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On top of that the mains voltage will vary too. It might be lower (or higher) than the "nominal" 110/220/240V
You have to design for the worst case scenario (and then some), so with Linear regs you will always be throwing away a fair amount of power if you properly design for those worst cases. If this keeps you awake at night, then you should be thinking about a DC-DC converter.
Dave :)
I was one who recommended considering the 15 volt winding to provide a (low current) 15 volt regulated output. This works pretty well if you have a bit of extra transformer (for low current supplies, you often cannot get a small enough transformer to be fully loaded), because a lightly loaded transformer puts out a bit of extra voltage.
But remember to take the low line voltage case into account when you calculate the regulator head room.
I think that you could use a 2 x 15V transformer for a +/-15V linear PSU. The LM317 and LM337 need Vin to be 3V above Vout. Assuming the smoothing capacitors are big enough, a 15V secondary would provide 15 x sqrt(2) - 0.7 - 15 = 5.5V.
Yet some people recommend a 2 x 18V transformer in this application. Why ? It makes the regulators work harder. Is it to avoid the bigger inrush current associated with larger capacitors ?
Thanks in advance.
-- André Majorel (Counterfeit: texamud@coarse.com ajanyh@telemeter.com) "La presse doit diffuser des idées saines." -- Serge Dassault, propriétaire de la Socpresse.
(usually factor for 2 diode drops at about 0.8V-1V)
No straight answer, as it hinges on the maximum current drawn, the transformer quality and how much your're prepared to cough up for smoothing caps.
Cheap transformer, cheap caps, 1A load then go for 18Vac. Cheap transformer, cheap caps, 100ma load then go for 15Vac. Good transformer, good caps, 1A load then 15Vac.
From a sim' I already have ("good" transformer and caps) ...
18Vac, 2200u, 1Amp. Gives 21.5V to 25.1V ripple pp. (pass) 18Vac, 10000u, 1Amp. Gives 24V to 25V ripple pp. (pass) 15Vac, 2200u, 1Amp. Gives 17.2 to 20.5V ripple pp. (fail) 15Vac, 10000u, 1Amp. Gives 19.5 to 20,5 ripple pp. (pass)Also bear in mind the capacitor ripple currents run at about 2.8Amps rms for a 1Amp load. The 10000uF's can usually handle this in a 35V rating but (say)
2200uF's may only be able to handle this up at about 100V rating. Cost of each capacitor is markedly different. Minimising the smoothing capacitor costs is a classic juggling act.regards john
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