UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

richrussell · 8th Jan 2015, 10:24 am

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.

8th Jan 2015, 10:24 am	#4
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	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.