Understand the 74HC595

Posted on Feb 5, 2014

The Triac drivers use the 5 VDC output signal from the 74HC595 chip outputs (Q1, Q2, Q3, Q4, Q5, Q6, and Q7) to switch the 110VAC gate signal to the Triacs. If you are connecting the IC chips directly to the PC, a desktop PC MUST BE USED to drive the electronics in a large system because a laptop computer does not have the voltage required to run

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a large quantity of 74HC595N chips. A laptop can be used in smaller systems (16 channels) but the voltage to the 74HC595N IC chips will need to be lowered since a laptops output voltage is lower than that of a desktop. Add on PCI cards tend to have a lower voltage. Also don`t expect to drive as many IC chips with a laptop as compared to your desktop. In some cases, the parallel port of a desktop will not provide sufficiently high pulses and the Vcc (5 volts DC) to the 74HC595N chips needs to be lowered so the 74HC595 can "see" the pulses from the parallel printer port. The symptom results in some channels working and others not and channels triggering out of order. Here is the catch, if you lower the voltage to the IC chips then the voltage from the outputs of the IC chips also gets lowered and the current from the outputs gets lowered. It may be necessary to lower the value of the limiting resistor from the 74HC595N chip output to the triac driver in order to properly trigger the triac drivers.

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