Wanting to calculate the value of the pull down resistor for a 555 trigger. For the formula should I use the typical or max trigger current (typ = 0.5 Max = 0.9 μA). I want the calculated value not an average PU/PD value. TIA


1 Answer 1


The decision you are facing is whether to use typical or maximum guaranteed values provided by the IC manufacturer.

This is a very common decision to make.

My assumptions: This project is conducted in the realm of non-profit/hobby electronics. The setting achieved with the resistors is therefore not "supercritical" across the whole temperature/supply voltage range. Learning and a certain gain in experience are the main goals - as is the fun.

If not so: You need to make the decision whether your circuit must always work somehow (from -40°C antarctica doing science stuff to 125°C within a battle tank in the desert - from a crappy out of spec switch mode supply or a high precision linear local source). Go with the maximum/minimum guaranteed values if so. Contact the DOD and get your 10B$ fund -> give me my fair share ;)

If so: Go with the typical values and check the Datasheets for more details on what "typical" does mean - 3sigma, 6sigma? As an example: Typical current is .5uA +/- 5% for 3sigma of the devices -> Design your circuit in a way, that i works as desired for .5uA +/- 5% and it will work with 99.7% of the parts supplied.

Please see: 3-Sigma Wiki

My suggestions: Go with the typical values and add series-trimmer resistors. Choose the compoents in a way, that on mid-trimmer-stroke the typical value is used and the spectrum from minimum guranteed to maximum guranteed is within the range of the trimmer. This approach is useful in several ways:

  1. Have fun and learn calculating these values. Forms the mind.
  2. You can trim your circuit to a very precise operational mode depending on the used components.
  3. You can adapt small changes in the operation mode by simply turning these trimmers.
  4. If you are a hobbyist/student: Keep on going - Calculating values is....lets face it: required but fun! ;)

Please see: Schematic with trimmers - 555


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