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12AX7
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Induction heating
Any interest in this subject? We can start a discussion...
Warning, it's a pretty electrical subject, I don't know how well versed you all are (or want to be).
This is where I am tonight:
Tim
(Edit: this pic has glowing  )
[Edited on 23-3-2005 by 12AX7]
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Alek
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The discussion about it is here:
http://www.trioda.com/php/forum/viewtopic.php?t=930&star...
unfortunatelly in polish language. But don't worry- there is some schematic diagrams
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Twospoons
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I'm interested. Especially as we could then grow cubic zirconia crystals.Cubic zirconia
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Eclectic
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http://www.richieburnett.co.uk/indheat.html
http://www.dansworkshop.com/Induction%20Heating.shtml
http://www.powerlabs.org/indheating.htm
http://www.hvguy.4hv.org/ih/indheat.htm
[Edited on 29-3-2005 by Eclectic]
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12AX7
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http://www.ameritherm.com/technotes.html
http://www.gocs1.com/Psionics/CHEAP-5Kv-CAPACITOR.htm
http://www.monmouth.com/~grimcorona/capacito.htm
http://www.anomalies.net/archive/Keely-Net/Energy/CAPACITR.A...
http://www.dansworkshop.com/Induction%20Heating.shtml
http://www.dansworkshop.com/Induction%20Heating.shtml
http://www.ameritherm.com/induction_furnace.html
http://www.hvguy.4hv.org/ih/index.htm
http://www.ibiblio.org/obp/electricCircuits/index.htm
http://foundry.psibersport.com/forum/viewtopic.php?t=67
http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat1.h...
http://www.powerlabs.org/indheating.htm
http://www.richieburnett.co.uk/indheat.html
http://www.schematica.com/555_Timer_design/555.htm
http://www.kpsec.freeuk.com/trancirc.htm
http://www.ibiblio.org/obp/electricCircuits/AC/AC_6.html
^^ Links from http://groups.yahoo.com/group/Electro_cast/
Tim
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tumadre
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induction heating
I am finishing up a homebrewed piece of crap furnace
I am using 20 2sc2625 transisters configured as a half bridge pushing a maximum of 100 amps @ 170 volts into the coil.
the 240vac is rectified into 300-340 volts dc across 400 volt capasitors (200volt capasitors in series) as to provide the connection for the coil.
modern heaters use a transformer and ac capasitors as to allow the use of SCR's to drive the circuit.
and most use currents of 600 amps or higher as to use a coil of 2-4 turns
I am using NO capasitor in series with the coil. so It is nesisary for the coil to have an inductance of at least 40 microhenerys to make a useable
duty cycle. because those tranistors have a turn on and turn off of 1 micro second, the max usable frequency is around 17khz to avoid high losses.
and the current cannot exeed 100 amps, so for my application,
a coil of 20 turns, 25 cm long and 15cm diameter is perfectly
useable.
I would be cooling the coil with a non electricly conductive liquid. so i could integrate a radiator, as the tubing ends at the tranistors.
contrasting a profesional heater where WATER circulates through the transformer/coil. that being a continuous tube.
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12AX7
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Nice to hear someone else is taking on the challenge. Pics/schematics/oscillographs?
FYI, modern heaters use IGBTs as much, if not more than, SCRs. Easier to handle, more efficient, a whole lot faster (wider bandwidth so especially in
the HF units). Them, and FETs, are much easier to use than BJTs or SCRs, too!
You're going to waste a *lot* of VAs skipping the capacitor. Figure a power factor of 0.2 or worse. That means, out of your 170 *
100 = 17000 = 17kVA capacity, you're reduced to, at most, 3.4kW. You could be getting more than 8kW out. Also, note that the horrible power factor
puts undue stress on your power supply capacitors, since the 90% inductive current that you fed in during one half cycle is dumped out in the next
half cycle (and restored back and forth).
The L-LC network I use seems to be pretty reasonable, and matches a high-amp coil to a reasonably dimensioned half bridge well. It's also completely
short-circuit-proof.
You'll probably be burning a lot of power (lesse, guess 2V saturation at 10A = 20W per transistor, that's um 400W... if that... count on noticably
less than the 98% efficiency I expect!). I suggest running the cooling water over the heatsink. With the dissipation requirement, you'll need those
transistors as cool as you can keep them.
Oh, and yes, water... don't give a second thought to it. Distilled water is more than resistive enough that you'll cause maybe 0.001% of loss to the
output power. It's AC, and fast for that matter, so electrolysis isn't a concern. Use a couple feet (<1 meter) of plastic hose where you need to
connect between hot or high and grounded pipes.
Tim
P.S. "transistors"; "capacitors"; "necessary"; "microhenries"; "usable"; "electrically".
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tumadre
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get the data sheet Here: www.alldatasheet.com
at 8-9 amps the Vce is 1 volt
and Vbe is about 1.7 volts at one amp drive
the reason for not using water also had to do with skipping the LC network-maybe not a good idea
I was expecting 200 watts lost in the transistors, with 100 watts lost in the emitor resistors- they are necessary because the Hfe is temperature
dependant. running the temperature high seems to be a good idea to me.
also if I could find a large enough ferrite core, I could use that to match the coil.
I have no shortage of 200 volt caps, 330, 470 and 680 uF
like 140 of them,
I also have around 40-70 of those x-rated .22 through .68 uF, 250 Vac mylar capacitors used for filtering out the EMI in computer power supplies;
btw a friend of mine said that due to the failure rate expected of them, they can be expected to withstand around twice their rated voltage, at an
amperage limited to what the 22 guage copper wire can stand
I don't know how effective they would be in this application though, due to the need to use about 60 of them in parallel.
[Edited on 15-7-2006 by tumadre]
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12AX7
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My tank contains 200 x 0.1uF 630VDC (250VAC) polypropylene "MKP" type capacitors. The little blue boxes. Ya, same sort of stuff you find across the
line (also mentioned in the data sheet).
I wouldn't trust the larger values, because although they may handle more current than a smaller one, you get strength in numbers. Try to find as
many 0.1 and 0.22uF caps as you can, rated for as high a voltage as you have.
If you're going to use a normally sized coil, your concern is current, not voltage. According to the data sheet, my capacitors can withstand 2.3
amperes RMS per capacitor, for a total of 460A.
Looking at the 2SC2625 datasheet, it looks like typical data is better than I remember. Suprisingly, hFE doesn't change much between 25-125C,
according to this datasheet. Remember to keep a few microseconds, maybe 5, inbetween switching events.
Tim
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tumadre
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I also discovered in my collection.
an h-bridge consisting of 40 rfp50no6 mosfets
using a 50 volt @ 400 amp power supply !good luck finding that
it could permit a 10 KW or even 15 KW portable unit consisting of 4 deep cycle lead acid batteries.
and the mosfets will power the coil at 100Khz no problem.
@400 amps running through 2 legs of the bridge, 704 watts lost to internal resistance
so at 400 amps peak current, maximum heating would be around 500-600 watts with additional energy lost in the forward voltage of the reverse current
diodes internal to the mosfets.
This would require a cap rated at 600 amps 
and a lot of copper
The unit came out of a 1700 watt APC uninteruptable computer power supply.
I mounted the tramsformer in a 400 watt APC UPS case and it wil power 100 amps at 18 volts continuous.
no over-heating with a good fan. I would sell it but it does weigh 30-40 pounds
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12AX7
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Ooh, wonderful! :drool:
*Searches*
Nurrr  that's RFP50N*zero*6, not "o"6.
But, honestly, good luck switching 400 amps. The thought occurred to me to research a series-resonant tank, that is, only the work coil and
capacitor, no Lmatch. But that means the work current flows through the bridge, which means constantly switching yea many amps a couple ten thousand
times a second.
I even posited the question to a forum where professional EE's reside and they basically laughed at it. Some chat, but the basic consensus is, it's
unfeasable.
It takes careful design to switch 500 amps in even 5 microseconds. Consider an inch of wire in free space contributes about 20nH (IIRC). Since V = L
* dI/dt, the voltage developed in switching 500A in 1us is 10V, about your gate "ON" voltage, or a third of your power supply. You can easily
overvoltage the transistor on switch-off, if the inductance to the nearby diode is too high.
And yes...you need lots of amps in your power supply capacitors.
Tim
P.S. I smell MIG welder, for that chunky transformer!
[Edited on 7-17-2006 by 12AX7]
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tumadre
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didn't think about that.
back to the 2SC2625 design.
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today
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One of my plans for a job is to build a large scale induction setup. I've done a test of 1mm nicro chrome wire and one battery can heat to 800C 60mm
of wire.
I would like to know can i add another battery in series or will it melt the wire, what about parerell.
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YT2095
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NiChrome wire across a DC source isn`t induction heating.
para you`ll just get the same effect but it`ll last longer, in series and you`ll likely burn the wire out.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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Quince
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I'm interested in this, after I finish some audio projects. I've only really looked at the schematics on 12AX7's website, and I'm wondering if the
complexity of some of them is really necessary, or it's more like squeezing out the last 15% of performance.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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12AX7
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Compared to what? You're welcome to try a chunky bipolar free-running oscillator, and get good results, but class C oscillators are typically in the
60-80% range, and I didn't find them very reliable (at Vebo < 6V for most transistors, it's a fine line between adequate excitation and blown
transistors). It's a lot easier to switch 10kW with $40 of IGBTs or $60 of MOSFETs at 98% efficiency than who knows how much for BJTs (although 10 or
20 x MJE13009 seems to be a good deal at Digikey). Or you can pay upwards of $500 for a 60-70% efficient vacuum tube system (tube, socket, cooling,
transformers, etc.), which so long as you keep it cooled to spec, I can guarantee will let you beat it over the head with a brickbat, electrically
speaking.
Tim
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Quince
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Well, I'm aiming for no more than 3-4 kW.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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The_Davster
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Hows about compared to a powerlabs type setup? With just a big cap, NST power supply and a sparkgap?
I have a 10000V 0.1uf capacitor which might be suited for one of these?
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12AX7
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I have some 4kJ worth of caps in a box somewhere, but that doesn't mean I can shrink dimes with them, they're electrolytic and would probably explode
if I tried it!
Now if it's a Maxwell labs or similar sort of construction, that's different, and very interesting indeed.
But as for spark excitation, it sucks. For one thing, at the ballpark efficiency Sam himself mentions, you'd be sucking down most of your house's (or
shop's) electrical feed just to get those precious few kilowatts. You still need to match load to source, which for high voltage means many turns (of
wire that can't be cooled with water) for the work coil, or a transformer (not very practical, and probably needs to be cooled too!).
Induction heating is most practical in the range of 2-10 turns of copper tubing, which is suited by roughly equal amounts of volts and amperes. Solid
state around 100-600V fits in pretty nicely.
Besides, a spark isn't elegant. It might've been in 1892, but not in 1950 when they had 833s and 4X1000As. Now that's elegant. Even more elegant,
but less visually appealing and more apparently complex, is solid state. The efficiency of SS is simply awesome, and what can I say, I'm an
efficiency whore when it comes to not buying heatsinks.
Tim
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The_Davster
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| Quote: | Originally posted by 12AX7
Now if it's a Maxwell labs or similar sort of construction, that's different, and very interesting indeed.
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As in a big oil filled monstrosity that weighs as much as my 2.5L bottle of sulfuric?
I emailed the company when I got(or rather 'dived') it....no PCBs....yay.
EDIT: This reminds me...I gotta get rid of my couple dozen or so PCB capacitors.(600V, 3-5uf)
[Edited on 2-10-2006 by rogue chemist]
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Quince
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Caps are cheap. Last year I bought four slightly used 3 kV film in oil, 40 uF each, for $40. That's 720 Joules for a decent price with virtually no
searching effort (i.e. went to eBay). With some effort around surplus places etc., probably could get 1000 Joules for $10.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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12AX7
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Ya, I got a board of electrolytics (totalling those 4kJ I mentioned) from eBay something surplus for a good deal, think it was like $20.
Tim
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fuse123
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what the maximum temp?
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12AX7
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Unlimited, in theory.
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cbfull
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I have a bit of experience with this type of circuit, mainly the self-tuning push-pull, half-bridge and full-bridge. Switching the large amounts of
power involved is not as scary as it sounds. The key is to get the transistors to switch when the sinusoidal current passes the zero point. This is
called zero current switching, or ZCS.
Another type of switching is zero-voltage, or ZVS. These conditions are key to building a circuit that does not turn transistors into firecrackers or
heaters. A self-tuning circuit will get pretty close to switching at zero all by itself, but I don't have an oscilloscope to see exactly what's going
on, so please excuse me if I am wrong.
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