Translate this page

Custom Search

DRIVING HIGH-CURRENT LOADS


The output drive-current for a 555 is 200mA maximum. The output voltage is 1.7v less than rail voltage.
A driver transistor can be connected to the output pin to improve the output current to 1amp (or more) and deliver an output voltage that is near rail voltage. Globes are a typical example of a high-current load. They require up to 6 times the normal current when starting. This is due to the cold filament having a very low resistance. The same applies to motors. They have a high start-up current requirement.
Any driver transistor can be fitted as shown in the diagram below:


Use a driver transistor for loads greater than 200mA

The CMOS 7555 has an output current capability of 50mA and will need a driver transistor for currents above 80mA.