Simple noise free stabilized power supply
230V in ~ to 12V / 125mAdc out.
This simple little power supply is designed for use with noise-critical instruments, like a Miniwhip active antenna. It is a well filtered linear circuit, contains no switching circuit, therefore does not generate any HF-noises.
It can be left switched-on, day and night, and then prevents condensation in a small outdoor instrument, like the Miniwhip antenna.
Although a double sided PCB will be available, and a bottom copper mask for a DIY pcb is given, a printed circuit board is not strictly needed. See below.
Specified transformer Tr1 and sot-223 stabilizer LM2940IMP-12 are both short-circuit-proof. A fuse is therefore omitted. The rectifier is bypassed with four capacitors to prevent rattling noises and cross modulation caused by possible penetrating RF signals. Penetration is prevented with High-Ui (5000) ring cores F1 and F2, which block RF signals on both input and output cables. You should wind as much as possible turns THIN twin cable through the holes of F1 and F2.
The PCB is designed for two horizontal PCB connectors for AC mains voltage and 12Vdc output, and its installation into a Hammnond 1254JGY ABS box.
Schematic for PCB v15.
Download the schematic for PCB v15 HERE.
Download the parts list for PCB v15 / 20190108 HERE
The C7 Euro connector mains cable is connected to a C8p PCB connector on the PCB.
The 5 pole DIN PCB bus delivers the 12V output to a 5 pole DIN audio plug on the output cable (1-2 = minus, 4-5 = plus).
The four fully plastic bushes
at the mid of each side must be removed (do NOT remove the metal filled bushes
at the corners) :
- Use a 9mm drill in a table drill stand
- Drill de round parts down until flat with the bottom
- Break the remaining plastic flanges away using pliers.
As currents are small. You therefore should use thin twin cable to wind the ferrite chokes F1 and F2, with much turns as possible through the hole for optimal choking effect.
Remember : three times through the hole of a ring core is nine times more effective than one time through the hole of a ring core.
Drill a few 10mm holes into each long side of the box to allow some airflow cooling.
In the short sides each a hole must be drilled, to enable ACmains and DCout plugs to be connected.
If no PCB is to be used (air wiring).
The whole must than be hand wired. You could glue the transformer, the bridge rectifier Br1 and the elco C5 upside down , together with the wounded ferrite rings, onto the bottom of the box. Use thermal glue or quick hardening two component glue. Use for IC1 an LM2940CT-12 (TO220), with a small heath sink. Screw IC1 to a long side of the box, on 10cm2 aluminum strip for cooling.
All connections can be made using thin insulated wire.
All ceramic capacitors should be soldered directly onto the pins of Br1 and IC1, with the shortest wires possible.
IMPORTANT : Please
do use C6 with
the stated value 330nF, for stability reasons of IC1.
Wire the following connections to a common ground point, which is the middle pin of IC1 :
a. Connect the minus of elco C5 only to the middle pin of the MC7812.
b. Connect the minus of rectifier Br1 only to the middle pin of the MC7812.
a. Connect the minus of output core F2 only to the middle pin of the MC7812.