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vasil

Sat Dec 27 2008, 03:47PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

I am starting to gather components for my future VTTC. The tube has 2.5 kW dissipation on the plate so I will need some more current. The main problem is that I have not access to a lot of MOTs and they will be probably different size, so they can have some power dissipation because different ratings.

I want to discuss with you two possible settings.

One of them is almost similar with the setup used by Dr Spark for his Quatro Pack. But because my MOTs will not be identical, I will rectify each chain of MOTs separately, as in the pricture:

In this way the voltage will be doubled and I will have some more current. There are some disadvantages: the internal resistence of the seried MOTs adds one to another decreasing the current. Another weak point is that I would use only half of the AC cycle.

The doubler used in the majority of the VTTC setups is the best option in my oppinion. The output voltage ripple above the 0 Volts, so I have always potential on the tube. I can not parallel in direct way the MOTs because they are different (so, a few tens of volts would burn in the weaker MOT), so I was thinking to a setup that allows to feed the tube with paralleled doublers. It uses a lot of diodes to keep the currenty on the correct way, but I feel that it could work:

I would have doubled voltage, doubled current (well almost) even the two MOTs are different...they are not allowed to short each other. The setup is drawn only for two MOTs, but it can use 3, 4, more xfrms, depending by the current capabilities of the diodes. I hope that it will work, because I dont think that I can achieve more than two MOTs :(

So, it would work?

Dr. Dark Current

Sat Dec 27 2008, 04:59PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

vasil wrote ...

One of them is almost similar with the setup used by Dr Spark for his Quatro Pack. But because my MOTs will not be identical, I will rectify each chain of MOTs separately, as in the pricture:

This is not a good approach. The secondaries will be only conducting one halfwave so they will be able to deliver less power (more resistive heating) and the cores can saturate from the DC bias.

Don't mess with complex circuits, the easiest way to extract the most power from a transformer already exists and is called a fullwave bridge rectifier. There are more effective ways but they involve power factor correction (passive or active).

Don't worry if your MOT's don't match, simply connect them in parallel, their reactance is enough to limit the "matching" current to a few tens of mA.

I would probably go with a SMPS, a simple half-bridge and a ferrite transformer, without any feedback etc. If you can get 4 similar cores from monitor or new TV transformers, you can make one "E" core (2x2) out of them which should be good for at least 4kW continuous. Some smaller IGBTs (~60A) should be enough to drive it.

vasil

Sat Dec 27 2008, 05:31PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

OK, thanx.

vasil

Thu Jan 15 2009, 07:34PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Found two MOTs. They are almost identical.

But with correct primary phasing and secondaries in parallel I have a 11 volts difference bewteen outputs, for 20 volts input from my variac, so it will be more than 100 V at 220 V. I can not simply parallel them.

I will choose another way. I will put the secondaries in serie and full rectify them. On the output of the rectifier I will put a small filter cap (a few hundreds pF), so the voltage wave will swing above 0 V line, almost like in a doubler output. (a staccato is also a possibility). So I will have more power at twice voltage and approx the same current as from one MOT.

Hope that would work.

Dr. Dark Current

Thu Jan 15 2009, 07:41PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

vasil wrote ...

But with correct primary phasing and secondaries in parallel I have a 11 volts difference bewteen outputs, for 20 volts input from my variac, so it will be more than 100 V at 220 V. I can not simply parallel them.

If it really will be 100V, then each of the transformers will be "dropping" 50V on its reactance. Now, the secondary leakage inductance is around 4H. Xl=2*pi*f*l=~1300 ohms. I=V/R=50/1300= approx. 40mA.

Does that sound excessive to you? That's approx. 1/10 the rating of a standard MOT.

EDIT: You can also take out one of the heater windings and wind a few more turns of primary in there to get the voltages ballanced.

vasil

Thu Jan 15 2009, 07:58PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Ok, thanx, I will try with them in parallel then. If does not work it is simple to re-wire the things. Adding the fillament winding is a good idea.

Dr. Dark Current

Thu Jan 15 2009, 08:12PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

If you don't want to mod the MOTs (this should be pretty easy though), you can connect a small transformer between the MOTs like this. The whole winding only needs to be rated for the difference of your voltages e.g. 5% of 220V which is 11V and wire thickness the same as MOT primary, so this should be an easy to make little transformer.

Or you can use any >10VAC >5A centertapped transformer's secondary winding.

vasil

Sat Jan 24 2009, 05:59PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

The forum lost the last messages?

Dr. Dark Current

Sat Jan 24 2009, 06:12PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

vasil wrote ...

The forum lost the last messages?

yes, see start page

vasil

Sat Jan 31 2009, 11:36AM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Sorry for the late news but I have so little time to spare with the projects..

I tryied the inductor variant suggested by Dr Kilovolt and achieved a 62 V reduction between the two outputs of the MOTs (from 92 V to 34 V), but it was the biggest core I had in the house and had not another one.

So I folowed my own variant with coupled doublers. The final schema is here:

The cap C is charged to the highest voltage from the two MOts and the currents just add one to another. The diodes allow the correct way for the current, so with a bit of luck I will not have parassite circulating current through the weaker MOT. The schema allow also to add more MOTs in parallel to get more power:

I built the source with the goddies I had in hands, today it was ready to experiment with:

For comparaison I just used my previous built VTTC:

-the output spark using a single MOT:

SINGLE MOT

-the output with the new source: longer and more powerfull sparks:

NEW PSU

Of course the final exam will be the test with the big russian toob:

But I have a feeling that I can hope that will be ok.

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