[sdiy] Source for good power supply schematics

[Bob Weigel]

--
Jesse Yoder wrote:

I am looking for good schematics for DC power supplies, one regulated
at +/- 12V and one regulated at +/- 15V.  I had to design power
supplies in electronics class in college, but that was a long time ago
and I really don't remember much of anything,

Thanks,
~~Ponyboy

---

Hehe..neither did the 'engineers' who did a project for us a while 
back..after charging us 10,000 for 'engineering fees' I'd shown them a 
basic layout; thinking THEY (the..engineers...) were doing to make 
decisions like WHICH 5V regulator should we use for this RV application 
with refrigeration and so on kicking in all the time...and how much 
capacitance does that device require.
      Instead..."Miller Dial" hired some  hacks who literally threw in a 
7805 with NO CAPACITANCE.  After we destroyed the first chip I went in 
and my jaw dropped as I saw the utter stupidity that laid before me.  I 
absolutely couldn't believe it. I threw in appropriate parts and had it 
up and running in a bit of course...but man what a bunch of fraudeulent 
nin-com-POOPS! 
      Anyway, yes it's very important to, on any 78-- series or 79-- or 
340 series 317/337 etc...to have a .1uF capacitor near the output and a 
good sized from end filter to keep power always above the 'voltage 
headroom requirement' of the regulator in worst case circumstances.  
Calculate roughly from the current demand of the circuitry, the 
acceptable ripple or change in voltage during a transient condition and 
supply current limitation the value of the capacitance there. 
       There are higher end regulators that are more reliable and better 
in harsh environments like the one we wound up using on the project 
above.  Can't even recall the number without looking.  But for most 
applications those above work fine.  A 78/7915 pair in the case of you 
15V rails need, 78/7912 for the 12V should be fine.  I think they work 
as long as you have 2.5V above the regulation point or like that but get 
the spec sheet and check for exact values on the chip you choose to use.
       The front end of the power supply; you'll need some kind of 
transformer up to the continuous/momentary current deliver needs and a 
center tapped secondary is nice because then you can just ground that 
and run a bridge rectifier on the outer two legs to get 120hz half waves 
to charge up the capacitors in front of the regulators.  Use an 
appropriate fuse set to blow if things exceed the max current you 
anticipate possible for any amount of time.  eg. if you have chips that 
are going to consume half an amp on each rail at 15V, then the 
transformer in an optimal case is supplying that on each leg continuous 
average at 17.5V on the capacitors plus a fractionth of a volt loss in 
the bridge rectifiers so say 18V.  Transformers are quite efficient and 
you can usually just rough out some 10% loss approximation or something 
and not be too far off, so say 1/5th the current at the mains of 2x .5A 
then =200mA.  So a 1/4A fuse medium time blow fuse would be optimal 
probably in a circuit like this for reliability, and a fast blow for 
safety if you don't mind it possibly blowing errantly due to mechanical 
stress and the initial charging of your capacitors on turn on stress.  
You'd want to decide the type of fuse to use based on your caps also 
obviously. Huge caps can blow fast blow fuses. -Bob

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