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Re: SCR coil questions..was Sam's HIGH POWER...
At 08:18 PM 1/23/99 -0700, you wrote:
>Original Poster: "becyn comunication" <becyn-at-hotmail-dot-com>
>>
>>If you have questions about this circuit feel free to ask!
>>
>>
>>OK, I will!
>Have you ever 'scoped the primary waveform? That 2 us current rise
>is probably pure wasted energy, drawing on the power supply and
>doing some strange things with the primary tuning until it all
>slams into full conduction. I was wondering if any ringing occurred
>at all during this interval?
I have a fluke 97 DSO and a Pearson model 410 current probe. The current
probe is good for 5KA. Sure it is pure wasted energy but it amounts to
approximately 32A for 2us. If all 1000v was across the inductor for this
interval this would amount to less than 0.064J. Compared to the area under
the first main 1000A current pulse it is minuscule. As for what it does to
primary tuning before saturation, I'm sure it does something weird but it's
hardly significant. There is no ringing until the core is saturated.
>Scr's are intrinsically slow devices, so one may consider that the
>primary reverse time may NOT be sufficient to turn the SCR off!
>Perhaps the reactor comming out of saturation is what shuts it down.
Oh yes! I've seen this in action! Actually the turnoff in this circuit is
dominated by the current stored in the saturated saturable inductor. This
current forms a loop though the big mystery diode and the SCR which decays
as given by a time constant L/R. The saturated L is pretty small and the R
is pretty small but the current is pretty high and takes a while to decay. I
have the recharge circuit connected such that it will try to desaturate the
core.
>Most diodes have a non repetative surge rating, which means that
>the device can take maybe ten times the operating current upon some
>event such as power up into a capacitve load. The key phrase here
>is NON REPETATIVE. What kind of life do you get from your 80A SCR
>pumping 1000 amps?
The diode has outlasted several SCRs. The main thing that has killed SCRs
is the core still being in saturation. The SCR barely gets warm during full
power operation.
>Hopefully your mystery diode is some high current "fast" type,
>otherwise it will experience catastophic failure due to its
>not comming out of conduction fast enough(reverse recovery). Seems to
>me that this one is MUCH more vulnerable than the SCR since it has
>no large inductor to protect it.
>
It has yet to have a problem. I killed some 80A diodes. I had 3 400V units
in series with no heatsinking. They got very hot and went kaput. I had no
dynamic or static voltage division scheme on these guys.
>Last question. Do you run the coil that close to your computer?
Sure I've been using a laptop for data acquisition. I have not connected
the resonator coil for most of the testing because I have yet to come up
with a satisfactory pulse transformer. I've tried several things so far and
the best has been an 8" pipe wound with 180 turns of copper tape for the
secondary and a 1 turn copper sheet metal primary. The designs I have been
considering recently are as follows: making a loop of copper pipe and
passing some hv teflon wire through the inside of the pipe several times.
I'v seen transformers for ham radio amplifiers use this technique. The other
is to use an autotransformer with the SCR circuit at the one turn tap and a
HV cap to ground at the final tap.
Any suggestions for impedance matching into base of the third
coil(resonator) would truly be appreciated!
I'll get busy and post those waveforms!
Eddie Burwell