I built the following single stepping circuit using a new 74HC00. The inputs for the two unused gates were connected to GND and power decoupled with 0.1uF as close as I could.
The circuit appeared to work but it sometimes produced strange transients (below is an example of one of the more extreme ones). The IC also drew 0.05A which was more than expected. I eventually constructed a copy of the circuit on a breadboard using a different 74HC00 which worked perfectly. No transients, and the power supply showed 0.00A. Dropping in the (desoldered) first IC and I got the transients and the 50mA current.
- Is this some kind of classic fault?
- Could it be due to my lack of proper ESD practices?
- Do you test ICs before?
Best Answer
The 50mA draw indicates that the chip has been damaged, and ESD is a likely cause, or trigger, for such damage. A damaged chip could exhibit strange behavior as you indicate, perhaps because of power supply bypassing that is insufficient for such a high fault current (it's possible that if you shunted the 100nF cap with 1000uF it would appear to work properly).
To make your circuit more bulletproof, I suggest adding 10K resistors in series with the gate inputs so that any kind of ESD coming from the switch will cause less current to flow through the protection networks on the chip. Like this:
simulate this circuit – Schematic created using CircuitLab