[Boatanchors] 12 volt supply question

[Jim Wiley]

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



On 11/30/2016 3:57 PM, Brian Clarke wrote:
> Hello Ray,
> There are several considerations:
> * Safety - computer SMPSUs run directly off the mains - there is no
> isolation. So, the Voltage on the first filter capacitor is enough to kill
> you.
> * EMR - how much noise does the SMPSU generate that your receiver objects
> to?
> * EMC - when you PTT, what happens to the Voltage regulation? Try this first
> with a purely resistive load corresponding to maximum transmitter current,
> eg, if you need 10 A at 12 V, the load needs to be 1R2.
> * SMPSUs are designed for a minimum current load - will your receiver demand
> this?
> * What happens to output Voltage and ripple when your transmitter draws full
> current, especially an FM rig or if you are into heavy-duty digital modes?
> Check with a CRO.
> The 12 V output on most computer SMPSUs is only good for about 2 A maximum,
> whereas the 3.3V and 5V outputs can deliver 20 to 30 A, or more. Only one of
> these two outputs will be used for Voltage regulation - not the 12 V output.
> If you want more current at 12 V, and you want it regulated, consider
> rewinding the 5 V winding and replacing the output capacitors to handle the
> higher Voltage; you will also need to alter the feedback loop to regulate to
> 12 V output rather than to 3.3 V or 5 V.
> 73 de Brian, VK2GCE
>
> On Thursday, 1 December 2016 10:17 AM, Ray asked:
>
> OK gang ..   Can someone provide a definitive answer to this question?   I
> see all kinds of conflicting opinions and would like to know what the
> thinking is here.  Or, if someone here is using them for that purpose.
>
> Computer power supplies.   Can they or can they not be used to power ham
> rigs such as two meter rigs, or even
>
> low power HF rigs?
>
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