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Old 8th Jan 2015, 1:53 am   #1
Oldcodger
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Default 555 used as a RS FF .

Normal 555 design suggests that on on Cos 555 (MONOSTABLE Mode), a 100uF cap is placed close to the IC. Has anyone used a 555 as a simple S/RS FF and needed the capacitor on the supply rail to counter transients, or not ?
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Old 8th Jan 2015, 8:17 am   #2
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Default Re: 555 used as a RS FF .

I think the transients are due to shoot-through in the output stage, so all applications of the 555 will probably need it. If your application is purely made of slow digital circuits you might be able to get away with less, but the output stage problem will still exist and could lead to odd behaviour.

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Old 8th Jan 2015, 8:45 am   #3
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Default Re: 555 used as a RS FF .

It's only the original bipolar 555 that really needs the decoupling capacitor. A design flaw briefly turns both output transistors on creating the current spike. The later CMOS parts corrected this flaw so don't need decoupling. However it's always good practice to put some decoupling across supply lines.

The dual 555, the 556, was badly affected by this flaw. Without the supply decoupling the spike caused by one timer changing state could easily trigger the other timer in the package. Guess how I found out!

Keith
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Old 8th Jan 2015, 10:24 am   #4
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Default Re: 555 used as a RS FF .

As above, originally you needed the capacitor to counter the spike from both transistors switching on together in the original 555/556. Around 400mA of current could flow as a result of this on the Intersil ones - enough to pull the supply down and cause problems. On the CMOS ones, it's usually less than 2mA so not worth worrying about most of the time.

Have a look at the data sheet for the specific variant of 555 you're using - the CMOS ones have quite different internals depending on manufacturer and model.
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Old 9th Jan 2015, 2:10 am   #5
Oldcodger
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Default Re: 555 used as a RS FF .

Thanks folks. I'd forgotten about the Cmos version. Probably go for that, as I only need one and using a 555 makes the PCB a bit smaller and easier. To elaborate- it's to square up car flasher pulses for a logic gate system.
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