Electronics > Beginners
Improving small unregulated power supplies
Zero999:
--- Quote from: Ian.M on October 16, 2019, 02:01:53 pm ---Availability!
You *TRY* finding a SMPSU that has a grounded inter-winding screen and absolutely no secondary side grounding at a half-way reasonable price. Then see how much extra such a PSU with say three outputs is. e.g. +/-15V for your analog circuits and +5V for your logic.
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You're right that a wall wart with an AC output is the most convenient choice for +/-15V.
You don't need a grounded inter-winding screen, just a PSU with a decent Y capacitor between the secondary and earth, which is quite common with laptop power supplies.
Grounding the secondary is a good idea and doesn't cause any problems, as long as you don't connect it in series with another grounded PSU.
Ian.M:
A 'Y' capacitor* from secondary to earth would be rather undesirable for a floating item of test equipment. When you initially make a connection from secondary side 0V to a ground referenced circuit, there will be a nice zap of current to charge/discharge that capacitor to the new potential. Not exactly desirable for a supposedly floating instrument.
That's why I specified an inter-winding screen, as the only way to return the HF leakage current from the inter-winding capacitance between the secondary and the 'hot' (switched) end of the primary whence it came without compromises# is to intercept it with screening before it reaches the secondary. Once you've got the screening, it might as well be grounded to minimize line frequency leakage current. Unfortunately inter-winding screens cost core window area so push up the size, weight and cost of the magnetics so are usually only seen in medical rated equipment, and purpose designed PSUs for precision equipment, where the extra cost is a minor factor.
* Is it a Y capacitor when it doesn't have to stand off line voltage, even if the part you use is rated as one?
# See above, & also, the leakage current causes bad things happen to sensitive devices under test if your suggested grounded secondary side, or grounded Y capacitor SMPSU ever looses its ground connection.
Zero999:
--- Quote from: Ian.M on October 16, 2019, 06:32:02 pm ---A 'Y' capacitor* from secondary to earth would be rather undesirable for a floating item of test equipment. When you initially make a connection from secondary side 0V to a ground referenced circuit, there will be a nice zap of current to charge/discharge that capacitor to the new potential. Not exactly desirable for a supposedly floating instrument.
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The value of that capacitor is tiny (the order of a few nF) and there'll only be a zap if the capacitor is charged, otherwise nothing will happen. In normal conditions, the capacitor shouldn't be charged, as it's only coupling the secondary side to earth.
--- Quote ---That's why I specified an inter-winding screen, as the only way to return the HF leakage current from the inter-winding capacitance between the secondary and the 'hot' (switched) end of the primary whence it came without compromises# is to intercept it with screening before it reaches the secondary. Once you've got the screening, it might as well be grounded to minimize line frequency leakage current. Unfortunately inter-winding screens cost core window area so push up the size, weight and cost of the magnetics so are usually only seen in medical rated equipment, and purpose designed PSUs for precision equipment, where the extra cost is a minor factor.
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Yes, that does sound expensive.
--- Quote ---* Is it a Y capacitor when it doesn't have to stand off line voltage, even if the part you use is rated as one?
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If it's just connecting the secondary to earth, then I suppose it doesn't have to be a safety rated capacitor, but the switched mode power supplies I've seen do use a Y capacitor. I suppose it's to ensure the secondary can float at high voltages, relative to earth, if necessary. I don't see why it's a problem. Any decent switched mode lab power supply will have a Y capacitor connecting the output to earth. It's never caused be any trouble and I've not heard any stories about it creating problems for anyone else.
Anyway, I forgot the original poster is only interested in this for hobby purposes, not a professionally designed project. To answer the question: yes, a switched mode converter can be added to the output of an unregulated wall wart.
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