about 555 astable circuit

[bosskardo]

Hi, I saw this sch on NE555P datasheet (https://www.ti.com/lit/ds/symlink/sa555.pdf):

But I've only seen it done like this:


I don't get why the first one is like that. Why is there a capacitor from the power source to gnd? And whats the point of R_L from output to power(I don't know much about 555 internals but could it be that its just opposite to NE555N and I've used NE555N and thats why they are different?)?

 

[betwixt]

The circuits are the same, just drawn differently.
RL is the load on the output of the 555, it isn't necessarily a resistor, it is drawn to show where whatever the 555 is driving is connected to. On one diagram it's a resistor, on the other its a resistor and LED.

Good circuit design always places a capacitor across the power pins of an IC. The reason is that the wiring between the power source and the IC has resistance and inductance. Even though their values may be very small, to some degree they limit the flow of current into the IC and consequently the voltage across it's pins will drop a little as it draws current. In the case of a 555 the current can be significant and vary rapidly so the voltage drop may be enough to upset it's internal operation. The capacitor works as a local energy reservoir and should therefore be placed as close to the IC as possible.

Brian.

 

[bosskardo]

Thanks
But about the0,01uF capacitor, should I use it with every IC (and what would be the optimal size)?
And why haven't I seen sth like this before? Are people lazy and leave it out?

 

[Audioguru]

Thanks
But about the 0,01uF capacitor, should I use it with every IC (and what would be the optimal size)?
And why haven't I seen sth like this before? Are people lazy and leave it out?

ALL electronic circuits should have a supply bypass capacitor. IC manufacturers assume that you know this.

Why don't you read about this capacitor in the datasheet for the IC? The datasheet of the LM555 says, "Adequate power supply bypassing is necessary to protect
associated circuitry. Minimum recommended is 0.1μF in parallel with 1μF electrolytic".

 

[m.mokhtar]

The capacitor is used for bypassing transients that may occur on the output of the power supply and you need it if the 555 are located far from the power supply, in general for switching circuits it is preferable to use it especially if the circuit includes many switching integrated circuits.

I have never used that capacitor for 555 and there was no problem.

 

[Audioguru]

Intersil's datasheet for their ICM7555 Cmos 555 shows a graph of an ordinary 555 drawing a supply current spike of 400mA (!) each time its output switches. A supply bypass capacitor can supply this current spike without allowing the supply voltage to drop very low. Cmos 555 ICs do not produce this supply current spike.