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Power Supply Check

The 8051 development board requires DC voltage between 8 to 15 volts to operate properly. The power source should be rated for 100 mA or higher. Higher input voltages, between 15 to 30 volts may also be used, but the heatsink may become hot if additional power is used for LEDs, an LCD backlight or other add-on peripherals.

Green power LED illuminated
Figure 1: Green LED Indicates Power

The first, quick check is the green LED that indicates the board is receiving power. If this LED is not illuminated, the power may not be connected, or may not be turned on.

The power input may also be connected backwards. The 1N5819 diode protects the board from damage due to reverse polarity power. But the board can not run without the power applied correctly.

Current consumption should be under 50 mA when all red LEDs are off, no LCD is plugged in, and no other circuitry is added to the board in the prototype area.

If the green LED, an additional check is to measure the DC voltage output from the 7805 voltage regulator. Figure 2 shows this measurement. The red multimeter probe touches the top pin which is labeled "+5V".

Measuring +5 volts under 7805 voltage regulator
Figure 2: Measurement of +5 Volt Regulator Output... DC voltage should be 5 volts, and AC voltage should be under 20 mV

The voltage should be between 4.7 to 5.3 volts, and usually it is very close to 5.0. This measurement can also be made with the multimeter set to AC voltage, to verify that the power is stable. AC voltage should measure less than 20 mV. If a larger AC voltage is measured, the DC voltage input may actually be AC voltage (common with "wall wart" power adaptors), or it may not be capable of supplying enough current for the 8051 development board to operate properly.

One final test is to measure the input voltage, at the connections on the circuit board. DC voltage should be at least 8 volts, and AC voltage should be under 0.2 volts.


8051 Development System Circuit Board, Paul Stoffregen
http://www.pjrc.com/tech/8051/board5/power.html
Last updated: February 25, 2005
Status: finished
Suggestions, comments, criticisms: <paul@pjrc.com>