+ Post New Thread
Results 1 to 10 of 10
  1. #1
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    more corona from tesla coil

    I recently made a tesla coil however its output corona ~20mm wire to wire with no toroid or spheroid of any kind, the corona is quite small given the secondary coil size. The ~20mm was made when the coil was laying on granite countertop so as to not conduct into other objects and the end wires were not arcing into the coil windings.

    Capacitors are Dawn-cap DTH rated to 30kv 35nF each but they dropped to 21nF and the other to about 24nF since I overvolted them with a DC high voltage module but they are self healing with very low ESR so the capacitors are not likely to be the issue since the spark gap is under 2mm which means the caps are charged to less than 8kv which is very tolerable of the two series 30kv rated capacitors even if they are damaged.

    Click image for larger version. 

Name:	Screenshot_20190227_000030.png 
Views:	3 
Size:	23.5 KB 
ID:	151377

    The secondary is not grounded.

    Specs were taken with an LCR meter
    Primary is
    14uF, 21uH 1mm Wire 228mm Length

    Secondary https://temcoindustrial.com/32-awg-c...l-gpmr200.html
    540nF, 31mH, 215R, 32AWG, 520mm length

    Click image for larger version. 

Name:	IMG_20190212_222306.jpg 
Views:	2 
Size:	493.1 KB 
ID:	151378

    the ignition coil and spark arrangement, used hot rubber glue

    Click image for larger version. 

Name:	IMG_20190212_222328.jpg 
Views:	6 
Size:	464.3 KB 
ID:	151379

    I tried varing the primary coil into a single flat winding with lots of space and also the coil driver frequency but it didn't help. The layout will not be changed, I prefer the tesla coil with 2 series capacitors rather than the more common layout with only 1 capacitor. The ignition coil driver is fine since it can make 3-4 inches of corona without being connected to the tesla coil. Maybe the capacitors are infact the issue but its doesn't seem to be the case.
    Last edited by Zak28; 27th February 2019 at 09:37.

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 81,302, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,328
    Helped
    4453 / 4453
    Points
    81,302
    Level
    69

    Re: more corona from tesla coil

    My first thoughts are that the coil is poorly constructed, the primary (blue) winding is covering about half the length, if you are expecting say 40KV corona, the insulation will have to be good to about 20KV. I would try a two or three layer primary with a little more space between it and the secondary winding to prevent flashover.

    However, your main problem is likely to be one of resonance, remember that a transformer converts a change in electrical current to a magnetic field variation then back to a voltage in the secondary winding. If the change isn't fast enough or there isn't enough primary current, the secondary will not show the voltage you are expecting. The idea behind using a spark gap is that it gives sharp edges to the primary waveform as it flashes over, if you are not seeing a bright and continuous arcing across the gap it will not produce much output. The ignition coil should be producing a continuous high voltage to allow the spark gap to operate and have a high enough output impedance that it isn't damaged by the conduction of the spark.

    The ideal spark frequency will be approximately "1 / (2 * pi * sqrt(L * (C / 2)))" where L is the coil inductance in Henries and C is the capacitor value in Farads. The result is in Hz.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

  3. #3
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    Re: more corona from tesla coil

    Quote Originally Posted by betwixt View Post
    My first thoughts are that the coil is poorly constructed, the primary (blue) winding is covering about half the length, if you are expecting say 40KV corona, the insulation will have to be good to about 20KV. I would try a two or three layer primary with a little more space between it and the secondary winding to prevent flashover...
    This can be an issue even thought the blue foam provides ~4mm of spacing away from the secondary which itself is covered by packaging tape. I will try rewinding the primary with more insulation to see if it helps with corona output.

    Quote Originally Posted by betwixt View Post
    ...However, your main problem is likely to be one of resonance, remember that a transformer converts a change in electrical current to a magnetic field variation then back to a voltage in the secondary winding. If the change isn't fast enough or there isn't enough primary current, the secondary will not show the voltage you are expecting. The idea behind using a spark gap is that it gives sharp edges to the primary waveform as it flashes over, if you are not seeing a bright and continuous arcing across the gap it will not produce much output. The ignition coil should be producing a continuous high voltage to allow the spark gap to operate and have a high enough output impedance that it isn't damaged by the conduction of the spark...
    The spark gap does not spark at all if the gap exceeds ~4mm rather the ignition coil just buzzes away and all the current seems to be flowing thru the primary windings instead of causing the capaictors to discharge thru the spark gap. However at ~2.5mm or closer the spark gap continuously sparks rapidly until there is too much ozone. The ozone is not an issue since I will likely redo the whole thing with a fan sucking the air away and out from the spark gap to quench it.


    Quote Originally Posted by betwixt View Post
    ...The ideal spark frequency will be approximately "1 / (2 * pi * sqrt(L * (C / 2)))" where L is the coil inductance in Henries and C is the capacitor value in Farads. The result is in Hz...
    Is the capacitance in the resonant formula that of the 2 series equivalent high voltage capacitors or the parasitic measured of the secondary coil?
    Last edited by Zak28; 27th February 2019 at 10:20.

  4. #4
    Super Moderator
    Points: 81,302, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,328
    Helped
    4453 / 4453
    Points
    81,302
    Level
    69

    Re: more corona from tesla coil

    You would probably get better results if you used an oscillating power driver instead of the spark gap. It would allow much higher primary current and also let you optimize the frequency by adjustment.

    The exact resonance is difficult to calculate since it depends on several factors that are difficult to measure, in particular the impedance of the secondary under discharge (corona) conditions and the properties of the core it is wound around. The formula I gave assumes each capacitor is 'C' (it is divided by 2 because they are in series) and the primary inductance.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

    •   AltAdvertisement

        
       

  5. #5
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    Re: more corona from tesla coil

    Quote Originally Posted by betwixt View Post
    You would probably get better results if you used an oscillating power driver instead of the spark gap. It would allow much higher primary current and also let you optimize the frequency by adjustment.

    The exact resonance is difficult to calculate since it depends on several factors that are difficult to measure, in particular the impedance of the secondary under discharge (corona) conditions and the properties of the core it is wound around. The formula I gave assumes each capacitor is 'C' (it is divided by 2 because they are in series) and the primary inductance.

    Brian.
    I will make a rotatory spark gap from a salvaged high speed 50mm fan and magnetically quench it.

    Or did "oscillating power driver" mean solenoids or something else?

  6. #6
    Super Moderator
    Points: 81,302, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,328
    Helped
    4453 / 4453
    Points
    81,302
    Level
    69

    Re: more corona from tesla coil

    I was thinking of a pulsed MOSFET or BJT to interrupt the primary current. Even a 555 timer and a MOSFET would work well. Ideally you would use a H-Bridge driver which would give four times the power.

    You can substitute the high voltage on the primary for high current, it makes it safer and more controllable for little extra effort.

    I'm not sure a rotary spark gap would help unless you could store the energy before the gap closes. It would mean using DC to charge capacitors then discharging them through the gap into the primary coil. A lot of work and you wold still need to generate a high DC voltage.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

  7. #7
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    Re: more corona from tesla coil

    Quote Originally Posted by betwixt View Post
    I was thinking of a pulsed MOSFET or BJT...
    I prefer spark gap since its rising and falling current edges are way faster than any semiconductors. Spark gaps can handle almost any pulse current unlike semiconductors also.

    Quote Originally Posted by betwixt View Post
    ...It would mean using DC to charge capacitors then discharging them through the gap into the primary coil. A lot of work and you wold still need to generate a high DC voltage...
    It doesn't seem as much work you have made it out to be since the ignition coil already makes the high power high voltage AC and all is needed is rectify I didnt know rotary spark gaps demanded DC charging the capacitors. This is my first tesla coil and I prefer it be spark gap.

    Is full wave rectification the way to go for this ignition coil? I have more than 4 of these https://www.ebay.com/itm/173401710935/

    The ignition coil doesent make high voltage beyond ~1khz and I run it at ~90hz therefore those diodes should be fast enough.
    Last edited by Zak28; 27th February 2019 at 23:42.

  8. #8
    Super Moderator
    Points: 81,302, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,328
    Helped
    4453 / 4453
    Points
    81,302
    Level
    69

    Re: more corona from tesla coil

    Spark gaps are fine but tricky to get working properly and they do 'burn out' over time.
    Bear in mind you can't charge capacitors with AC, I was considering allowing a DC voltage to reach high enough potential as it charged capacitors that it flashed across the gap and conducted current into the primary coil.

    The rectifiers might work but you need to find out the peak voltage from the ignition coil first then at least double it to get the required PIV rating for the diodes.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

  9. #9
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    Re: more corona from tesla coil

    Quote Originally Posted by betwixt View Post
    ...The rectifiers might work but you need to find out the peak voltage from the ignition coil...
    The coil can make > 65Kv but why would it matter if the caps would be charged to below the 20Kv reverse voltage rating of the diodes? The spark gap limits the voltage doesent it?

    •   AltAdvertisement

        
       

  10. #10
    Advanced Member level 2
    Points: 2,211, Level: 10

    Join Date
    Aug 2016
    Posts
    578
    Helped
    6 / 6
    Points
    2,211
    Level
    10

    Re: more corona from tesla coil

    Issue resolved, the aliexpress capacitors are bad, replacing them with just a few brand name Maida caps with much smaller capacitance yeilds far more corona with a smaller high voltage secondary.

    Click image for larger version. 

Name:	IMG_20190227_193205.jpg 
Views:	4 
Size:	475.1 KB 
ID:	151396

    Are these capacitors usable for the this tesla coil without any diode rectifiers or rotary spark gaps?

    https://www.ebay.com/itm/183664576130/

    Its what I have but I put them in the circuit to see if the film caps were the issue.
    Last edited by Zak28; 28th February 2019 at 05:09.

--[[ ]]--