0
\$\begingroup\$

I am using Maxim MAX 6866UK28D3L+T in my Design with TI SE555 Timer IC , and having some problem of Flase Triggering.

Circuit Explain:- WDI_IO is Drive from Micro controller , which Continuous toggle this pin, when by any reason this pin is not toggle form 200Sec (approx ) then Reset pin is go low for some micro second. this Reset is open Drain output and drive a 555 Timer IC. So as in normal function Reset of U3 will be high , 555 will not trigger , and out of 555 will be low, that make Q3 off and Q2 On and Drive the SSR1 on. and whenever WDI no toggle till Watch dog time defined in datasheet ( approx 200 S) then Reset of U3 go low, than trigger the 555 IC, and Out of U8 555 , goes High ( Width is calculated by R35 and C38), that make Q3 on and Q2 offm and make the SSR1 off for this time.

Issue:- Whenever I turn on the Power , 555 got Trigger by somehow.

Trial:- I isolate the both section by removing R36, that time during power on 555 never trigger. so I thought some glitch or noise coming form MAX 6866 IC.

Trial 2:- so Connect R36 and again , I tried to change value and never got success Change R34 to 1K Change C42 to 0.1uf, also tried with 22pF changeR36 to 1K, also tried with 82 ohms

Any suggestion to prevent power on false trigger.

\$\endgroup\$
1
\$\begingroup\$

"Issue:- Whenever I turn on the Power , 555 got Trigger by somehow. " You have designed the the 555 circuit to trigger on power up (R34,C42,R36).

On power up C42 is discharged (0V across it) so it will pull the trigger input low to begin with.

Simple solution - remove C42

A more reliable solution is to add a power up reset circuit (RC delay on pin 4) and use an edge trigger circuit at pin 2 so pin 2 is pulled up to 5V on power up.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Tried with C42 removed , but did not helped. \$\endgroup\$ – Short Circuit Jul 27 '17 at 8:49
  • \$\begingroup\$ ...and that's why I put up the more reliable solution because you have not shown the whole circuit. An intermediate solution would be to simply add the RC delay to pin 4 as shown keeping the 555 in reset as it powers up and ignoring the trigger voltage. \$\endgroup\$ – JIm Dearden Jul 27 '17 at 10:41
  • \$\begingroup\$ When I remove the U3 IC, then i observe no power on triggering. U3 is suspect now for me. \$\endgroup\$ – Short Circuit Jul 27 '17 at 10:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.