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Old 14th November 2012, 11:42 AM   #11
vasko is offline vasko  Czech Republic
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You guys rock. I gaped it, and changed 470k with 1M, and now I have 2-3 x higher voltage.

Elvee, can you point me to some tutorial on web where I can read about smps transformer calculus? What you just wrote.

What I noted UC3843 is not suitable to give much power on low voltage, because of its I-sense pin 3, I've put there 0.1R, and it can't give more than 30W on 16V.

Only problem I have now is that circuit wont operate below 14V, which is strange, UC3843 should work above 7V. I don't know what is the cause?
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Old 14th November 2012, 12:09 PM   #12
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by vasko View Post
Elvee, can you point me to some tutorial on web where I can read about smps transformer calculus? What you just wrote.
There are many sources, this for example:
2001 Magnetics Design Handbook - MAG100A
Designing high-current chokes is easy | EDN

Quote:
Only problem I have now is that circuit wont operate below 14V, which is strange, UC3843 should work above 7V. I don't know what is the cause?
You lose ~1V in your FR102, plus an unknown amount due to ohmic losses in the 100µH, and the combined drop plus the ripple could be sufficient to trigger the UVLO at 8.2V
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Old 14th November 2012, 04:24 PM   #13
vasko is offline vasko  Czech Republic
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Thanks for links.

I tried supply with this PCL82 SE amp, and it works perfect. There is absolutely no noise.

Click the image to open in full size.

I use it with guitar on input and 32R headphones or 8R speaker. I'd like some more volume on headphones, but I insist that triode is first to pick up signal, and pentode on output, so some mini FX loop seems to be logical solution. I'd like to add BJT or FET between triode and pentode, on 180V, with current mirror perhaps. Do you have some circuit for suggestion?
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Old 14th November 2012, 05:01 PM   #14
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by vasko View Post
I'd like to add BJT or FET between triode and pentode, on 180V, with current mirror perhaps. Do you have some circuit for suggestion?
For what purpose? Additional voltage gain?
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Old 14th November 2012, 05:14 PM   #15
vasko is offline vasko  Czech Republic
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Yes.
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Old 14th November 2012, 07:55 PM   #16
Elvee is offline Elvee  Belgium
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I don't think it is reasonable: the first stage already has quite a high gain, near that of a µ-amplifier, and anyway the whole amplifier is closed-loop: just increasing the loop gain will lead you into troubles, since there is a transformer in the loop.
Also, the external gain will remain almost unchanged anyway.
Moreover, inserting a semiconductor gain stage would alter the tube character of the amplifier. If you go that way, it is simpler to simply get rid of the glass completely.....

Your best bet is to alter the feedback ratio, by reducing the 100ohm cathode resistor for example.

If you are concerned by the loading from the pentode, Miller effect, etc, you can add an active anode load acting simultaneously as a µ-amplifier and as SRPP impedance adapter.
You will get a marginally higher gain, a lower output impedance, and a minimally intrusive addition from a tube perspective.

Anyway, I am not a tube guy, and you should ask a moderator to move this part of the discussion to the tube section, people there will be in a better position to advise you.

Here is the comparison between the original and the actively loaded version, with and without load.
The MOS is a small, low capacitance one, it could also be a high voltage jFET with adapted resistors values.
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File Type: png AmpActiveLoad2.png (98.7 KB, 109 views)
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Old 14th November 2012, 08:16 PM   #17
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Quote:
Originally Posted by vasko View Post
What to put? 1M?



Core is cold.

There is no gap. Should I make it on all 3 pillars of 'E'? To put 1mm paper between?
Yes that's what I did, I used some cardboard on all 3 legs with a SMPS and it was ok.
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Old 15th November 2012, 07:03 AM   #18
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by vasko View Post
...I gaped it, and changed 470k with 1M, and now I have 2-3 x higher voltage...
Flyback PSU always needs air gap, there are many info sources, you just need pacience to go trugh them.

On the other hand, UC3843 has the noninverting imput of it's error amp connected to an internal voltage of 2,5V, so that is the voltage level needed at feedback pin witch is the inverting imput of the error amp, so let say the 200V output with the lower side of the divider a 10K pot plus a 10K resistor, that makes 20k, now 2,5v divided by 20k Ohms equals 125 micro amps that is the current of the divider, now for 200V on the output the upper resistor must be like this: 200V-2,5V=197,5, that is from the output 200 needed we substract the 2,5V needed for the error amp, and we got 197,5V left, now we divide that to the current of the divider and we get 197,5/125u=1,5M Ohms, so that is how the divider should look for the 200V needed, any other voltage levels can be easy calculated this way, you just need to know the voltage level at the EA imputs and establish a divider current witch is also the EA imput current and from that you can choose a the wright resistors, i would take out the 10k resistor between the Fb pin and the divider, it only makes the upper resistor needed that much higher.
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Old 16th November 2012, 03:15 AM   #19
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Weird, the output voltage range is 61.25 to 122.5V. That should be a 1M resistor, not 470k, so that the range is 127.5 to 255V.

The turns ratio is correct for 6.3VDC and 200VDC, respectively.

Wonder why I didn't catch that when I drew the thing!

Tim
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