Microcontroller Beginner Kit -
5 Volt DC & -10 Volt DC Power Supply
Vcc = 5 Volts DC
The schematic above shows how to
build a simple -5 Volt DC and -10 Volt DC power
supply using a MAX232CPE chip and a 5 volt supply.
A 5 volt supply is easy to build using a common DC
wall adapter (9 to 20 volts DC or so) and a 7805
chip (as in our 5 Volt Kit). The MAX232 chip is
intended to be used for communicating with a PC
through a serial port. (See the Data Collection Kit
for more information on communicating with a PC.) A
serial port requires a negative 10 volt signal to
work properly so the most important thing that the
MAX232 chip does is generate a -10 Volt power source
for those signals. But negative voltages are also
needed for other applications such as operational
amplifiers (op-amps). The MAX232 chip can be used
for this as long as the current requirements are not
With no load, the outputs are about
-5 Volts and -9.5 Volts. (The -10 Volt Source does
not quite reach -10 Volts but it is still referred
to as the -10 Volt Source.)
For the -10 Volt Source:
At 4 mA the voltage drops to
about -8.5 volts.
At 10 mA, the voltage drops to about -6 volts.
For the -5 Volt Source:
At 3.3 mA, the voltage drops to
about -4.6 volts.
At 9 mA the voltage drops to about -3.5 volts.
The graph below shows how the -10
Volt Source drops as current increases.
Note: The -5 Volt output and the -10
Volt output are not independent. If you are using
both outputs, the voltages will drop off faster.
Since the chip can not provide much
current, one common way it is used is in a double
opamp configuration. The first opamp acts as a
buffer, inverting the signal to a negative signal
then a second opamp reinverts the signal to positive
and supplies the power, drawing current from the
positive voltage source, Vcc.