7
revolution
Revolution Education Ltd. Web: www.picaxe.co.uk
Version 1.3 04/11
AXE401.PMD
AXE401 PICAXE-28X2 Shield Base
3.0 Power Supply
The shield base is primarily
designed to be used with an
external ‘plug in the wall’ regulated
power supply that provides 9-12V
DC on a 2.1mm, centre (tip)
positive connector. A suitable
power supply for UK use (only) is
part PWR009A.
A minimum of about 6.5V is required via the 2.1mm power connector, and voltages
above 12V may cause the regulators to get hotter (some warmth is normal). Do not
use a PP3 9V battery via this connector, this type of 9V PP3 battery is designed for
long term minimal current use (e.g. in a smoke alarm) and so is simply not suitable
for long term use of any type of shield base.
The board can also be powered via batteries (e.g. 4.5V from 3 x AA alkaline cells)
via the 5V power header. Connect the red positive wire to ‘5V’ and the black
negative wire to ‘GND’. Alternately, if assembling your own board, leave off the 5V
regulator and solder in a battery clip instead (see the minimum battery powered
circuit assembly example).
The shield base contains both 5V and 3V low-drop out regulators, each rated at
500mA. Therefore for 3V work there is generally no need to build a separate 3V
regulator onto the shield. However if the shield already has a 3V regulator fitted this
may still be used.
3.1 Why the 5V / 3V Jumper H1?
Most shields are designed to work with a 5V system voltage, so jumper H1 should
be left in the 5V position. However when building your own circuit using a proto
shield with a third party 3V only device, for instance an XBee or GPS module, this
third party device may be 3V tolerant only and hence damaged at 5V. So rather than
build 5 to 3V conversion circuits on to the proto shield it is simply much easier to
run the entire system voltage at 3V to start with. This is no problem for the PICAXE-
28X2, which can be used at any system voltage between 2.1V and 5.5V.
3.2 Can I power the board from the USB port of my computer?
We are not keen supporters of the USB powering method. The majority of PICAXE
shield projects are only connected to the computer whilst programming, so
generally require a separate power supply system for the actual project use anyway.
Therefore we suggest that you might as well use this power supply all the time!
Many users also fail to recognise the power limitations of USB (e.g. when a laptop
goes to sleep), and the current limitations when using the more complex shields
with additional on board regulators or motors. However if you so desire you may
naturally choose to use a USB breakout board or USB breakout cable to provide 5V
to the board (via the power header).
H3
Default positions of the 4 jumper links
3V3
H1
5V
H4
H2
H1 - 5V - Top
H2 - RXD - Bottom
H3 - TXD - Bottom
H4 - LED - On
LED
AXE401 PICAXE Shield Base - Power Circuit
C5
100n
RG1
LD50V
+
+
+
H1
D1
1N4001
RG2
LD33V
C4
100n
C3
22u
C2
22u
C1
22u
R8
330
L2
Vin
V5V
V3V3
VIC
GND
Reset
V3V3
V5V
GND
GND
Vin
Power
Header
CON1