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4.5 Higher power output
The kit provides around 250mW of output power using a 5V supply and a single BS170 transistor
(depending on the band). The transistor gets warm to the touch. If higher output powers are desired, this
can be achieved by using a higher supply voltage for the
Power Amplifier (PA). The microcontroller must still be run
at a voltage not exceeding its 5.5V rating (5V is
recommended). To facilitate running the PA at higher
voltages to provide more power, the PA supply connection
is a separate pin.
At higher supply voltages and power outputs, the heat
dissipation in the BS170 will increase and its temperature will be higher. Provision is made on the PCB for
two more transistors in parallel with the first. This will slightly increase the power output even at 5V supply,
but more importantly at higher voltages it should share the heat dissipation between the devices.
It may be necessary to fix a heat sink to the transistors if the temperature rise is excessive. The transistors
are located near the edge to the board to allow a heatsink. Experiment is needed in this area, with PA
voltages up to perhaps 12V.
4.6 LCD backlight options
a) Full brightness: For basic operation connect A0 to A1 using a wire jumper (made from a
capacitor lead offcut, for example). The blue LED backlight of the LCD module will be at full
brightness. This is shown as the Red line (left). In this case, do not fit any resistor at the R2
position.
b) Reduced brightness: You may find the LED backlight of the LCD module too bright for
your liking, or if you wish to operate from battery power, perhaps you consider that it
consumes too much current. In that case, fit a wire jumper between A1 and A2, and use a
suitable valued resistor at the R2 position: experiment, e.g. starting at 100-ohms. 180-ohms
has been reported to work well. 220-ohms or 270-ohms may also suit your taste.
c) Software variable brightness: The U3 firmware contains a "Backlight" configuration
setting which can be set from 0 (backlight OFF) to 9 (maximum brightness). This is achieved
using pulse width modulation. To enable this setting, you must connect A0 to A3 using a wire
jumper (do not fit any resistor at the R2 position).
d) Switched backlight: If you do not wish the LCD module's LED backlight to be on all the time, then
instead of the wire jumper, you could connect wires to an on/off switch. This could be useful for battery
powered operation, to reduce current consumption. For example, you could switch the display on to set up
the kit, then leave it switched off during operation. The official QRP Labs U3 Case and accessories kit
includes a spare switch which could be used for this purpose.
4.7 Si5351A outputs
For possible use in other projects, the connection points labelled Clk0, Clk1 and Clk2 provide direct
connection to the Si5351A module's outputs. They are 50-ohm impedance and provide a 3.3V peak-peak
squarewave. Do not draw excessive loads from these connections.
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