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Calculate losses within a reg. PSU / matching Transformer

fred sonnen

fred sonnen

pfm Member
I build regulated PSU to get 500mA@5V respectively 400mA@17V and wanted to know the matching Transformer.
I got a answer I do not understand. Is anyone able to translated it for a Dummy ;-)

"The basic rule is that you'll get 1.4 times the AC rms voltage if you rectify and smooth the voltage. One more thing to consider is the open load voltage of the transformer. This voltage is higher for small transformers. If you read the datasheet of the transformer you can find out what the max output voltage to be. The goal for you is the have 5 V more at all time at the LM317/337 regulator. If you omit this pre-regulator then you'll gain 2.5 volts."

quite simple for a Pro, for me.....
Thank you ;-)
 
The answer quoted above explains: rectification of AC to DC, and that each regulator needs 1.25v and ~3.5ma each to regulate. The transformer choice depends on if doing full wave bridge or full wave center tap rectification.
 
I build a SSR03 PSU.
ssr03r0_overview.jpg

And the BOM said I used 2 Voltage regulator:
LM317 positive, floating
LM337 negative, floating

The webpage explaind: "The full-wave rectifier bridge is built up by four...."
 
From the text you have posted I think you need 5V more at the input of the regulator than the voltage you have coming out of the regulator. Regulators work a little better if you give them a little more input voltage than what they require (up to a point)

Open load voltage of the transformer - I think it means that a small transformer produces more voltage than its specification when not loaded (voltage regulation) but that voltage will decrease when a load is put on the transformer. Larger transformers have better voltage regulation and so will produce roughly the same voltage as their specifications with no load - and the voltage decreases very little when the load is applied.

Hope I have that right and it makes sense. :)
 
fred sonnen said:
I build regulated PSU to get 500mA@5V respectively 400mA@17V and wanted to know the matching Transformer.
I got a answer I do not understand. Is anyone able to translated it for a Dummy ;-)

"The basic rule is that you'll get 1.4 times the AC rms voltage if you rectify and smooth the voltage. One more thing to consider is the open load voltage of the transformer. This voltage is higher for small transformers. If you read the datasheet of the transformer you can find out what the max output voltage to be. The goal for you is the have 5 V more at all time at the LM317/337 regulator. If you omit this pre-regulator then you'll gain 2.5 volts."

quite simple for a Pro, for me.....
Thank you ;-)
Click to expand...

That explanation is correct... near as damn it anyway. 3V is enough voltage overhead.

OK well you will need two transformers ideally or a dual secondary transformer. Ideally you want 6.5 -7V transformer for the 5V supply and Schottky rectifiers. Then 16V transformer for the 17V supply, normal rectifiers will do. Lots of smoothing next. us e the closest voltage up from those figures that you can find in available transformers (probably 8 or 9V and 18V).
If you want to get both outputs from one transformer with one secondary then it's going to have to be an 18V one and you will need to waste a load of power as heat with a large wirewound resistor in order to supply the 5V reg from the 23V or so DC from the rectified and smoothed 18V transformer output. This is not ideal! Fine for sound quality but lots of heat to get rid of and very inefficient.
 
The plan was to use 1 TT for the analouge proportion of the project.

So, continuous power rating: 100VA
Primary: 0-240V @ 50Hz
1st. Secondary: 8-0-8V / 2A
2nd. Secondary: 20-0-20V / 2A
3rd. Secondary: 20-0-20V / 2A

Then 3x SSR03 to get 1x 5V/500mA respectively 2x 17V/400mA
right?
 
OK that's not too bad then. Use 0 - 8V from the 8-0-8V winding (the other 8v on transformer is not needed), and do the same with the two 20-0-20V windings, ie use 0 - 20 for each 17V reg again leaving the remaining 20 unconnected. You could use just one 0 - 20V to power both 17V regs in fact. (They are two separate +17V regs yes? Not +17 and -17?)
 
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