A cheap 12V / 38A power supply for a 100W transceiver
Modern Switched Mode Power Supplies are very efficient. They are small, stay cool, their fan is silent, and they are much cheaper (60 Euro) than transceivers brand power supplies. They generate noises, but that problem is to overcome.
I use a Thermaltake "Hamburg" PC power supply for feeding my Kenwood TS-57D. After the adaptions below, listening on my loop only a few meters away, i could not find any noises nor carriers from this SMPS on 80m nor 40m.
This power supply, bought at Conrad.nl, is designed for the German market. I therefore trusted that it really complies with European noise
generation regulations. It switchesnot at 60kHz, but at 99kHz, so it generates less harmonics on our frequency bands. After 10 minutes continues 100W carrier transmission, the case
was still cold, and the fan still noiseless running at low speed.
This SMPS delivers only 12.1V, which cannot be changed, as the sense connection must be connected to +3.3V. My TS-570D did not show to be hindered, as modulation reports indicated. Only the display illumination dimmed a bit at SSB peak power. A positive result from the low supply voltage is, that less heath is generated in the transceiver.
All output wires were removed and replaced by two thick + and - 12V wires.
Changing the SMPS output, wiring and denoising.
- Open the cabinet.
- Shorten the green output wire, and connect it with ground (-12V).
- Shorten the brown (3.3V sense) wire, and one orange (+3.3V) wire. Connect them together and insulate it.
- Unscrew the PCB.
- Remove all other output wires (yellow, red, black, orange).
- Interconnect all +12V points on the PCB.
- Interconnect all -12V points on the PCB.
- Solder a small 100nF / 63V film capacitor between a +12V and a -12V.point.
- Solder the new +12V and - 12V output wires. Use thick wires (6mm2) here. I used twin speaker cable for each.
- Screw the PCB in the housing.
- Check for short circuits.
- Close the cabinet.
- Check that the fan wires do not run near fan blades (acoustic hum generation).
- Both parts of the cabinet must make good electrical contact (preventing radiation of RF noise).
The goal is to isolate the complete SMPS for all in it produced RFnoises :
- The 230V~ input
- The 12Vdc output
- The metal cabinet
This is accomplished by a "floating" SMPS :
a. "cutting" both 12V wires separately for RF by means of ferrite
b. "cutting" the 230V~ wire, by means of ferrite
c. physically cutting the safety ground wire at the SMPS, to free it from mains ground.
d. insulation of the cabinet from all metal surroundings.
The SMPS cabinet is then still grounded for 50 Hz mains power via the -12V output wire and the grounded transceiver.
All wires should be wound as many times as practical possible through the holes of the ferrite cores.
The best ferrite for this project should have a Ui of 3000 or more.
The ferrite cores should have the biggest hole practical, for winding multiple turns thick wire.
As a second choice, use 28B1020 ferrites (12.8mm hole, mouser.com).
1. Cut the 3-wire Euro mains cable at ca.5cm distance from the Euro apparatus plug.
2. Wind the free end of a thin 2-wire mains cable as many times possible through a big ferrite core.
3. Connect the 2-wire cable to the 3-wire cable. The yellow / green safety ground wire is NOT connected.
4. Insulate the connection.
6. Directly near the SMPS cabinet, wind the + 12V wire pair as often as possible through a big ferrite core.
7. Directly near the SMPS cabinet, wind the - 12V wire pair as often as possible through another big ferrite core.
8. Directly near the transceivers cabinet, wind the +12V and -12V wires both two times through a big ferrite core.
9. Install the SMPS insulated from all metal.
For safety reasons the transceiver cabinet must be connected to ground (preferably to a noise-free antenna ground).
After disconnecting the SMPS ground connection, and installation of the ferrite clamps, the SMPS is floating for RF currents and -voltages.
As the power supply metal casing is internally connected to -12Vdc output wire, for 50Hz mains voltage the SMPS still is safety-grounded to the grounded transceiver.