[Boatanchors] 12 volt supply question

RAY FRIESS rayfrijr at msn.com
Wed Nov 30 22:56:39 EST 2016

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 <brianclarke01 at optusnet.com.au>
Sent: Wednesday, November 30, 2016 8:24 PM
To: 'Jim Wiley'; 'RAY FRIESS'; 'boatanchors'; 'Old Tube Radios'; boatanchors at puck.nether.net
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
*Filter capacitor
*Series inductor, usually the transformer primary

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
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"

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

- Jim, KL7CC

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