Hi Dave,
The thermistor I used is rated Rcold=1.0 Ohm, Rhot=0.015 Ohm. I see this
on eBay, looks like what I have:
http://www.ebay.com/itm/2-PCS-SG405-RTI-SURGE-GUARD-NTC-THERMISTORS-1-Ohm-30A-/121126970034?pt=LH_DefaultDomain_0&hash=item1c33bae2b2
I found the datesheet at
http://pdf1.alldatasheet.com/datasheet-pdf/view/601139/AMETHERM/SG405.html
I don't think it's possible to parallel NTC devices unless you have a way
to closely couple them thermally. But this is the part needed, and the
price is right. A lot easier than the relay/resistor approach.
Regards, Gary
On Fri, Jan 2, 2015 at 8:34 AM, David Rieben <drieben@xxxxxxx> wrote:
> Hi Brian and Gary,
>
> 19 ohms variac input DC resistance? That sure doesn't sound right to me.
> If this were the case, your variac would be more like a (resistive)
> rheostat instead of an (inductive) variable transformer, not to mention a
> ~750 watt heater! I would check the resistance range of your DMM with a
> resistor of a known value to verify its accuracy. Like I said before, until
> the core magnetizes and produces the counter EMF to the 60 hz AC input, the
> only resistance of the input of a variac should be that of the copper
> conductor wire of the variac coil itself, which should be only a small
> fraction of an ohm. Be that as it may, I believe a circuit breaker reacts
> quicker to momentary over current conditions than does a typical slow-blow
> fuse, so the momentary inrush over current may be able to trip a 15 amp
> breaker before "having enough time" to melt an 8 amp rated fuse, especially
> if its the typical slow-blow type of fuse. However, I would think that a 10
> amp rated
> variac should be equipped with something like a 12 amp rated fuse?
>
> It would seem to me that the described "Hairpin circuit" is nothing more
> that a standard SG Tesla coil circuit with which the primary/secondary coil
> is replaced with pretty much a buss bar, which would be a dead short to low
> freq AC or DC, but would still offer significant inductive reactance to the
> mutliple 10's or 100's of kHz produced by a resonant LC spark gap circuit
> to produce some measurable (though not very practical) output. But, as far
> as all of the free energy stuff that many of these sites go into, you have
> to filter through them and discard the "free energy" nonsense into the "BS
> file". ;^~
>
> Also, Gary, what type of NTC thermistor would be suitable for inrush
> current protection for powering up a variac? I did a quick eBay search and
> many of them were rated 10K or even 100K and <1 amp and/or part of
> temperature sensors. I did come across these Chinese jobber pieces:
>
> http://www.ebay.com/itm/151092289740?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
> of the disc type construction. Would one (or two paralleled) of these be a
> decent match for the input to a 22 amp rated variac?
>
> David Rieben
>
>
> On Thursday, January 1, 2015 10:27 PM, Brian Hall <brianh4242@xxxxxxxxxxx>
> wrote:
>
>
>
> Thanks Dave and Gary,
> I checked my variac and it is rated for 10 amps, and as it turns out I
> only have an 8 amp fuse in it which still works. When I put a DMM across
> the variac dialed to 0 I got 19 ohms resistance. I do have some ceramic
> incandescent bulb holders that I might try putting inline with a bulb
> between the variac and wall, or just plugging in further away from the
> breaker box, as you suggest. Even a 50' extension cord I have on a spool
> might do the trick. Thanks for the tip on this 60 cycle and core saturation
> timing issue.
>
> I did try to find a website that showed this 'hairpin' circuit that was
> NOT spouting free energy B.S. all I wanted to show was the illustration of
> the circuit, but unfortunately many of the google image top results for
> that diagram link to sites that immediately blast 'free energy' nonsense at
> you right away. After several minutes of searching I picked that one for
> the sake of time. and did not scroll down enough to see its other claims.
> My apologies for that.
>
>
> Tesla referred to the experiment as one with two stout copper bars in his
> lectures.I refined my search and came up with a google books reference,
> this one with the diagram starting a brief section about impedance
> phenomena: as you know turning lights on with Tesla coils is fun, and this
> circuit is another means by which he turned lights on.
>
> https://books.google.com/books?id=PmrsAgAAQBAJ&pg=PT271&lpg=PT271&dq=tesla+two+stout+copper+bars&source=bl&ots=lGiP_6SItj&sig=pH7gpfrtcPWONpSJWLxuf44Jxh8&hl=en&sa=X&ei=OQSmVMHkCYeYNrGBhAg&ved=0CCwQ6AEwAQ#v=onepage&q=tesla%20two%20stout%20copper%20bars&f=false
>
> ----------------------------------
>
> Brian Hall
>
>
>
>
> > Date: Thu, 1 Jan 2015 16:27:58 -0500
> > From: glau1024@xxxxxxxxx
> > To: tesla@xxxxxxxxxx
> > Subject: Re: [TCML] Hairpin circuit triggered circuit breaker without
> the spark gap firing
> >
> > Dave is absolutely correct - My 15A Variac would often trip the breaker
> > when switched on, even when dialed down to 0%, if it was plugged into a
> > receptacle in the basement near the breaker box. When I plugged it into
> a
> > more distant receptacle, the additional wiring resistance limited the
> surge
> > current and didn't trip. The solution was to add Negative Temperature
> > Coefficient (NTC) thermistor in line with the input of the Variac. These
> > devices have a resistance of some fraction of an Ohm at room temperature,
> > passing enough current to magnetize the Variac core and generate heat in
> > the NTC device to cause it to lower its resistance to a much lower (and
> > negligible loss-wise) resistance, so a high-temperature/low resistance
> > equilibrium is reached.
> >
> > I must say however that the web site you cited spouts nonsense. I've
> never
> > heard of a "hairpin circuit", but when somebody tells you "it goes
> against
> > everything that we have been taught about electrical energy", be very
> > suspicious. Everything about Tesla coil operation is compatible with
> the
> > math and physics that we embrace, and anyone who says otherwise needs to
> do
> > more than post a YouTube video.
> >
> > Regards, Gary Lau
> > MA, USA
> >
> > On Thu, Jan 1, 2015 at 8:43 AM, David Rieben <drieben@xxxxxxx> wrote:
> >
> > > Brian,
> > >
> > > You didn't mention the size of your variac, but my guess would be that
> > > this incident had nothing to do with your Hairpin circuit, but was much
> > > more likely due to heavy inrush currents from powering up your variac
> at
> > > just the right (or wrong) time along the 60 hz sine wave of your mains
> > > electrical service. Until the core of a variac (or any other inductive
> > > load, like a transformer or a motor) is magnetized to the point of
> > > developing the appropriate CEMF due to inductive reactance, the very
> low DC
> > > ohmic resistance of the copper wire coils inside the variac are
> essentially
> > > a dead short and it can take a few milliseconds for the core to
> magnetize
> > > and the inductive reactance to "kick in". I often trip my 20 amp
> service
> > > breaker on initial power up of a 20 amp or larger rated 120/140 volt
> > > variac, regardless of what the variac dial is set at or whether there
> is a
> > > load connected to the variac or not. From my experience, this doesn't
> seem
> > > to be an issue with
> > > smaller variacs, say < 15 amps rating, probably because the smaller
> > > guage wire in the coil has sufficiently more DC ohmic resistance to
> prevent
> > > the inrush currents from momentarily rising to sufficient levels to
> trip
> > > the service breaker. You can verify the very low DC ohmic resistance by
> > > simply placing leads of a DMM, set in resistance mode, across the
> input to
> > > your variac, as the resistance will read < 1 ohm, which would
> translate to
> > > hundreds of momentary amps if the variac is energized at or near the
> peak
> > > of the input sine wave of the mains supply. To remedy this problem, you
> > > could add a resistive load (like a 100 watt light bulb) in series with
> the
> > > input to the variac at power up, then sidestep the light bulb resistor
> with
> > > a straight conductor after initial power up. I've just been too lazy to
> > > build a setup with a relay or contactor to do this automatically ;^)
> > >
> > > Also, as a side note, a 15 amp breaker sounds more like a lighting
> circuit
> > > than an outlet circuit, as outlet circuits are usually on 20 amp
> breakers,
> > > at least in most electrical code jurisdictions of the US. But a 15 amp
> > > breaker, as opposed to a 20 amp breaker, would only worsen this
> problem.
> > >
> > > David
> > >
> > >
> > > On Wednesday, December 31, 2014 3:28 PM, Brian Hall <
> > > brianh4242@xxxxxxxxxxx> wrote:
> > >
> > >
> > >
> > > After drawing a few arcs from my simple desktop SGTC, I hooked up the
> > > traditional hairpin circuit (as seen at the top of this page
> > > http://www.transformacomm.com/en/tech/tesla-hairpin-circuit.htm but
> with
> > > just a short copper bar across the top, there were no bulbs to light)
> > > Basically I added another cap in parallel, removed my primary coil and
> > > then where the primary coil connections would be, I connected a 1/2"
> O.D.
> > > 2' long copper tube to each side. To connect the tubes at a right
> angle I
> > > used two 90 degree elbows and an 8" length 'stub out' copper tube,
> also at
> > > 1/2" O.D., at the other end of the 2' long tubes, for essentially a
> long
> > > inverted U shape. They were just press fit, no soldering of any kind.
> > > The spark gap was set to trigger at about 80 volts on my variac, which
> I
> > > used to control power to my 10kv 23ma OBIT. The two caps were each
> 10kv at
> > > .01 microfarads.
> > >
> > > Now the freaky part - the variac is both unplugged and switched off for
> > > safety in rewiring from coils to bars, along with of course being at 0
> > > volts, and the instant I plug it in... I noticed a light go out on the
> far
> > > side of the basement - of course I have not yet mentally connected the
> > > dots. The spark gap which worked minutes earlier has not a single
> spark
> > > fired as it had when I was using this primary part of the circuit for
> my
> > > SGTC, which I also tried with the two 10kv caps in parallell. Still
> with
> > > nothing happening, I switch on the variac and turn it up, and no sparks
> > > were in the spark gap when there ought to be - something's off. I
> turned
> > > down the variac and switched it off, unplugged it, and was quite
> puzzled
> > > and went to turn on the overhead light - to find it didn't work. The
> 15
> > > amp circuit breaker had been tripped!
> > > After resetting the breaker, everything else still works thankfully -
> what
> > > on earth could have happened that my variac, even it it was switched
> on,
> > > was at 0 volts and still that triggered the 15 amp breaker? One
> thought
> > > is that instead of going through the spark gap, the current just went
> > > through the caps and copper bar (even with 0 or less than 10 volts
> > > applied?) and back fed through the transformer and variac into the
> wall,
> > > triggering the breaker instantly - but if that was the case, then
> swapping
> > > out the bars for my primary coil (which is exactly hot it was earlier)
> > > would have done the same thing, right?
> > > What do you think happened? I am hesitant to repeat the conditions
> > > because my furnace may be on the same circuit, and it is going to be
> very
> > > cold the next few days. I disconnected the caps and just used the
> variac
> > > and transformer to arc across the spark gap, that part still works
> fine.
> > > 15+ amps going through all that in what was probably less than 1/2
> second
> > > would be... very strange?
> > > ---------------------------------- Brian Hall
> > >
> > >
> > >
> > >
> > >
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