LINEAR PSUs

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sinus trouble
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LINEAR PSUs

Post by sinus trouble » Sat Apr 14, 2018 11:49 pm

Hello Necks! Its been a while!
I would like to share my findings with others regarding basic AC to DC conversion in unregulated PSUs, Now whilst I have used applications successfully many times in the past, I must admit I have never actually looked deeper into what is going on? (Basic electronics and I should know better)

Anyways I have based my observations on a toroid RS 671-9047
DATA.PNG
The transformer as seen has dual secondary windings rated @ 25vac 4.50a each so I should be able to rectify this and supply 25vdc to my circuit right??? Wrong!!! Its more like 38vdc!!
20180414-0001.png
Under 'no load' test the AC voltage swing is around 80V peak to peak! Just under 40V once fed through the bridge rectifier! (inc diode losses)
20180414-0002.png
So you may ask? Why rated @ 25vac? well it seems that 25vac is an RMS value! Without going into to the math, the smoothing capacitor will charge to the peak voltage and NOT the RMS!
I hope this helps!
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I am as stupid as I look! :|

Albert H
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Re: LINEAR PSUs

Post by Albert H » Sun Apr 15, 2018 2:19 am

25V AC rectified and smoothed will be about 35V ( usually a diode drop less than 1.414 times the secondary RMS voltage). Off load, it may be a bit higher (especially with smaller transformers with poorer regulation factors).
He who laughs last, thinks slowest! :lol:

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sinus trouble
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Re: LINEAR PSUs

Post by sinus trouble » Mon Apr 16, 2018 12:50 am

Quite right Mr Albert!
Taking an average bridge rectifier and filter capacitor into consideration, I have used as a basic rule the output voltage would be 10v higher than the AC voltage supplied! In lower current applications would follow some kind of regulator so irregular outputs would be less critical!
What caught my attention is where this extra energy came from? And how manufacturers rated their products!
A capacitor cannot charge to a higher voltage than the supply therefore the peak voltage had to be higher than the rated output! It leads me to believe that a transformer rating is an RMS/loaded value
I am as stupid as I look! :|

Shedbuilt
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Re: LINEAR PSUs

Post by Shedbuilt » Tue Apr 17, 2018 1:12 pm

Yes Sinus. As you've seen, rated transformer secondary voltages are always RMS, and yes; that rated voltage, should be at the full rated load. As Albert alluded to, smaller transformers tend to have poorer regulation factors. Also, in my experience / opinion, older transformers tend to have better / tighter regulation (ie less sag under load), than more modern transformers. This is partly due to core saturation (basically a transformer with the same ratings is built with a smaller core than used to be the case; partly to save costs, partly to save space and weight. Take the same thing and give it "better" specs; ie "sales engineering", but allow for more sag ie "design intent"), but also partly due to quality control of the core construction, quality of windings etc.

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Re: LINEAR PSUs

Post by sinus trouble » Thu Apr 19, 2018 11:00 pm

Cheers Mr Shed
As my preference of power supply I like that I still have a lot to learn! Despite its flaws, I can rely on it! Switch mode PSUs are great when working have let me down many times in the past!
I am as stupid as I look! :|

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