Unexpcted Behavoir of Small Capacitors

[msuffidy]

In the attached diagram I would expect the capacitor pointed out is a noise reducing capacitor. I always expected capacitors would be connected sort of in parallel to a rail, and not in series with a rail. In this case, is this functioning sort of as a conductor? Does this differ from electrolytic caps? I don't think this is some sort of on low bias?

[tooki]

A capacitor in series will block DC, but will permit AC to flow, so we call it a DC-blocking capacitor, or a coupling capacitor. (When you select “AC coupling” on an oscilloscope input, this is precisely what it’s doing: putting a capacitor in series.)

A series capacitor and a resistor to ground, as shown here, is a CR high-pass filter. It blocks low frequencies and permits higher ones to pass.

[tooki]

Take a look at this. I watched it the other day for fun, but it actually explains it REALLY well:

[T3sl4co1l]

Coupling capacitor or differentiator.

Likely the latter, which, amplified by a logic gate, will act as a one-shot timer.  Likewise C2 acts as a (much larger, longer time constant) coupling cap, presumably to affect DC offset (sync?) of the output.

Huh, and that's direct into the other gate's output?  Will that do anything?  ...Were 74LS outputs able to be dragged negative?  They might be, I'm not sure offhand.  Never tested it.

So, it's a bit of a weird circuit, but I guess if it works, it works.  It surely won't work with 74HC, or probably other 74xx families; definitely use the LS as shown.

Note that, you still need a capacitor somewhere near the +5V/GND of this chip.  It can be shared with others (in case this is a fragment of a larger schematic), or connected by a short length (say up to 10cm), but otherwise should be onboard, and maybe with an electrolytic (10uF say) too (say for connections over 50cm in length).

Tim

[SeanB]

LS still has input and output clamp diodes, simply because input side is a transistor base emitter junction, and then there is a parasitic substrate diode for negative transients. Put too much current into them and you latch up the parasitic SCR structure intrinsic to the chip, making it go somewhat non functional, and if the power supply can provide enough current it will let out smoke.