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SE555: Leakage Capacitor on RC Time Constant Causing Increase in Timer Output

Azhar
Prodigy 110 points
Part Number: SE555
Other Parts Discussed in Thread: LMC555, TLC555

We are using 555 Timer IC with P/N# SE555 and the RC combination used to get 60Sec Output from SE555 are RA=2.7MΩ RB=2.7MΩ and C=10uF. We are using SE555 in Monostable mode. SE555 IC is supplied with VDD Voltage of +5V DC. The Trigger Pin is driven through MOSFET. A bypass capacitor of value 0.01uF is connected to VDD Supply. And Output Pin is pulled up with R=4.99K at +5V DC. Do you see any challenge with use of 10uF Capacitor? Because with use of one batch of Capacitor we get 555 Timer IC output as 68 Sec and with second batch of Capacitor we get output as 110 Sec. We are tested this on more than Qnty 20 PCBA for same electrical circuit the value at the 555 Timer IC is not same and it ranges from 68 Secs to 170 Secs.

Is there any way we can test the waveform at discharge Pin?

What value of RA, RB and C you recommend for 60 Sec output?

Does this combination you recommend RA=4.99MΩ RB=4.99MΩ and C=5.6uF?

What is maximum current value that discharge pin can take?

Any better alternate of 555 Timer IC than SE555 that you recommend for 60 Sec Output in same package?

Do you recommend to use Solid Tantalum capacitor to lower the leakage current ? 

Thanks & Regards,

Azhar

  • 0
    TI__Guru**** 165376 points

    Hello Azhar,

    Use a low capacitance FET probe to measure pins 6 or 7. Other types of probes could change the timing.

    Most monostable circuits use a RB of zero. The discharge pin is made to handle a capacitor discharge. If a lower discharge current is desired, then RB (or RX in the FAQ) could be added. 1k ohm more than enough , maximum current is 2/3 * VCC / R.

    TLC555 and LMC555 have much lower "leakage" current than SE555. TLC555 even has a TI-Tina simulation model

    There is a e2e FAQ for monostable timers here , https://e2e.ti.com/support/clock-and-timing/f/48/t/857914

    The capacitor will biggest source of variance , but there is a way to eliminate the capacitor completely. TI also has nanotimers that do not use a timing capacitor.