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Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
My objective here was to build a high voltage AC source that would work well with a C & W multiplier. The transformer part of it has a secondary coil made from the secondarys of three 1970s' vintage ignition coils connected in series. The laminated iron cores where replaced with ferrite rods witch run cooler. The primary coil was wound with 300 turns of 10 gauge magnet wire. Leakage inductance is very high and in away that is a good thing because it prevents you from overloading the oscillator which draws 3 amps at 115 volts with no load at all and draws 7 amps when you are drawing a good ark.
I'm using the same Royer oscillator circuit here that you may have seen used for fly back drivers and induction heaters but here I am powering it directly with 115 volts from the wall outlet going though a full wave rectifier into a capacitor for a voltage of about 150 volts DC. The MOSFETs I am using are FCA47N60 but any high voltage MOSFET with a good current rating should work. It should be noted that this circuit as it is will not work at frequencies much higher than the audio range. With the 6 uF tank capacitor this one resonates at about 2 kHz. The inductors I am using are the primary windings of two MOTs with the secondary coils removed. I'm sure something smaller would work since the inductance is not critical.
This setup can be run easily for 10 to 20 minutes at a time without overheating anything. The primary coil is about the only thing that heats up while the secondary coils are in an oil bath that should keep them cool. The MOSFETs run very cool. One word of caution, If you ground the end of the secondary coil don't connect it to the ground of your power cord. The MOSFETs don't like that for some reason. Having a direct fed power supply like this can cause some strange things to happen.
I just thought I should though in some facts about ignition coils. On the secondary of one of my coils I measured the diameter of the wire and the resistance of the winding and I came up with .0025 inches which is 42 gauge and 11 k Ohms resistance. The resistance of 42 gauge wire is 1,659 Ohms per 1000 feet so that means the length of the wire used to wind the coil was 6,630 feet (that is over a mile! ) So now you can say that your 69 Chevy has over a mile of wire in it. But anyway I went on to figure the number of turns in the secondary buy taking the average circumference of 3 inches and dividing that into the wire length for 26,520 turns. Although these canister type ignition coils are not used in new vehicles you can still buy them as after market replacement parts. I got one of them off of E bay recently for about $15.00 and measured the resistance on it and it was only 9,000 ohms so I would have to assume it has fewer turns than my vintage ones.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi roger, very interesting project, though not exactly as I imagined it!
I thought one could drive an ignition coil by a very normal mazzili driver with IRFP260's, only using two inductors and an increased tank cap to set the frequency to some 1-2kHz. 60V input would be enough to maybe even roast the ignition coil; with 155V square wave from a half bridge I could get arcs bigger than you could from a single coil, and with IRFP260 mazzili you could push up to 200V sine which is likely more efficient for the coil.
One word of caution, If you ground the end of the secondary coil don't connect it to the ground of your power cord. The MOSFETs don't like that for some reason. Having a direct fed power supply like this can cause some strange things to happen.
Well, I guess this proves just how unstable this configuration of a royer circuit is at high voltages!
For a low frequency royer oscillator, I would recommend including some significant resistance in *series* with the very mosfet gates, since turn on/off times aren't so critical anymore. I think you could safely use up to even few kiloohms at just 2kHz, along with perhaps some ferrite beads too. This should help improve the stability of the circuit.
Additional ponderings incluse:
- Have you found what is the actual self resonant frequency of your series-coil resonator? You could probe it the same way as you would a tesla coil, with primary shorted.
If you're using 3 ignition coils in series along with ferrite rods in them, I assume the resulting resonant frequency should be quite low, perhaps in low kHz or so. I doubt it's as low as 2kHz, but it'd be nice to check it. If you could tune your royer oscillator to drive it at resonant frequency, you could increase your voltage output drastically (you probably don't want to use full mains voltage for that in start!). You might even get some streamers but I can't put any guarantees on the survival of the coils!
- Also, your mot's don't seem like they have any airgap in them which would cause them to saturate under the load current... but the effect of saturation in the end reduces inductance in similar way an air gap would so it might not even be bothering the circuit at all. You could check out the current ripple with your scope (use an isolation transformer for power and a current shunt). If ripple is significant then you should perhaps consider adding some air gaps to your inductors.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
Hi Marko, On the resonant frequency I figured it to be around 2 KHz based on calculations but I haven't tried to measure it yet with a scope and a signal generator. Perhaps someone here with a good ear for music can tell what the frequency is by watching the video. Of course you would have to divide the audio frequency by two to get the electrical frequency. Maybe someone here has one of those fancy electronic guitar tuners and they could check it and tell us what it is.
As to getting the secondary to resonate, I actually did that somewhat accidentally by putting a 10 inch diameter top load on the thing. The voltage output doubled but I started to get some arcing going on in the oil bath. If I could insulate it better it would be a cool way to make a low frequency Tesla coil.
Thanks for the suggestions on the gate circuits and the inductors. I have been wanting to try some different things with the inductors anyway. I just used the MOTs because they where handy.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
Patrick - yes it goes down about an inch below the primary. I might have been better off to use a little bigger diameter tube. Perhaps a 4 inch PVC pipe. The existing primary coil form is just under 3 inches in diameter.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
RogerInOhio wrote ...
Hi Marko, On the resonant frequency I figured it to be around 2 KHz based on calculations but I haven't tried to measure it yet with a scope and a signal generator. Perhaps someone here with a good ear for music can tell what the frequency is by watching the video. Of course you would have to divide the audio frequency by two to get the electrical frequency. Maybe someone here has one of those fancy electronic guitar tuners and they could check it and tell us what it is.
As to getting the secondary to resonate, I actually did that somewhat accidentally by putting a 10 inch diameter top load on the thing. The voltage output doubled but I started to get some arcing going on in the oil bath. If I could insulate it better it would be a cool way to make a low frequency Tesla coil.
Thanks for the suggestions on the gate circuits and the inductors. I have been wanting to try some different things with the inductors anyway. I just used the MOTs because they where handy.
Roger
Well, the coil sounds pretty much like mine did when running at 2kHz (I actually expected purer tone from yours since it's fed by sine wave).
Arcing in the oil bath sucks, good it didn't do any lasting damage?!
I proposed another contraption very similar in principle, but involving a single layer coil (or a coil composing of multiple separate "pies"), possibly with a ferrite rod inside and a fairly sized capacitor in parallel with the coil. The primary would be wound on the ferrite rod inside and this could be driven by a mazzili circuit or some other way. This would be easy to construct and could provide a fair amount of HFHV power for CW multipliers and like without being too large itself.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
Marko
I proposed another contraption very similar in principle, but involving a single layer coil (or a coil composing of multiple separate "pies"), possibly with a ferrite rod inside and a fairly sized capacitor in parallel with the coil. The primary would be wound on the ferrite rod inside and this could be driven by a mazzili circuit or some other way. This would be easy to construct and could provide a fair amount of HFHV power for CW multipliers and like without being too large itself.
Marko
I like the Idea of using "pie" or "pancake" coils and having them separated by some type of insulation but it might be hard to construct.
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Modified Ignition Coil Powered by Royer Ocsillator
