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Hi & Low periods are too long (especially Time Low) in a discrete 555 timer

The attached discrete 555 timer circuit does not result in the calculated times for hi and low periods.
The time hi is slightly longer than calculated and the time low is off by quite a bit. (long)


I replaced the discharge transistor with a "virtual" device with no junction capacitance and the time low did decrease.
However, it is still much longer than calculated.


Any ideas?




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Hi there,


I've noticed this type of problem too and to be honest, this is one of the situations where circuit simulation cannot make a substitute for bench and lab work.


One thing that you can do to get more accurate results is to reduce the time step. To do this, you should:

  1. Click on Simulate>>Interactive Simulation Settings
  2. Change Maximum time step (TMAX) to something smaller like 1e-6 (this will give you more accurate simulation results while sacrificing simulation speed)
  3. Try simulating
  4. If you are still not satisfied, click on Simulate>>Interactive Simulation Settings again, go to the Analysis Options tab
  5. Click on Use Custom Settings under the SPICE Options group
  6. Click Customize
  7. Play around with the options under the Transient tab. You can find explanations for each option if you click the Help button.

Hoped that helped. There is no hard rule to get accurate simulation results. The settings depend very much on the nature of your circuit as well.

Software Developer
National Instruments - Electronics Workbench Group
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