New HF Driver Idea

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Richard Hull
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Re: New HF Driver Idea

Post by Richard Hull »

While HF bridge driven systems are ideal in a number of ways, if you plan on leaping into this effort, you will need to be armed with a minimum of diagnostic and test gear. An old triggered scope can be found relatively cheap, as noted, at a hamfest. You will be buying a lot of silicon to replace blown silicon or gobbling up a lot of energy and producing a lot of waste heat without an o'scope.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: New HF Driver Idea

Post by Liam David »

I put 15V AC(60Hz) into the large wires that I thought were the secondary and got 70V out on the two springs. This seems way to low of a turn ratio to output multiple tens of kV at 60Hz in, so it must be a HF transformer.???
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Rich Feldman
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Re: New HF Driver Idea

Post by Rich Feldman »

You are off to a good start, Liam. Actually connecting and measuring things.

Did you really measure 15 volts AC between the two ends of thick-wire winding? (You said secondary, but probably meant primary.) Or is 15 volts just a value on label of the other transformer? Was the 70 volt output measurement AC, between springs, or DC from that rectifier stick in picture? Have you told us the electrical resistance between each spring and the core, or the other spring?

Now is the time to learn about your transformer (and electricity?) using low voltage at 60 Hz.
Then you will know what you need when planning to make, buy, or operate a HF driver.

When you are set up to properly measure primary AC voltage,
the next step is to measure primary AC current. No expensive new instrument is needed, and you can get advice here. ( I think current in wires is more fundamental than voltage, and should be taught first. )
Then with a variac, or a small assortment of low voltage transformers or primary ballasts,
you can chart the primary current vs. voltage at 60 Hz, and find out the saturation limit. Then we can revisit your turns ratio determination.

Meanwhile, we know you have taken the core apart and observed the mating surfaces. It's important to keep them clean and flat, and clamp them back together tightly, to avoid having an air gap. 0.001 inch of air would be significant. An electrical resistance measurement between two clean places on the outside of the core can positively distinguish ferrite from laminated metal.
All models are wrong; some models are useful. -- George Box
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Liam David
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Re: New HF Driver Idea

Post by Liam David »

I attached the 15V to the thick wires with the crimped-on bolt connectors. This primary has a resistance of 0.02 ohms. The springs, or the secondary, have a resistance of 110.6 ohms and this is where I got 70V AC out. From the outside, the core has inconsistent continuity, sometimes masking contact and sometimes not. I left out the rectifier stick to simplify things. The picture above is an old picture. I have since taken the square "case" off and separated the two coils. I'll plot the current vs. voltage.
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Richard Hull
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Re: New HF Driver Idea

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A 110 ohm secondary is all you need for a full ID that this is an HF system. If 60hz you would have to have about 10,000 or more ohms. End of discussion on what this is. Now, what is its optimum drive frequency. Got a signal gen and a power amp and scope. One of the next best steps, if you are serious about using this.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: New HF Driver Idea

Post by Liam David »

I hooked up a variac to the primary and turned the voltage to 5V at 7.3A. The output voltage was over 400V, compared to using a 15V 60mA power supply and getting 70V (because the output of flyback style transformers depend on the input current???). Over ~10V, the current was too much for my multimeter to handle (10A) and I blew the fuse.
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Rich Feldman
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Re: New HF Driver Idea

Post by Rich Feldman »

Good work there.
Looks like you have a good set of tools, and got realistic voltage ratio and current measurements. Your toy can become lethal. Fuses are there to protect the tools and toys, and flammable materials around them.

If you repeat the experiment with "15 V 60 mA" transformer, which gave you 70 V on the springs, I bet you will find the thick primary voltage to be much lower (around a volt?) and current to be much higher than those nameplate values. Today's wall warts with AC outputs are, IMHO, still simple transformers with high-ish output resistances. Open circuit output voltage is probably well above 15 volts. The wall-wart will see your thick primary as almost a short circuit. Its inaccessible thermal fuse might put up with that for a minute or two.

As Richard said, your secondary resistance values are much too low for a 60 Hz transformer. Andrew's report that it was used with an H-bridge driver confirms that it wants a higher frequency. The apparently steel core makes it an odd beast, but high core losses would be tolerable for the short duty cycle of an XRT.
I support the idea of experimenting with an audio amp before bridge jumping. A ballast resistor (12V lamp?) between amp and primary might help to protect the amp.
All models are wrong; some models are useful. -- George Box
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Andrew Robinson
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Re: New HF Driver Idea

Post by Andrew Robinson »

I must be invisible :) as I have mentioned multiple times in this and other threads and during the sale of this transformer to Liam that it is a high frequency tranny. I even posted pics a few days ago of the exact H bridge that used to drive this exact transformer. Not sure why people were still discussing this... Thank you Richard for attempting to finally put this question to rest once and for all :)
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Re: New HF Driver Idea

Post by Andrew Robinson »

I still have the small oil filled RMS cap that went with the H bridge (now with NC State) and the larger bank (Now with Bill Zinnikas). I'll pull those specs today or tomorrow as it may shed a little light on freq.
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NicKolas Garrett
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Re: New HF Driver Idea

Post by NicKolas Garrett »

Hey again, it seems pretty well decided that this is a medium frequency transformer. Unless you can actually see laminations in the core, you should by no means assume that it is made of transformer grade steel. It is much more likely to be ferrite. With a dc to perhaps 100khz sweep you can determine which frequency it will respond to best. But in all likely hood, anything above 10khz will work as well as any other frequency. Personally I'd pick about 50khz so I wouldn't have to hear it whining. _I_ specifically suggested powering THE SECONDARY not the primary for safety reasons. If your step up is more then 4:1 ratio you run the risk of serious and potentially fatal electrocution. You got lucky. And I want you to be safe, I believe we all want that. So please be careful. Any further measurements would be much safer if you just go ahead and solder a 10k resistor across those springs for now. This will cause a dominate voltage drop across the resistor instead of your fingers!
Another thing to realize here is once you've taken it off the core, only the voltage ratio is true. You've lost most of the coupling the transformer had before and so most of your measurements will be quite invalid. Good luck! I'll keep checking in to see if I can offer any advise or what have you.
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Rich Feldman
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Re: New HF Driver Idea

Post by Rich Feldman »

Hi Andrew. I think we've all seen your posts here and know it's a HF transformer. Unique in my experience because of the steel core.

I advised Liam on some 60 Hz measurements because that's where I would start with that transformer. Use the tools on hand. Learn design details of the particular coil and core. Get some practice with basic AC power and measurements. Now he knows the turns ratio, and can think about HF bridge voltage and current requirements.

The frequency to approach saturation (and the likely original design point) is proportional to bridge voltage.
Can't apply full bridge voltage before this tranformer is vacuum potted in oil.
Especially for a novice, let's tackle this in steps.

Primary magnetizing current for near-saturation is independent of frequency. I'm guessing tens of amps, and we can probably make a pretty good prediction using Liam's 60 Hz tools on hand. With a temporary sense coil we can learn the absolute turns numbers, and from core dimensions we can determine the maximum volts per turn per hertz. Also the forcing and sensing tool ranges can inexpensively be extended beyond 10 amps, keeping voltages "relatively" low.

This from a transformer engineering enthusiast without much HF driver experience yet. Liam may be more focused on advancing his fusor.

[edit] As Nicholas and other have said, when transformer is out on the bench it's safer to drive the secondary. What's the primary voltage with 120 V 60 Hz on the secondary? A volt or two? (Take steps to limit the secondary current, or work up to it with variac.) Can you wind a couple dozen turns of thin wire around the existing coils, and measure the voltage on that?
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NicKolas Garrett
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Re: New HF Driver Idea

Post by NicKolas Garrett »

Before you do much else I kind of insist that you acquire a *cheap* ~20 Mhz oscilloscope! You are not going to get vary far with inverter construction without one unless you go with a foolproof design like the mazilli circuit I posted. And i agree, you definitely do not want to power that transformer unloaded period, but certainly not when it is out of oil. At this point to repot that transformer you are going to have to use some of that nifty vacuum tech you have to draw air out of the transformer(windings spaces ect.) The moment you "un-submerged" it, it became unpotted. So i'd start working on that soon. While it is a good idea to determine the saturation limit of the V/HZ/turn so that you do not reach it in operation, I'd recommend you operate it far from that point. These things tend to destroy them selves quickly when abused.
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Re: New HF Driver Idea

Post by Eric Slighton »

Hi NicKolas,

I am experimenting along similar lines and would like to share thoughts.

Currently I am using a Mazzilli Driver ready-made from China (1000W at high frequency and up to about 100V), and a high frequency transformer rated for 40kV at 30mA. The circuit is virtually identical to your schematic except that it uses a pair of IRFP260 MOSFETS in place of each single MOSFET in the diagram, and it has eight 0.33uF capacitors in paralell. The two inductors are 100uH. my work coil is 2.5uH. With the 2.5uH work coil, the resonant frequency is around 62kHz. The transformer is similar to a flyback transformer but relatively beefy.

Driver: http://www.aliexpress.com/item/1kw-ZVS- ... 85575.html
Transformer: http://www.amazing1.com/transformers-hi ... uency.html

Nominally, this should be sufficient for fusion, but I doubt real life is so simple. One thought I have is that if the output is too weak, I could set up two and synchronize them to be 180 degrees out of phase. Then put the rectified outputs in paralell.

Unfortunately, I seem to have damaged the driver while experimenting with it as an induction heater, so I am on hold while I figure out what is wrong. It used to put out a nice 62khz sine wave at 3x the input voltage driving a 2.5uH coil. After damage it puts out 1/2 sine wave (positive only), leaving a high net DC amperage, so I am afraid it will simply burn out the primary winding of the transformer if I connect that. The work coil is 1/4" copper tubing and it gets hot quickly. [Any idea what might be wrong with the driver?]

Regards,
Eric Slighton
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