DC Power Tesla Coil

[Jeg]

Hallo again.

I still get at point B the same value with point A.

Well, i tried with
1. no de-qing diode,
2. other MOT as charging coil, i tried with open/close primary coil of the charging trans.
3. Smaller Cap at the end (5nF)

I also tested without cap, and still the reading after the charging coil is 2,2KV. The same as at point A!!!!!

Do you think that i have to ground something at this schematic? I just grounded the PCB input Line filter and limiter! Nothing else!

Tnks

[TinselKoala]

Are your bridge diodes working properly? What is the part number of the diode you are using?

My suggestion is to break it down by steps. Completely disconnect everything to the right of the main MOT. Leave the left side alone with the two MOTs as chokes, that is OK. Test the open-circuit voltage output of the main MOT. It will be AC... it had better be AC! Be careful here. What is your _rms_ voltage reading?
Now hook up the FWB only, and read its output. This should be "pulsed" DC at twice the line frequency and the peak voltage should be substantially higher than the AC rms reading. It would be excellent if you had a Variac, so that you could do both these tests at low input voltage first.

[Jeg]

FWB consists of 24 X 4007 diodes. Each diode has a voltage rating up to 1000V.

I just tested it again by the help of an oscilloscope. FWB work as expected. A rectified signal at 100HZ. Capacitors smooth it at a decent level. RMS is 2,2KV. Then i connect the charging coil and the final capacitor and nothing changes!!! Nothing at all! Is there any possibility to need more inductance? I am thinking to try two mots at series as a charging coil. Any other ideas?     

[TinselKoala]

OK... so you've confirmed that the FWB may be working properly, under no load. It is not how I would have made the bridge -- I would have used higher current diodes and I am surprised yours are holding up, actually. You raise the voltage handling when you put them in series but the string can still only handle the current of a single diode, and the 4007 is rated at 1 amp continuous rectified current. I think in full operation these diodes will fail pretty quickly. May I suggest that you construct your bridge from heavier diodes? I recommend the sturdy workhorse 6A100 (data sheet attached below.) They cost under 50 cents US each, as low as 27 cents in single units from the right supplier, or as low as 17 dollars for 100 pieces.

(I am assuming you are using 6 in series for each leg of the bridge, but if you are using 2 parallel strings of 3 in series for each leg, that increases the current handling to 2 A but interesting things will start happening as diodes fail....)

I appreciate your answer but I still can't get the info I need.
1. What is the AC output, unloaded, with nothing attached, from the main MOT? AC rms please.
Now hook up the bridge, and nothing else.
2. What is the DC output, unloaded, with no filter or anything else on the output, from the FWB? Peak DC output, the max height of the ripples, please.
3. Now hook the filter capacitor only across the FWB output and measure the voltage again. This should be your "decently smoothed" DC level and it should be nearly the same as the max DC ripple height.



Now... as to what I would do... Personally I would forget the "theory" and dump everything to the right of the FWB and feed the unfiltered, unchoked output directly to the primary tank circuit. As I showed in my single-mot DC blown SG coil. I think this would work fairly well with a properly configured rotary gap and I know it would work great with a multiple-element air-quenched gap. Of course you will still need current-limiting on the input, so I would suggest buying a good Variac. They really are indispensable for this kind of working.

But that isn't helpful to you, I know. Let's see if we can get the readings I asked for above. There may be nothing wrong with the 2.2 kV reading you are getting, or we could be seeing an underperforming bridge, or a really leaky capacitor. So let's look at the voltages from the beginning.

Have you got your spark gap built? Can you give the details of what you intend to use for a gap?

[Farmhand]

Hi Jeg and Tinsel, It is strange that the charging circuit is not working, again Tinsel is correct, if the charging circuit is not working there must be a problem so disconnecting it might show where the problem is. I would take a picture of my primary circuit but it is quite busy under the coil and would be difficult to trace the wire's and bits. One problem with using a MOT for a charging inductor is that the secondary is connected to the core/body of the transformer so the entire MOT core is at HV, I had to raise mine up off the wood base and sit it on wax blocks otherwise it arced into the wooden base and lost power but still tried to work, the other option is to disconnect the end of the secondary connected to the core but I don't think that way is without problems.

I didn't get the the charging circuit working properly until I used the rotary spark gap so that I had a reliable break rate. Your kinda jumping in the deep end using a charging circuit in a medium powered transformer as a first attempt. The charging inductance is related to the break rate and the HV caps it charges. It's a "Dark Art" with few hard and fast rules. The best way to gauge output voltage is to setup the circuit without the charging inductor or the De-Q-ing diode but with a safety gap across the Power MOT secondary about 1.5 to 2 mm and set a small working gap but keep increasing it till it stops and the safety gap fires, then add the charging inductor and de-q-ing diode and see if the working gap fires, when you can get a 3 or 4 mm spark on the working gap but the safety gap stops firing and is smaller then you know the charging circuit is increasing the voltage to the HV discharge caps. The actual gap in the spark gaps needs to be adjusted to "conditions" there is no actual distance can be given only approximate.

Open circuiting a MOT can damage the insulation if high enough voltages are reached, safety gaps prevent that happening, but using only one MOT the gaps will fire from HV to ground which I don't like to do, my gaps go from HV positive to HV negative not to ground. Is it possible the MOT insulation is damaged and the inductor is not really being utilized.

The charging circuit needs to have a certain amount of inductance, it is a bit flexible and it can work without the de-Q-ing diode but won't work if the FWBR is kaput or if the charging coil is shorted internally, diodes do weird things and they can test OK but not work properly, these circuits are hell on some parts and at times momentary currents can get to ridiculously enormous magnitudes.
I can't even test a Microwave oven diode, my DMM just says it's open circuit, if it showed a short then the diode is busted, but in between open and shorted funny things can happen also.

I use Microwave oven diodes for the FWBR and then I use two microwave oven diodes in series for the "De-Q-ing" diode. I found when the charging inductance was not enough
the supply can drive the primary directly through the spark gap which seriously heats the FWBR diodes as well as other things and uses a lot of power.

Just now I am not prepared to use my coil, I'm right out of space and the ground is dry as can be, if I fire up the coil with no ground connected it rattles the sub woofer connected to my computer by way of signals through the built in amplifier in the speaker system.   I won't use it unless I get good ground conduction. It'll rain within the month.

Don't allow it to frustrate you, you'll get where your going if you remain safe. Safety first, fun second.

Cheers