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Old 14th March 2019, 07:36 PM   #181
mondogenerator is offline mondogenerator  England
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Eli,

Is something like UF5408 suitable? If I recall they're fast recovery types, not Schottkys, but the PIV looks suitable, and they look beefy enough to handle the currents I'd expect in a smallish PSU.
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Old 14th March 2019, 08:02 PM   #182
kodabmx is offline kodabmx  Canada
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I use UF5408 all the time... They are perfectly fine.

I'd probably use these if I wanted Schottky... STPSC2H12D STMicroelectronics | Discrete Semiconductor Products | DigiKey
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Old 14th March 2019, 08:15 PM   #183
mondogenerator is offline mondogenerator  England
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That's good to know haha. I have a bunch of UF5408 although I typically use 35A bridge rectifier as I've yet to destroy one
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Old 14th March 2019, 08:26 PM   #184
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by mondogenerator View Post
That's good to know haha. I have a bunch of UF5408 although I typically use 35A bridge rectifier as I've yet to destroy one
I've cracked one in half with a short LOL

Your tag line is great BTW. I've connected a 10,000uF 150V cap backwards... A 5W 47R resistor got orange hot before the power was cut. It still works and so does the cap! "They don't make'em like they used to, eh?"
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Old 15th March 2019, 12:04 AM   #185
rpi is offline rpi  United States
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Was my calculation for R5-R6 correct - 7 Ohm 12W? It's different from 100 Ohm in the reference schematic. How about that 255V, 150V difference for B++?
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Old 15th March 2019, 12:27 AM   #186
tikiroo is offline tikiroo  New Zealand
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Your schematic is wrong for R5 and R6. They should be 100 ohm and tie togethor to form an artificial center tap in the middle. This centre tap then goes to earth (or in the case of the schematic you linked to, to one of the output valve cathodes which will be at some positive voltage). The two transformer wires go straight to the heaters set up for 12V as you showed in one of your previous schematics.

The B++ is the screen voltage. I haven't read the rest of this thread, but if someone recommended 150V then should probably stick with that. 255V will allow more power but will probably need a different load impedance.
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Old 15th March 2019, 01:36 AM   #187
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good idea
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Old 15th March 2019, 03:14 AM   #188
rpi is offline rpi  United States
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Thank you tikiroo, what should be the wattage of the R5, R6 ?
I'm still not sure how to calculate R3, R4.
Is it OK to switch this power supply on without any load?
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Old 15th March 2019, 04:41 AM   #189
VictoriaGuy is online now VictoriaGuy  Canada
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Quote:
Originally Posted by tikiroo View Post
Your schematic is wrong for R5 and R6. They should be 100 ohm and tie togethor to form an artificial center tap in the middle. This centre tap then goes to earth (or in the case of the schematic you linked to, to one of the output valve cathodes which will be at some positive voltage). The two transformer wires go straight to the heaters set up for 12V as you showed in one of your previous schematics.
I don't agree.
I think the idea as drawn on the schematic above, is to choose the value for R5/R6 to make sure that the heater voltage is correct (12.6 V RMS -10% / +5%). If the AC line voltage and the chosen transformer are working together to provide the correct voltage, R5/R6 can be omitted.

The 'artificial center tap' is a separate issue, and I agree it's useful and should use 100 or 200 ohm resistors.

Some of the resistor values cannot be calculated easily (or accurately) from simulations in my experience and as the schematic indicates, should be found by experiment.
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Old 15th March 2019, 05:56 AM   #190
tikiroo is offline tikiroo  New Zealand
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rpl was referring to another schematic linked to in his post #178 (where the 100 ohm resistors were used for a centre tap) then confusing them for the differently located resistors in his schematic. So I guess I chose my words badly when I said his schematic was wrong . As you say he may or may not need resistors in both locations. If needed for voltage drop to get down to 12.6V they will likely be under 1 ohm with a rating of a few watts.
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