Electronics > Beginners
Transformer voltage for dual power supply (+12V / -12v)
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drussell:

--- Quote from: Hero999 on August 25, 2018, 10:44:38 pm ---If a bipolar power supply is required, it's better to configure the transformer as centre tapped and use one bridge rectifier, than a bridge rectifier on each secondary, because it will have one less diode drop.
--- End quote ---

Indeed.  It would be silly to use two bridge rectifiers in this case and it was my assumption that the OP intended to use just one since a +12/-12 supply was specified.  Obviously if you need dual floating supplies (which can be easily done with a transformer with two secondaries rather than one centre-tapped winding, without modification) then you need them separate but in this case, a single bridge is the hot set-up.  :)
belzrebuth:
Some great points here; thanks everybody.
Thing is that I've already have the PCB for this power supply so I can't use two bridge rectifiers or 7812/7912.
I believe the reason the designer used LM317/337 is to get as close to 12V as possible.

The way I see it if I end up using this transformer is that I should up my capacitance to 12,000uF and use schottky diodes for rectifying..


Does anybody have a recommendation for a schottky bridge rectifier I can use in place of the KBU401?
I often buy from TME.eu but I don't think I've found something there..
belzrebuth:
I'm going with this capacitor
https://www.tme.eu/gb/details/lgu1v153melb/105c-snap-in-electrolytic-capacitors/nichicon/
Price is good considering its size.

Most bridge rectifier packages state Vf of about 1V.

I've already have a KBU4J and FBIU4D7M1 here..
The KBU4J has a Vfmax=1V.
Ian.M:
Post a photo of the PCB.  I'd bet the best option is to use a chassis mount bridge or paired Schottkys in TO-247 or similar packages on silpads and either wire them back to the bridge footprint on the PCB or link out the bridge to turn the PCB's AC in terminals into + and - DC in.

N.B. packaged Schottky bridges are rare like hens teeth.  Generally when you need that high current, low Vf and fast recovery its a switching supply with a CT secondary so dual common cathode Schottky diode packages are pretty common.   High current common anode ones (that would be used for a negative rail) are a lot rarer, and series (half bridge) are rarer still.

I couldn't find an appropriate common anode one in stock at Mouser: quite a few show up in the search but when you check the datasheets they are actually common cathode.

Mouser have ONE type of series pair in stock: https://www2.mouser.com/ProductDetail/IXYS/DSSS30-01AR (100V 30A) $6.18 each, with a max Vf drop of 0.5V @3A @25 degC.  Its Vf drop decreases significantly with increasing temperature, so ideally you'd let it run rather warm, with the heatsink area determined by your max load current.

However at that price (and you'd need two) and relatively high Vf, you'd probably be better off with four individual Schottky diodes even though they'd be far more of a PITA to mount.
belzrebuth:
PCB photos here:

http://musicfromouterspace.com/analogsynth_new/POWERSUPPLY2009/POWERSUPPLY2009.php

I could use four individual diodes but  yes, it would certainly be uglier and more difficult to mount.
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