[Boatanchors] [BoatAnchors] 12 volt supply question

[Bill Henderson]

When a minimum load is needed on a computer supply I am converting I will
often use an LED and double it as a visual indicator of power "on".

Has worked fairly well so far.....

 - Bill H. / va3hwa



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On Fri, Dec 2, 2016 at 2:12 AM, don davis <dxguy at earthlink.net> wrote:

> The reason you don't need min load applied externally to the PS is that
> each
> output has its own bleeder/min load applied.  All modern switch mode power
> supplies have outputs that will drift up due to high speed rectifying
> diodes
> (reverse recovery energy) causing switching spikes when they transition.
> Without some small load the spike peaks get rectified and the output caps
> charge up and they stay charged since only leakage will bleed the charge
> off
> (slowly).  Many years of designing these tells me I usually needed ~1% of
> nominal load to stay within eol +/- 5% voltage reg at no load.   In the
> example you provided each 5 volt output has 100 Ohm resistors dissipating
> 0.25 W which should be enough to keep no-load voltages under control.  Note
> the coupled inductor in the outputs, it helps keep the light load condition
> voltages under fairly good control.
>
> Ben's right that the noise level of these "depends".  If these are very old
> they might follow MIL-STD-462 / 462 RQMTS.  If so they might have good
> Common-mode rejection (as shown in your example) and sealed EMI filters
> with
> cavities, isolation between PRI/SEC, good ground bonding, etc.  Isolation
> from mains or PRI side depends on the end-user RQMTs.  Most space
> applications require ~ 1.0 MOhm or more, and in fact most have total
> isolation.  Most switchers nowadays use pulse xfmrs to send the control
> signal from SEC to PRI.  Opto-couplers aren't used in space anymore due to
> catastrophic failures of the HP-xx pattern part on orbit a long time ago,
> dunno if comm'l stuff uses them anymore.
>
> 73 de don ad6pb
>
> -----Original Message-----
> From: BoatAnchors [mailto:boatanchors-bounces at theporch.com] On Behalf Of
> Ben
> Hall via BoatAnchors
> Sent: Thursday, December 01, 2016 4:34 PM
> To: boatanchors at theporch.com
> Subject: Re: [BoatAnchors] [Boatanchors] 12 volt supply question
>
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