Hi,

Can someone explain why varying the Vcc for a 555 timer chip, say from 6 volts up to 15 volts, cause a change in the pulse frequency (measured on the output pin) when the 555 is configured as in the astable mode? None of the documentation I've read on calculating the frequency ever seems to talk about the voltage, however, from experimentation, I can clearly see a big change in the output frequency from below 20KHz to up around 30KHz as the voltage increases. The basic configuration of the circuit used
r1=680R
r2=62K
c=330 pF

I have read that both R1 and R2 should be > 1K, so maybe my r1 (680R) might be what is causing this odd behaviour.

Can anyone shed some light on why this frequency change is occurring?

 

It's not that odd, all 555's do it that I know of, part of being analog. I think it's because the internal transistors shift their bias points slightly as the voltage changes, the analog guys here might know the exact reasons better.

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resistors has to be of the same value, they form a voltage divider to serve as preset for internal upper and lower comparator. 1/3 and 2/3 VCC voltage will be fed to the comparators. their output will set and reset the flip floop which inversly gives the ouput.

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If R1 is the resistor tied to Vcc, then make it about 20K, then set your desired freq with the other resistor. Yeah, I know that will affect the duty cycle, but it will improve the freq stability over voltage.

btw-78L05s are really cheap.

btw- try a CMOS version. It will have less voltage variation.

 

According to National Semiconductor data sheet it shouldn't be changing:

http://www.national.com/ds/LM/LM555.pdf

edited for brevity.

Al

 

Having built and tested many 555 circuits in the astable mode I think that MikeBrady needs to check his equipment. I built the circuit as described(today), using standard 555's and some cmos versions. The basic frequency as measured on a frequency counter was very close to 33khz. The frequency did vary from 5V to 12V. In no case did the frequency vary greater than 350hz between the two voltages. This relates to little over 1% error. I think this is will within the spec's of the 555.

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The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

Thanks for the feedback. I tend to agree with k7elp60, that the results I am getting are caused by the components/equipment. I'm pretty confident that my oscilloscope is working correctly, so I'll take another look at the components I'm using. Anyway, it's good to know that the frequency shifts I was seeing are NOT expected, which means I can continue to use the frequency calculators without having to factor in the voltage settings.

 

Looks like a reasonable theory. The discharge transistor may not be able to cope with the higher currents....so indeed, go with at least 1k.

 

I guess you have used a lousy qualitity cap.

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Hi,

I have to agree that the frequency should not be changing unless something else
is wrong such as way off resistor values or something just plain bad.

The 555 is set up with those comparators that work at 1/3 and 2/3 of Vcc, and
if you go over the equations for the frequency (with the right circuit of course)
you get a frequency that does not vary with Vcc voltage because the trip points
are always at 1/3 Vcc and 2/3 Vcc no matter what Vcc is, and that means that
BOTH set points change as Vcc changes, which cancels any tendency to change
frequency. Im sure in the real world this 1/3 and 2/3 isnt exactly exactly exact,
but it's close enough such that there should not be too much change anyway.
Certainly not as much as 50 percent which is what seems to be happening here.

Perhaps you could post a schematic too so we can see exactly how you have this
thing hooked up. I say this because there are ways in which you can connect the
555 that force the frequency to change with Vcc and so it could be a matter of
how it is connected up.

Just to note, there is a neat little circuit that you can make with a comparator
that acts the same way as the 555 just about...made with three resistors and
one capacitor and one comparator section (like that of an LM339). This circuit
too works by switching at 1/3 and 2/3 Vcc and it too has very stable frequency
output.

 

Well guess I'll put my foot in my mount and say I was wrong and wander off now =) Good luck.

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MrAL has confirmed what I 'thought' I already knew...the voltage shouldn't really be affecting the frequency at all. I've been doing a bit more experimenting and can believe I can rule out the oscilloscope as the source of error - I tested the scope with a signal generator tool for a similar frequency, and it gave a very accurate result. So, I am now looking at either
a) my circuit being wrong
b) the components I have not being accurate enough

I'd like to post a diagram of the circuit - apart from doing it in Visio (which I already have installed) is there a good (free) tool that anyone can suggest I use to create the diagram? I can also post an image of the oscilloscope output easily enough 'cos its a digital scope I am using.

BTW, re. suggestion by MikeMi

I already have a steady voltage...I have a very nice variable voltage power supply I built which is as steady as a rock - I was intentionally varying the voltage and observing the output from my oscilloscope - thats what prompted the start of this thread.

 

That's only for the wierd configuration in figure 14.


You could always draw it by hand and scan it. Or there's Pspice, which has a free version and also a cct simulator. There's also LTSpice, which I think is free. et al of course.

Someone suggested a lousy cap, sounds like it's worth checking out; swap the cap out for another (less lousy) one. Also make sure that it's sufficiently rated for the maximum supply voltage.

 

The best part of the forum is this:
It confirms things.
We were all told 555's have a reliable output frequency over a fair range of supply voltage, but when doubt cropped up via one of the posters, we thought: "maybe the 555 isn't that stable."
Now we know - it is.

 

Depends on the whole circuit, are there diodes? There was another thread here where someone was getting frequency drift with a different type of circuit because the diode forward voltage changes with increase in VCC.

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