555 timer glitch

OBW0549

Joined Mar 2, 2015
3,147
Electronic components such as capacitors, resistors and inductors all have a tolerance associated with them, an error band surrounding the part's design value within which the actual value of any given part is guaranteed to lie. For example, the actual value of a 100 Ω, 5% resistor may lie anywhere between 95 Ω and 105 Ω. The actual value of a 1 μF, 20% capacitor may be anything between 0.80 μF and 1.20 μF.

Expecting any two identically-marked capacitors to be exactly the same value is unrealistic. So the fact that your output frequency changed when you replaced one 1 μF capacitor with another 1 μF capacitor is hardly a head-scratcher.
 

AnalogKid

Joined Aug 1, 2013
8,254
You don't say what the intended output frequency is (???), so this is guesswork. Assuming it is around 50 Hz, 5 Hz is only 10%. If your capacitors are +80^%/-20% tolerance, or even if the caps are +/-1-%, then a frequency change of only 5 Hz is excellent. For tighter interchange performance, use 1% tolerance resistors and a 1% tolerance film capacitor. Mouser has some at very reasonable prices. To lower the capacitor cost, decrease the cap value to 0.22 uF and re-calc the resistors. I would not go any lower with a bipolar 555.

ak
 

SamR

Joined Mar 19, 2019
1,635
Since there is typically more error in capacitor values and the output frequency is calculated on both the cap and resistor values I like to use a small multiturn potentiometer for the resistor if I need a precise output. Then I can tweak it for the ~exact frequency I want. Usually not an issue.
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
Electronic components such as capacitors, resistors and inductors all have a tolerance associated with them, an error band surrounding the part's design value within which the actual value of any given part is guaranteed to lie. For example, the actual value of a 100 Ω, 5% resistor may lie anywhere between 95 Ω and 105 Ω. The actual value of a 1 μF, 20% capacitor may be anything between 0.80 μF and 1.20 μF.

Expecting any two identically-marked capacitors to be exactly the same value is unrealistic. So the fact that your output frequency changed when you replaced one 1 μF capacitor with another 1 μF capacitor is hardly a head-scratcher.
Thanks for the refresher course. I forgot about the tolerance varyible.
 
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