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The σ24 power transformer board



σ24 ("sigma 24") is a companion circuit board for the σ25 positive regulated power supply, σ26 negative regulated power supply and ε24 power switch driver circuit. It allows a standard EI-30 form-factor miniature transformer to be board-mounted and to supply power to the above circuits. Both single-secondary and dual-secondary transformer configurations are supported. The two secondary windings on a dual-secondary unit are connected in parallel.

σ24 can also be used to mount an EI-30 transformer for use with other circuits if such a transformer is appropriate.

The following is a photo of the top and bottom sides of the σ24 circuit board, and a photo of an assembled board.

WARNING: Building the σ24 will require that you work with AC line voltage, which is a lethal shock hazard. If you don't know what you're doing, please ask someone who is experienced to help you. (See the disclaimer).


The AC input terminals should be connected to AC mains. The AC output terminals should be connected to the AC IN terminals of the σ25 or σ26 board, or to the AC1 and AC2 terminals of the ε24 board. There is no need to connect any portion of this board to AC or chassis ground.

Parts list

The following is the parts list to populate the σ24 board. The listed power transformer is appropriate for an ε24 circuit. For the σ25 and σ26 you should read the "Transformer selection" section at their respective websites to determine the correct transformer for your application.

QtyPartDescriptionVendor part numbersNotes
MouserDigi-KeyAlliedFarnellRS Components
1PCBσ24 printed circuit boardAMB audio shop-
1T1Power transformer 12V, EI-30 encapsulated673-030-7136-0567-1031-5-ND70037414 --for 115V AC mains
---1689049347-3401for 230V AC mains
1J1Phoenix Contacts 2P .200 screw terminal block 1729128651-1729128277-1247-ND700544053041440193-0586-
1J2Molex KK 254 2P header 22-23-2021538-22-23-2021WM4200-ND701906061462926483-8461-
1-Molex KK 254 2P crimp housing 22-01-3027538-22-01-3027WM2000-ND701906491462825679-5363-
2-Molex KK 254 crimp terminal 08-50-0114538-08-50-0114WM1114-ND701906151462641172-9178-
--imperial #4-40 or M3 machine screws, standoffs and hex nutslocal or online hardware store-

Compatible transformers

σ24 is compatible with EI-30 encapsulated transformers with a single primary and and either single or dual secondaries. With a dual secondary transformer, the two secondaries are connected in parallel. Tranformer vendors:

The circuit board

The σ24 board is made of high quality FR-4 glass epoxy, single copper layer with top-side silkscreen and bottom-side soldermask. The dimensions of the board is 2.0" x 1.2" (50.8mm x 30.5mm). The thickness of the board is 0.062" (1.58mm). The dimensions and mounting screw locations of the σ24, σ25, σ26 and ε24 boards are identical, therefore they can be "stacked" together with 1" long standoffs. The following image shows the board layout:

Clean the bottom (copper) sides of the blank σ24 board with paper towel and isopropyl alcohol or electronics flux remover before soldering any parts on it.

Additional information

Copyright © Ti Kan
All rights reserved. Commercial use of this design is prohibited without prior permission.