10
Input Voltage
Witty Pi 3 newly adds an on board LDO voltage regulator (LM29150) and hence it can accept
voltage higher than 5V (if you input power via the XH2.54 connector).
The LM29150 is 26V tolerated, but it doesn’t mean you can really use 26V to power your Witty Pi 3.
LM29150 is a linear regulator, which means the dropped voltage will be dissipated as heat and the
heating power can be calculated by:
P
heat
= V
drop
* I
out
For example, when using only Raspberry Pi 4 and no other devices attached, the I
out
is about 0.5A. If
we input 9V and let the regulator to regulate it to 5V and power Raspberry Pi, the thermal dissipation
power will be:
(9 – 5) * 0.5 = 2W
This is rather high power and it needs to be dissipated soon enough, otherwise the regulator will
become hotter and hotter until it eventually overheated and gets damaged. You will need to install
external heat sink to use higher voltage.
Also please consider the efficiency, which will drop significantly when input voltage increase. When
using 6V the efficiency is about 83%, while using 9V the efficiency will be lower than 56%.
Our suggestion is to use no higher than 8V, when no external heat sink is installed. These power
sources would be ideal to power Witty Pi 3:
6V Lead-acid battery
1.5V battery x 4 (in series as 6V)
7.4V Li-po battery (2S)
5~8V solar panel
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
5.3V 6V 7.4V 9V 12V
Efficiency
Efficiency
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