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Microcontroller Beginner Kit $149.00 
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A simple negative power supply
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 too high. 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.For the -5 Volt Source: At 3.3 mA, the voltage drops to about -4.6 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. For more information on the MAX232 chip click here for the Data Sheet. Links Main Tutorial Page
This page last updated on April 7, 2003. |
