555 timer overshoot problem (astable mode) (rising edge) - Page 1
Forum section: electronics / projects

author: basantab
author_rank: Contributor
timestamp: December 29, 2017, 12:36:03 pm
content: Hello,A basic Astable mode 555 timer configuration gives overshoot as shown in the picture below.First pictures are without power supply (9v) decoupling capacitors and last two pictures are with decoupling caps for pwr sply. it does not make much difference though. The rising edge is only the problem and falling is not.I also tried with bench pwr supply and 9V battery. with bench pwr supply it is even much worse than shown in the pictures. (psu pictures are not included)I tried both bipolar and ceramic caps in power supply decoupling.I tried compensating probes(just in case) but no gain there.I tried shorter wires but its 14 khz so no gain there as well.tried couple of other same model IC (just in case that ic is not working well) But...yes no achievement there also.I will go deep into, like changing capacitors types and so on now and internal schematics but still if anyone knows about this then could you please lead me to idea behind this..Oh yes i do not wanted to use comparator or histeresis and so on to make it rectangular look like. Just with this ic what its made for.thankyou

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 29, 2017, 04:24:03 pm
content: Your 555 output is an emitter follower. As the voltage rises toward VDD, less and less sourcing current is available and then you are subject to the impedances of the probe, wires, etc.Load your output with a 10K resistor to ground and try again.

author: Audioguru
author_rank: Super Contributor
timestamp: December 29, 2017, 05:47:25 pm
content: I do not see a supply decoupling capacitor on your circuit board.A 555 produces a 400mA shoot-through supply current spike each time its output switches. Then at least one or more supply decoupling capacitors mounted directly at the pins of the IC are needed to supply the current during the spike. The inductance of a wire from the IC to the power supply does not pass the 400mA fast enough.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 29, 2017, 08:12:55 pm
content: I ran an experiment. 10K will not help...makes it worse.A 330pF cap solves it (not a critical value)Here are two waveforms, before and after.Supply is bypassed by .1uf cap at the pins.

author: mikerj
author_rank: Super Contributor
timestamp: December 29, 2017, 09:31:12 pm
content: The CMOS 555 variants don't suffer from this shoot through problem (or at least not to any noticeable degree), one of the advantages of using them.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 30, 2017, 02:37:13 am
content: CMOS is always better.https://www.amazon.com/Analog-Circuit-Electrical-Computer-Engineering/dp/0199765073

author: basantab
author_rank: Contributor
timestamp: December 30, 2017, 06:13:38 pm
content: Quote from: Audioguru on December 29, 2017, 05:47:25 pmactually i have not shown that in the circuit but the last two pictures are with supply decoupling capacitor..there was no overshoot with TS555 timer(cmos) after i put decoupling capacitors and clipped the output and supply wires to shorter in length.. :-)but that Ca555ce still has little has overshoot or kind of bump (seen in pictures)even after decoupling caps..thanks audio guru for reply...It really helped for TS555

author: basantab
author_rank: Contributor
timestamp: December 30, 2017, 06:38:45 pm
content: Quote from: mikerj on December 29, 2017, 09:31:12 pmyep with Ts 555 ic it was much cleaner and eventually sorted out with bunch of caps in supply but ca555ce is just for high and low output requirement..