Re: V5.1 and Nextion 5”

Murray Wills (ZL2IQ)

Thank you Evan for the time you have taken to help me out.

73 Murray ZL2IQ


From: <> On Behalf Of Evan Hand
Sent: Monday, 5 August 2019 11:11 PM
Subject: Re: [BITX20] V5.1 and Nextion 5”



What Mike is saying is that the 5" screen may be too much for the 7805 5 volt regulator in the Raduino that comes with the uBitx.  There are a number of options, everything from modifying the Raduino regulator circuit, to simply adding another 7805 5 volt dc regulator. 

The 13.8 volt power supply is much more than the uBitx needs.  The 13.8 volts is good for the finals, however the rest of the board prefers the specified 12 volts.  That is why there are two power connections on the uBitx board.  The 7805 5 volt regulator on the Raduino runs off of the uBitx board supply, not the finals supply.  As currently connected it needs to drop that 13.8 volts to 5 volts.  The difference is dissipated as heat by the regulator.  The amount of heat  depends on the current draw through it. The recommended power supply for the display by Nextion is 5 volts at 1 amp.  Much more than the stock Raduino setup can provide.  

Here is the Nextion spec sheet: 

Based on the current requirements I would suggest that you add another, separate 5 volt power for the Nextion display. This can be based on a 7806 type of regulator like one that is in the Raduino, however it will need a dropping resistor, heat sink, and the appropriate filter capacitors to work.  Another option is to by an assembled 12 volt to 5 volt reducer.  The watch out here is that it could generate noise as most are pulsed based circuits.

Here is an example datasheet for the 7805.

See figure 1 on page 7.  It gives typical input and output capacitor values, though does not cover a dropping resistor. 

Since the input is relatively high compared to the minimum required, and the power dissipation is dependent on the voltage drop times the current (nominal 13.8 - 5 = 8.8 volts times the max current of 1 amp = 8.8 watts).  Much higher than the design dissipation even with a heat sink.  Adding a voltage dropping resistor to get the dissipation lower is suggested.  The minimum specified input voltage is 7 volts.  I would use 8 volts to account for any variations in the input supply.  That means we need to drop the 13.8 volts to 8 volts with a resistor at the current draw of the specified 1 amp.  For this I would use a 4 or 5 ohm power resistor rated for 4 or 5 watts respectively.  Since this is the DC part of the radio, wire wound works well. The actual value of resistor would be 5.8 ohms and 6 watts.  You can go lower in value, just means more heat to be dissipated by the regulator, but do not go higher as at some point the input voltage may drop below minimum.

You will need to add a heat sink on the regulator.  This can be the case if it is aluminum, and you install the appropriate insulators for the mounting.  I would go with a separate heat sink similar to the one on the finals.

Another suggestion is to put a voltage drop between the 13.8 volt supply and the uBitx board.  This involves separating the red and brown power supply wires (per the supplied connector).   To that end I would recommend adding 2 4n4000 series diodes to drop the 13.8 volts to 12 volts to the RED wire.  4n4000 series diodes are rated for only 1 amp, so you will need to separate the finals supply and connect it directly to the 13.8 volt source.  If you do this modification as well, will need to go back and recalculate the voltage and resistor values for the Nextion display above.  I am using a 4 ohm 4 watt resistor in my setup, with the diodes to drop the voltage to the main uBitx board.

Please take the above with a grain of salt.  There may be errors, so verify.

Good luck with the build.


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