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My first DRSSTC

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loneoceans

Sat Jan 07 2017, 10:48PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

futurist wrote ...

What should I do?

Running with a lower capacitance increases the primary impedance and with the same primary current will increase peak voltage. However, you can typically run the coil with lower peak current with a slightly longer on duration. Finding a good balance is a fun part about coiling so you should experiment around to see what works.

You can quickly calculate the primary reactance using X_c = 1 / 2pi*f*C. Multiply this by the peak current limit to get your peak voltage. Typically I size my MMC generously and use the AC voltage rating. So the ideal way to solve this is to simply increase the ratings of your capacitor bank by using more small caps. Unfortunately this can usually get quite expensive and there really isn't any easy way around it other than finding good deals on capacitors. Alternatively you can get it running for now with a lower current limit / lower bus voltage / lower on time and see how it performs, then add more caps later as your budget allows.

zzz_julian_zzz

Sun Jan 08 2017, 08:10PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi Futurist,

in my opinion,
your options:

1. configure the bank to 0.2uF/6kV
a. compute the Z (surge Impedance) as per Loneoceans
b. divide 6kV to the surge Impedance, you will get the max current that you should not exceed in order for you to not over voltage the cap
c. set the OCD with this current - decrease by some percentage maybe 10% for allowance
d. if on your runs, the OCD led lits, then this means you are reaching threshold and this tells you to reduce the ON time signal from your interrupter
e. if current is still high with the interrupter on a very low value, you could cheat tune the primary to a fres near the secondary freq - where I think notching will occur(see this in your Oscope) - what will happen is that the primary will need more ON time to get to the current you want, slows the ring up on the primary

2. config the bank to 0.112 uF - this will make your tank at high impedance, causing very slow current ring up - and of course your coil will thirst to longer ON time. , however, you would need higher inductance.. you will expect higher voltage in this configuration but again, V accross cap can be capped with the procedure above

3. Use your 0.45 uF cap, try it first- but as you can see from where you copied the design (S. Ward), it uses very high coupling BUT with obvious protection against racing sparks - do add some non conductive/magnetic materials between the two circuit/coils.. Steve's coil is the proof that High K can be acheived. Higher K means slower current ring up and Magnitude is even lower, avoiding OCD being triggered.

EDIT: oops, I didn't digest the formula from Loneoceans, but I think that is the formula for Capacitive Reactance. you can check in wiki 4hv. for Z formula. thanks!

Hydron

Sun Jan 08 2017, 10:06PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Though you've already found caps for a new MMC, I just had a quick look through the two .45uF/4kV MMC coils you linked, couple of notes for future reference and others reading the thread:

- Steve's was in the early days of DRSSTC coiling, when you ran everything way past it's ratings, capacitors included. He described the MMC getting "uncomfortably hot to the touch"!! Now that there is the knowledge that the expensive CD 942C caps aren't really needed for DRSSTCs (better suited for spark gap coils), you can build a MMC with much lower cost caps and run much closer or within the ratings.

- Mads (Kaizer) ran his coil with 500A limit, this is about right for a 0.45uF/4kV cap at these frequencies. You can start by doing the same, get the coil up and running while you build the new MMC.

One last thing, putting thick insulation between primary and secondary can make voltage stress worse (insulation dielectric constant >1 means that there is _more_ voltage stress in the airgap that is left), so is not always a good idea.

Uspring

Mon Jan 09 2017, 01:12PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

I'd go initially with the 0.45uF configuration. 6 strings can take more current than 4. There are coils out there, where you cross your fingers for every single run. Other ones you can turn on and leave for a holiday. You'll probably end up somewhere inbetween.

Big caps and correspondingly lower primary impedance imply shorter ring up times. The fewer primary turns (and maybe a low coupling) lead to less primary magnetic field seen by the secondary. This makes it difficult to get enough energy into the secondary. That means requires careful tuning. I tune my coils, so that the transfer of energy to the secondary limits primary current, not the OCD.

The 942C are pretty tough, but on the long run I'd try to get more of them as your funds allow. Makes the coil perform better and more stable. Increasing coupling will also help.

futurist

Mon Jan 09 2017, 02:11PM

Registered Member #61400 Joined: Sun Jan 01 2017, 01:01PM
Location:
Posts: 33

I have ordered these for the new MMC

It's cheaper than another 12 of 942C caps, and per datasheet 8 caps in series and 4 strings in parallel will give 0.34 uF at 16 kV DC (5.6 kV AC) with Irms = 100 A, which is even better than 942C caps so I hope they'll turn out fine

Since I'll probably receive them before I have a scope available I probably won't try the coil with the 942C caps. I'll keep them for some future project.

futurist

Sun Jan 29 2017, 05:53PM

Registered Member #61400 Joined: Sun Jan 01 2017, 01:01PM
Location:
Posts: 33

Some updates I did on the coil:

- added 4.7 ohm 1 W gate resistors
- added 1N5819 Schottkys in parallel with the gate resistors
- got a proper high voltage 10 Mohm MMC balancing resistors
- made a new MMC (0.34 uF 16 kV DC)
- debugged UD2.7 - TL3116 was soldered backwards, thankfully nothing burned out
- phased the GDT, had to add a few turns based on loneoceans scope shots

I scoped the primary current and the C1 E1 voltage, first on 30 V bus, then increased to max. 340 V (MOT as dummy load)
I didn't measure the ON-time, but I lowered it until it didn't trigger the OCD (~400 A)

Max. C1 E1 voltage is ~160 V measured by osciloscope (using Vmax option) at ~340 Apk - I didn't even touch the variable inductor on the driver
If I increase OCD ~500 A Vmax rises to 260 V at 340 V bus

My secondary and the toroid are unfortunately at another town so I'll probably have the coil complete in the next few weeks

I hope the measurements are fine
Is this looking good or am I missing something?

nzoomed

Sun Jan 29 2017, 10:41PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

futurist wrote ...

I have ordered these for the new MMC

They do look a very good capacitor indeed, and way cheaper than the 942C caps that nearly cost me $400 (NZD)
I am interested if these are easily available, as he is the only seller on ebay with them.

Hydron

Mon Jan 30 2017, 10:38AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Looks great, judging by the scope shots the switching is nice and clean and you've done a good job. I'd say it's ready for first light!

zzz_julian_zzz

Tue Jan 31 2017, 03:34AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

one quick note:
those screws - used as primary coil fastener, MAY cause arcs.. possibly..

futurist

Tue Jan 31 2017, 09:42AM

Registered Member #61400 Joined: Sun Jan 01 2017, 01:01PM
Location:
Posts: 33

Yeah I'm not too happy with the primary..
Next time I'll have to figure out some better way of making primary supports

@nzoomed I've been searching a little and it appears that many people use exactly those capacitors
Like here

,
he uses them in his almost every Tesla coil

They are even more expensive than 942Cs on mouser, I'm waiting for sellers response if he still has some left

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