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Registered Member #4532
Joined: Sun Apr 22 2012, 03:16AM
Location:
Posts: 3
I am building a smaller sized DRSSTC based upon Steve Ward's DRSSTC-3, and I currently have 4x CDE 942C20P15K capacitors. I was wondering if an arrangement of (1) 2 strings of 2 of caps in series yielding 4kv at 150 nF or (2) 1 string of 4 caps in series yielding 8kv at 37.5nF would give better results for my tank capacitor?
Specifications of my coil just in case they help to answer my question: 4.5x11" secondary with 1100 turns of 30 AWG wire 8" diameter primary with 5 turns of 1/4" copper tubing 3x10.5" toroid made from aluminum dryer ducting 4x CDE 942C20P15K tank capactior 4x IRG4PC50UD IGBT's in a H-Bridge Steve Ward's Universal DRSSTC Driver and interrupter
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
A quick plot of your coil data into JAVATC shows that you need to tap your primary coil at 2.2 turns with a 150nF MMC and at 4.6 turns with 37.5nF.
As you are using TO-247 package IGBTs, I would suggest going for highest possible impedance, 37.5nF MCC with about 5 turns on the primary would spare the IGBTs from a much higher peak current that they would see with the larger MMC.
Run longer on-times to get a decent spark output, this is the safest way to go if this is your first DRSSTC, you can make a low impedance, super high peak current coil with a MMC blasting many joules per bang when you have more experience and a lot of money :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you want to spare your devices from high peak current, use a current limiter. Steve Ward and others call it OCD but I prefer to leave that for obsessive-compulsive disorder.
Then if your coil is set up wrongly, it'll just make undersized sparks instead of exploding, and you can tinker with the setup all you like.
For what it's worth, I found that my coil performed much the same with tank capacitors of 50 and 100nf. The variable that made the greatest difference to performance was coupling. The tighter the coupling, the more sparks I got per amp of primary current.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Steve Conner wrote ...
For what it's worth, I found that my coil performed much the same with tank capacitors of 50 and 100nf. The variable that made the greatest difference to performance was coupling. The tighter the coupling, the more sparks I got per amp of primary current.
This is really interesting - so if you were to design a new coil, what would your MMC config design focus on as a primary requirement? Max Voltage, Max Current, operational reliability, heating ?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The main design parameter I use nowadays is size of each individual capacitor compared to a can of beans. This is the last MMC I built using 1 string of 8 industrial GTO snubber caps. Each unit is rated at 6uF, 2500V DC, 4000V peak. I forgot what the peak current rating is, but it is well into the thousands of amps.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Steve Conner wrote ...
The main design parameter I use nowadays is size of each individual capacitor compared to a can of beans. This is the last MMC I built using 1 string of 8 industrial GTO snubber caps. Each unit is rated at 6uF, 2500V DC, 4000V peak. I forgot what the peak current rating is, but it is well into the thousands of amps.
It has a 5000Apeak rating, same as my same type capacitors at 4uF 3kVDC, 4KVDCpeak. I use 5 of them in a string for 0.8uF at 15-20kVDC rating.
To help myself and other do quick surveys and design of the MMC I made a calculator, I fulfill all the parameters in red text under advanced results:
I design for reliability since, when building a large DRSSTC, everything cost a great deal of money, so no need to blow it all up because you want to save money on the MMC, I just faced the fact that if something is worth doing, it is worth doing right.
So keep an eye out for the temperature rise of the MMC capacitors :)
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A question on DRSSTC tank capacitor arrangements
