Sounds like maybe I should just scrounge my well stocked junk boxes and build a supply from scratch. I am sure I have all the components .... diodes, transformer, filter choke and caps. Just like we did in the "old days" HI HI
________________________________ From: Brian Clarke <[email protected]> Sent: Wednesday, November 30, 2016 8:24 PM To: 'Jim Wiley'; 'RAY FRIESS'; 'boatanchors'; 'Old Tube Radios'; [email protected] Subject: RE: [Boatanchors] 12 volt supply question Sorry Jim, You need to take your exceptions to world reality, thus: The only thing almost correct about your assertions is your first sentence and only for the USA. Everywhere else in the world, safety is taken seriously, and the SMPSU case is connected directly to the mains Ground pin in the mains plug, usually an IEC chassis connector. The computer SMPSUs that adorn our various computers, printers and other digital devices all take the mains directly into the SMPSU box or printed wiring board. Inside that box or on that printed wiring board are: *EMC filters, unless of the SMPSU is of Asian origin *Mains fuse *Rectifier *Filter capacitor *Series inductor, usually the transformer primary *SCR And all these are operating at your mains Voltage multiplied by sq rt 2. So, in Japan, 141 Vdc, in USA, 163Vdc, and in almost all other parts of the world, where 93% of the world's population lives, 325 Vdc. In some of the earlier SMPSUs, there was a switch to shift from direct rectification to Voltage doubling, almost entirely for the US market. In slightly later SMPSUs, this switching was automatic, based on sensing the mains input Voltage. In modern SMPSUs, the duration of pulses fed to the SCR takes care of any input mains Voltage variations; hence, the universal SMPSU running on between 90 and 264 Vac. These items are separated along the transformer core by an isolating strip of circuit board. On the secondary of the transformer are various low Voltage windings followed by cheap-as-chips half-wave rectifiers and their associated filter capacitors. All provision of low Voltages comes directly from the SMPSU box or printed wiring board. Some mother boards have Point of Application regulators or switchers to provide a local Voltage. Only one of the output Voltages is regulated in the SMPSU. A sensing circuit comprising two resistors in series is across this regulated output, the junction of the resistors if fed to the control IC, eg, a 494 or later chip. The output of this chip is fed via an opto-isolator to drive the SCR on the primary side. All the other output Voltages are then related to this regulated output via the turns ratio in the transformer. In some SMPSUs, the 12 V line may be regulated via a 7812 or similar chip - but this is by no means universal. 73 de Brian, VK2GCE On Thursday, 1 December 2016 1:35 PM, Ray said: I have to take exception to some of this. The mains power is not directly connected to case ground. In all the desktop computer power supplies I have ever fiddled with, the rectified raw AC goes directly to the switching converter, and nowhere else, which then delivers several different high-frequency AC voltages to the rest of the circuitry. Those voltages are rectified and filtered (and regulated) before they connect to the computer. The "raw" AC from the line never reaches the computer "innards". Let me try this another way: The rectified and brute-force filtered DC runs the switching inverter (for lack of a better word). The high frequency AC from the SECONDARY of that transformer is what is used for the various voltages inside the computer cabinet. If you check with a VOM, you will find that neither side of the AC line is directly connected to the cabinet or motherboard ground. The high frequency inverter transformer performs the "isolation transformer" function. A person still has to deal with all of the other issues that were raised, including dealing with voltages approaching 200 volts (or more) DC inside the power supply, but getting electrocuted by a direct path to the incoming AC line is almost certainly not one of them. This does not mean that a person cannot be injured by voltages found inside a desktop power supply. This can definitely happen, so caution is important. If you are not familiar with working on power supplies, of any kind, the best advice is to leave that sort of thing to someone who knows what they are doing. The author of the article had mentioned using the 12-volt portion directly for smaller loads and modifying the 5-volt section (with appropriate component changes) for larger loads. As I said, look up the QST article for more info. Also, and again as I pointed out, this applies to the "standard" configuration AC operated DESKTOP system. Laptops and other portable equipment may have other configurations where the above comments do not apply. - Jim, KL7CC _______________________________________________ Boatanchors mailing list [email protected] https://puck.nether.net/mailman/listinfo/boatanchors
