked it over for mistakes. If I have erred,
or was not clear on something, please let me know. Use common
sense, and don't expect the first attempt to work out. On my
first attempt I managed to destroy a HV winding during the
unpotting, as I did not know where the windings were located on
the core. But once you see one core unpotted, with minor
differences, you have seen them all.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12