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In Reply to: RE: A Question About Your Switcher posted by Triode_Kingdom on November 21, 2016 at 09:45:44
There are two numbers on that part that I can see ..... one is KA7500B and the other is HYC 576 H.
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Thanks Vinnie! The KA7500B is the SMPS controller for that supply. I'm thinking now there might be a better way to do all this. Here's what I have in mind...
The lighting transformers are essentially high power, square wave oscillators with a step-down transformer. They're small and cheap, and they were my first effort at ultrasonic heating nearly 15 years ago. However, there are two drawbacks. First, they oscillate at a fairly low frequency. The frequency can be raised, but not as high as I would like. The limit is created by the oscillator transformer core itself, which saturates during oscillation. I've tried other cores, but haven't been able to push the frequency above 65 kHz or so. Worse, the frequency is load sensitive, and the newer models I bought want to run at 45-50 kHz when loaded near their rated limit of 5 amps. The second downside to these is that they're not regulated. The square wave amplitude varies as a function of AC mains voltage.
The DC switchers you bought are somewhat similar, but they add a rectifier and filter at the output. In addition, because they utilize a separate controller, there's no oscillator transformer to limit the frequency. The controllers I've seen (including the KA7500B) have one or more pins for setting oscillator frequency with a resistor and capacitor. The datasheet for the KA7500B states a 300 kHz upper limit, and while I think that might not be practical from the standpoint of efficiency, 150 kHz is probably a reasonable limit. That would be a huge improvement over 65 kHz! The controller also provides good voltage regulation, usually accomplished with an opto-isolator driven from the DC output.
So, with all this in mind, I'm thinking it might be better to use the DC supplies for ultrasonic heating. It should be a simple matter to cut the PCB traces at the (+) and (-) output terminals, and to rewire those terminals directly to the secondary of the internal transformer. That would provide a regulated, high frequency square wave for the filament. The DC circuitry could simply be left unused, in parallel with the transformer secondary, in order to provide regulation feedback to the controller.
If I get time tonight, I'll dissect one of the little 3A switchers I bought to see if they're similar. This could open up a whole new method for ultrasonic heating!
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Buy Chinese. Bury freedom.
Well, it turns out that the "3A switcher" is actually only a 1.3A switcher. I remember now that I bought these just to power a few 12SL7s and the like. It's similar in operation to the large unit you purchased, but with less sophistication. Primarily, the oscillator is discrete, so there's no controller IC. On the other hand, it oscillates like yours using a RC time constant (no oscillator transformer). In theory, it can be easily modified to run at higher frequencies. I also found the frequency doesn't change as a function of load, and that the circuit uses an opto-isolator to regulate the output voltage. Unlike yours, it has no output filter at all, and the input filter is not overly effective. So, it spews a lot of high frequency garbage out both ends.
Here's the switcher next to a thumb drive for scale:
The internal view indicates only two inductors - the common mode input choke on the left, and the output transformer near the center. The opto-isolator is just to the upper-right of the output transformer, and the transistor on the rear wall is used to chop the transformer primary.
Here's the output with a 1.2A load (10 ohms). The main wave is 66 kHz at roughly 10 mV. Notice how much dirtier this is than the larger units you bought with their nice, built-in output filters.
This is a delayed sweep view of the switching spikes in the photo above. I'm pretty sure this is the output transformer ringing. This energy is 25 MHz at about 40 mV. When the switcher is running, this junk floats onto everything, including the line cord.
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Buy Chinese. Bury freedom.
Well, I wish I had waited to read this before I ordered two of them to use on the filaments of the 26's. Oh, well we'll see what happens. Maybe I can tack one of my filters on it and clean it up a bit. I wonder at what rating they put the other goodies back in?
Edits: 11/22/16
As I explained before, your filter won't clean it up. There's some improvement due to the additional shunt capacitance, but a common mode choke does virtually nothing in this situation. All this differential output noise will just pass right through.
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Buy Chinese. Bury freedom.
I understand what you are saying, but it sounds better with my filter than without, and until I hear something I don't like I think I will settle for that. I can always do more later.
OK, I just wanted to be sure I was getting the point across. Maybe tonight I'll go into the small switcher and see how high I can raise the frequency. I have no idea what the response of the various components might be. The limiting factor is probably the output transformer.
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Buy Chinese. Bury freedom.
Well, the joke's on me. The "transistor" on the back wall is actually a 5-leg PWM controller. There are three versions of the part; "L," "M," and "H." Mine is "M," which means (what else?) the frequency is fixed at 66 kHz. I could buy the 100 kHz "H" version (actually a KA5H0265R) on eBay from Hong Kong or Malaysia, but that doubles the cost of this little switcher. Guess I'll just have to resign it to my original intended use of DC for my 12SL7s. Proving that higher frequency ultrasonic heating is feasible will have to wait for another day.
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Buy Chinese. Bury freedom.
I will be really curious to see how the 3 amp model works on the 26. Mine are due to get here today or tomorrow. Thanks for the post.
As with the 813 supply, the primary concerns will be that neither output terminal is grounded and the output voltage is reasonably clean. Maybe when you get them, you could open the case and tell me which controller they use? I'd like to order the 5A versions for some experiments, but it's a crapshoot regarding the ability to modify the frequency. Too bad they don't include the controller type in the auction descriptions.
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Buy Chinese. Bury freedom.
It didn't make it in today's mail, but I am hopeful for tomorrow. I will let you know when they get here.
That all sounds over my head TK. Have fun figuring it out and let us know what you find.
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