[time-nuts] power supplies

[ed breya]

Power supplies are indeed mostly commodity items, but absolutely 
essential. I have a large collection of OEM open-frame units, in linear 
and SMPS, and a small collection of commercial bench supplies, and some 
home-made ones.

You can of course get a bunch of bench units and set them up for every 
possible need, and with a dazzling array of readouts and adjustments. 
What matters is what you mostly need to do, and what you want not to 
happen. If you have to work on one thing at a time, with one to a few 
voltages, it's pretty straightforward to set up and be ready to monitor 
what's going on. If you have a bunch of things that use standard 
supplies, that just need to run simultaneously, I think it's better to 
just have simple supplies that put out a bunch of standard voltages for 
them, with no fancy controls or measurement capability.

I've designed and built and used many SMPSs over the years, which is why 
I prefer good old-school linear supplies. If you don't need high power 
density or levels, you should go with linear, regardless of whether it's 
a commercial bench unit, OEM, or slapped together yourself. It will 
start out much cleaner, involving less grief when it comes to noise 
control - almost a guaranteed issue in time-nut type situations. You 
can't avoid SMPS noise entirely, since we're awash with their signals 
all around us, from PCs and equipment and appliances and practically 
everything else, but at least you can try to not add too much more right 
in the middle of your work. Don't get me wrong - I love SMPSs and 
related technology, and all the stuff they enable us to have and enjoy. 
Life would be a lot different and less fun (but quieter) without them.

If you just need an assortment of supply voltages at modest currents, 
one way is to put together an OEM linear supply like a standard +/-15V & 
+5V one, with some three-terminal regulator circuits at other desired 
voltages, and means to hook them up to the items. Refinements like 
OVP/RVP etc can be added, but of course add some complexity.

One thing I always do, especially carefully for high-value or 
one-of-a-kind items, is add intrinsic protection against any anticipated 
powering faults like over-voltage and reverse polarity. No matter how 
fancy your supplies are, it's still possible to make a mistake in hookup 
or settings, and ruin the device being powered.

The same applies to the power supplies themselves. With a bench supply, 
you don't necessarily know what would happen to it and its load, if you 
say, accidentally short a +12V supply to the +5V. Would it damage the 
supply and its load? If you fully know the characteristics and specs and 
what's behind the front panel, you can predict the outcome. If you built 
it yourself, you know that that you've (hopefully) anticipated the 
possibility and provided for this event, or, the supply may not be 
protected, but you've already added intrinsic protection to the load device.

So, it can get complicated, and as always, it depends...

Ed