General PSpice and 555 timer question regarding precision

Question

I have been experimenting with values recorded using a multimeter versus the voltages (RMS) simulated using capture. I simultaneously setup a circuit using a breadboard and drew exactly the same circuit in PSpice. I don't really have a particular circuit in mind. But I am sure I can set something up if it would help explain my problem.

Most of my component values have been extremely close to one another, such as resistors and capacitors. However I have hit a sour patch with the 555. It is not within a reasonable range. Not even close. The last test reported an error of about 45%. The breadboard pin 3 output voltage doubled the RMS PSpice voltage. Not good!

Is this normal? Is there anything that can be done to tighten up the PSpice component tolerances for ICs? I would like to use the program to its fullest, if it will let me!

Answer 1

A circuit diagram of what you are doing is close to obligatory - wise men who at their end know dark is right because their words had forked no lightning still make obvious mistakes - the rest of us the more so.

LM555 circuit diagram here

There is no reason [tm] that P-S.P.I.C.E.* cannot model a 555 like timer to have a high degree of precision if the SPICE model makers wish it to.

SO the fact that it doesn't either suggests that either:

• They don't wish it to.
  This would be the case if eg they considered that the 555 IC did not produce overly accurately reproducible results between ICs - you can look at the datasheet to see if this may be so.

or

• There is a bug in your P-SPICE.

OR

• You are using values or parameters that are outside the reasonable range.
  1k <= Ra <= 100k

What value of external resistors are you using?

555 accuracy will be affected by the internal threshold levels in the ramp up / ramp down window comparators which in turn depend on fabrication issues. IF the maker has chosen to make these accurate they can be so. If not then they probably wont be.

The LM555 datasheet suggests that the untrimmed accuracy of a '555 is "not too bad."

• Timing error monostable mode 1%/3% typical/max

• Timing error Astable 2.25% tpical no max - suggesting just maybe up to 7% max based om monostable spec.

  • Timing erreor 150 ppm /degree C. (-.015% per C or 1.5% over 100 C range (simplistically).

  • Depending on actual circuit, IF your main power supply has significant noise significant errors could be introduced.

The best bet so far is that you are doing something wrong.
Next best is broken SPICE.

---

• Spice is an acronym so should be, at least, SPICE, if not acyually S.P.I.C.C.E.
  Simulation program with integrated circuit emphasis, AFAIR.