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Re: SCR & PRIMARY CONNECTIONS (fwd)
---------- Forwarded message ----------
Date: Fri, 9 Nov 2007 10:17:14 -0000
From: Chris Swinson <list@xxxxxxxxxxxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: SCR & PRIMARY CONNECTIONS (fwd)
Steve, all,
I picked a few lines from the postings....
"The IGBTs have
some nice advantages over SCRs namely they turn on fast and they can be
switched off."
This implys SCR's can't be turned off ?!
"This
> works OK except that the SCR doesn't turn off until the primary cap is
> almost empty. No first notch quenching:-(. "
Why would you want first notch anyway, if you transfer all the energy fast
enough, IE first half cycle, then you do not even get as far as the first
notch. The SCR turning off when the tank is almost empty sounds perfect to
me.
"In order to get first notch with
> a 100KHz coil a Tq of 5uS would be necessary."
You do not need 100khz with an SCR. I've ran them at a few khz but never
pushed them beyond that. I doubt they could turn on/off fast enough. running
them at 100hz is more than good enough.
"As you head towards lower voltages
> and higher currents and more capacitance keeping the inductance low enough
> to hit the target resonance frequency becomes difficult. Not to mention
> how
> absurdly careful you need to be in order to minimize parasitic
> inductance."
Yes this is a real problem! I have to aim for 0.01ohm total primary
resistance, my first attempt was 0.070ohm and was not good enough.
"The book was vague about the actual
> circuitry involved but it did allude to a circuit that (1) provided back
> EMF to shut off the SCR and (2) provide a trigger pulse to fire the SCR
> for
> the opposite polarity."
Yes, this is the circuit I used. If you switch too fast the back emf is not
enough to turn off the SCR so it latches up. It was a car ignition coil
driver.
" I think SCRs may work in a SISG arrangement, but IGBTs are they way to go
> for DRSSTC."
I was told IBGT have a lot of internal inductance and should not be used in
resonant circuits. They do seem to take almost anything put to them though
so not sure how true it is!
You just replace the spark gap with an SCR. In fact you can get solid state
spark gaps (£2000+) which are just a big SCR, I think 50KA/uS 8kv and up.
Running a SCR at 100hz is no problem really. Though the voltages suffer so a
larger tank cap is needed and super conducting primary circuit! The reason
why my coil is taking so long to rebuild as I do not think anyone has even
attempted it before ?
In anycase, It was my point about using the SCR next to the primary to
obtain smaller length connections and less wiring!
Cheers,
Chris
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, November 09, 2007 3:06 AM
Subject: RE: SCR & PRIMARY CONNECTIONS (fwd)
>
> ---------- Forwarded message ----------
> Date: Thu, 8 Nov 2007 16:43:19 -0700
> From: S&JY <youngs@xxxxxxxxx>
> To: 'Tesla list' <tesla@xxxxxxxxxx>
> Subject: RE: SCR & PRIMARY CONNECTIONS (fwd)
>
> Yes, SCRs have been used. Here is a quote from Eddie Burwell, circa 2000:
>
> Original Poster: "Eddie Burwell" <eburwell@xxxxxxxxxx>
>
> Well, from my experiments with SCR based coils IGBTs look like the hot
> ticket. IGBTs like SCRs are four layer devices and have "fixed" voltage
> drops. This means the power lost in the switching device goes up linearly
> with increasing current rather than I^2R as does a mosfet. The IGBTs have
> some nice advantages over SCRs namely they turn on fast and they can be
> switched off.
> I tried using saturable reactors to effectively speed up the turn on
> time
> of the SCRs which works but causes other problems ( like holding the SCR
> on
> for an excessive period of time.) So I ditched the saturable reactors and
> went to a 365A 1800V 60uS Tq SCR. This sucker is rated for 8000A peak.
> This
> works OK except that the SCR doesn't turn off until the primary cap is
> almost empty. No first notch quenching:-(. In order to get first notch
> with
> a 100KHz coil a Tq of 5uS would be necessary. Unfortunately 5uS SCR are
> kind
> of rare and don't usually exceed 600v. As you head towards lower voltages
> and higher currents and more capacitance keeping the inductance low enough
> to hit the target resonance frequency becomes difficult. Not to mention
> how
> absurdly careful you need to be in order to minimize parasitic inductance.
> If your cap has as much inductance as your primary then your primary will
> only see half the voltage. This sort of thing can happen when your primary
> has only one turn. So higher voltage is better!
> A 1200V IGBT is sufficiently fast and can be turned off, but there
> remains
> one question. How far can they be pushed past their current rating for a
> pulse? A 100A SCR can do 1000A for a brief pulse. Could a 45A IGBT make it
> to 450A? The ratings on IRG4PH50U say 45A cont. 180A peak but there is no
> indication in the data sheet as to how the max peak current relates to the
> with of a pulse (for infrequent pulses). If They can hit 450A look out!
> With my last experiments in solid state coiling I could pull about 18"
> with a 1 joule cap fired 400 times per second. The SCR was barely warm.
> quality caps are an absolute necessity! I started with some CDE SCRN222s.
> Their internal connections were so poor the terminals got hot although the
> rest of the cap was cool. With 10 joules per bang an IGBT coil should
> start
> to really perform!
>
>
>
> And here is another posting from 1998:
>
> Date: Sat, 17 Jan 98 22:25:48 PST
> From: Al Syme <asyme@xxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: SCR power supply
>
> To All
> A while back I ran across a book on induction furnaces. In the book was a
> short discussion on using SCR's in a circuit like a multivibrator to
> provide power for an induction furnace. The book was vague about the
> actual
> circuitry involved but it did allude to a circuit that (1) provided back
> EMF to shut off the SCR and (2) provide a trigger pulse to fire the SCR
> for
> the opposite polarity. I don't know what the upper frequency limit for
> this
> type of circuit is, but I'm thinking that it may be a way of generating
> power at high enough frequency to make it cost effective for us coilers on
> a budget. If any of you have run across this application, any information
> would be helpful.....Thanx Al Syme KG0QJ
>
> - - - - - - - - - -
>
> I think SCRs may work in a SISG arrangement, but IGBTs are they way to go
> for DRSSTC.
>
> --Steve Y.
>
>
>
>
>
> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Thursday, November 08, 2007 10:45 AM
> To: tesla@xxxxxxxxxx
> Subject: RE: SCR & PRIMARY CONNECTIONS (fwd)
>
>
> ---------- Forwarded message ----------
> Date: Thu, 8 Nov 2007 15:28:05 +0000
> From: "Lau, Gary" <Gary.Lau@xxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: RE: SCR & PRIMARY CONNECTIONS (fwd)
>
> I don't believe that it's even possible to use an SCR in a Tesla coil.
> SCR's are very slow devices. And functionally, they're definitely not a
> drop-in replacement for an IGBT - they behave very differently.
>
> Regards, Gary Lau
> MA, USA
>
>
>
>
>
>
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