I do have an old Soviet "green radio" R-109m which can cover 21.5 to 28.5 MHz which makes it pretty attractive for playing in the field spooking CB-users :-)
But, a big problem on those old radios are the batteries; they used two sets of NHK-14 each consisting in two batteries in series to get + and - 2.4V.
Since I got this radio I used a simple common ground divider and a 6V/6Ah Lead battery but the big problem is easy to spot: the battery will be quick depleted by the current that goes through the resistors.
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I tried to find on Chinese sites a module that can convert a voltage between 9-15V to +/- 2.4V with common ground but I couldn't find one and I did this through an extensive period of time, around 10 years.
So, being a brave ham, the next logical step was to make my own power supply...
While doing one with linear regulators like LM317 / LM337 could solve the job, the power dissipation would make them unsuitable, being just an expensive version of resistor board I tried first.
Another solution would be to use one LiIon/LiPo cell per each power line (one for positive, one for negative with common ground) but those cells have +4.2V when full charge and that could be too much for the transistor-based DC/DC converter that is built in in these radios!
I now that first hand, I took out the smoke from a few of them when I was in the army!
Also, the voltage from the batteries is going to various places in the radio so the positive and negative rails will have different currents so each one must be maintained under 2.4V.
So, with the help of a few nice people that offered some valued advices, I made my first Switched Mode Power Supply from scratch.
Of course, the first iteration didn't get too well; ita failed to keep both voltages well regulated when I draw a bigger current on one of the lines, so I had to made a second one.
This time, everything went perfect!
The PCB's looks awesome. I ordered 5 pcs.!
... and it's works too!
Next step will be to make a proper case to put the fuse, the switch and so on.
Some specs:
-Frequency: 50 KHz (+/- 5%)
-Tested input supply voltage: 7.5 to 17V.
-Input recommended voltage: 10 to 17V.
-Current draw when no load attached: 4mA
-Observed efficiency: better than 90%.
-Output voltage (settable): +/- 1.4V to +/- 5V.
-Max, Output current: 2x 2.5A for short periods, 2x 1.5A sustained.
I did some tests at 100 KHz and it is working well too and, by choosing the right capacitors, a +/- 12V can be achieved without any problems, making it a nice power supply for other old "green radios".
Important thing is that now, I can power my military radio from a large range of batteries, from RC LiIon packs to car battery!
Here is the temperature after some time in a stress test.
Input: 13V
Current: 2.5A.
No radiators.
By choosing the right wiring of the transformer and cooling, the SMPS can be used to power a broader range of radios with higher current or higher voltage (with right capacitors in the secondary circuit).
All relevant documentation is on Github.
Waiting for the case, I bent an aluminium strip and made a little radiator and did some stress test for a longer time.
Gate signal in blue trace and Drain signal in yellow trace.
Load:
Transistors:
Diodes:
So, after a painfull process of learning some elements of CAD I did some sort of a case, and sent it to a small factory to cut the aluminium and bend it on some machines...
