One of the discussions on the BITX20 IO list over recent weeks has been a variable power supply to control power out on different bands. This seemed like the wrong approach to addressing variable output of the µBITx to the editor of ubitx.net (where the problem of inadequate drive or inadequacies in the PA design should be addressed directly) until he spotted this circuit diagram drawn up by Walter W9KJO, based on earlier suggestions. It is simple and straight forward with the TIP142 device at the centre of the “buck” function. 24v input can be adjusted downwards to set the power output of the µBITx PA stage.
Walter says, “It controls voltage nicely. Really helps limit output power while working digital.”
However the TIP 142 really generates some heat. He has a heat sink on the device, but the heat sink will need to be much larger to be safe.
I’ve been working on a similar concept but trying to avoid any modifications to the ubitx board ( I don’t trust my soldering at micro levels!) The concept is to use a buck boost module such as this one https://www.ebay.co.uk/itm/Auto-Step-Up-Down-DC-DC-Power-Supply-AT30-Converter-Buck-Boost-Replace-XL6009-UK-/272694639853 to provide varying voltage to the PA alone. By setting the buck boost potentiometer at 24V with the input at 12V and the bridging it with 4 switched potentiometers selected from a from panel control. One could be a low power tune function. As the b/b module is so inexpensive even if you make a hash of soldering to the underside of the module board it isn’t the end of the world! The front panel control can be labelled with the bands covered. It needs a b/b module capable of the power levels anticipated, say 3A. On paper the one above will do it. I experimented today with a smaller one but it seemed to get very hot above 1.5A – no substantial heat sink like the one above. The concept could be expanded later for relay control if the constructor is happy to solder onto the filter control lines as outlined in other articles. One problem could be hash from the switching action requiring some additional filtering on the output.
Just an idea for you electronic experts, I’m just a retired mechanical engineer! I’m still putting my ubitx together so any comments please?