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I'm using 555 timer in astable mode. I've used Multisim to generate it ant this is what I've got:
I'm little confised by the 100 ohms Rl resistor. Do I need it there ? If I do, why ? I've tried with and without it, and it works for me both ways ... I'm using the 555 to drive another CMOS IC input. Thanks |
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The 100 ohm is A load - an example - what load you really use is up to you. If you are driving a CMOS IC you do not need anything else connected to the OUT pin. The 555 has a "push pull" or bipolar output stage. This means that it drives "actively" both high and low, so you get an output voltage whether there is a load connected or not. The 555 load is generally connected to the OUT pin when you want a square wave output. There are other places you can connect to if you are doing less usual things. LM555 / NE555 / xx555 datasheet - Fairchild Active low and high drive can be seen here:
More normal block diagram view. Here you cannot tell that the output pin is actively driven high and low.
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You don't need this 100 ohm resistor because the IC drives the pin high or low as required even if nothing is connected to it. |
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