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555 timer operation

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its multisim 9 so should do a good job, I think though sometimes it tried to take so much reality into theoretical consideration that it runs you round in circles doing things you wouldn't expect. But I can't see what the problem would be with this circuit, last time I had a problem it was because it refused to turn on a led unless the resistor in series was able to supply at least 10 mA in the real world the led would have just lit dimly

hi,
LTSpice is a free download and is well supported.
 
problem solved: like i said multisim can be an arse, it needed a ground point despite the scope having the - connected to neg and the pos to the output
 
problem solved: like i said multisim can be an arse, it needed a ground point despite the scope having the - connected to neg and the pos to the output

hi,
LTSpice needs a 'ground' reference also.

I make the frequency of the 555 approx 550Hz.

Note: the pot will vary the mark/space ratio, but also changes the frequency about 50 to 70Hz.
 
yes I noticed the frequency varies, the only explanation I can think of is that the supply voltage may be dropped by a volt or two on the output so depending on how much time the cap charges versus discharging will cause a shift in frequency, perhaps adding diodes on the discharge part will help rebalance the circuit
 
Why you don't use the classic 555 PWM circuit
 

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I beleive my design allows pretty much 0-100 % range I don't think that circuit will achive this. the thing is the "traditional way" means that the discharge resistor will also be carrying current coming from the charging resistor as this is permanently connected to VCC so you will have trouble having full range and predicting it. my way (used also by others) means that the supply is cut off when the cap discharged and so does not bias the circuits function
 
I'm with you Thunderchild. I hardly ever use pin 7 for discharge. I much prefer to drive the cap from pin 3. It's more instinctive and gives you a lot more options for duty cycle...

Just watch your output impedance as it can affect charge times, normally thats not a problem as I use the pin3 output to drive a NPN transistor base via resistor so the output impedance is pretty reliable.

And it's lower parts count, just one resistor and one cap for a 50:50 duty cycle osc which is most of the 555 circuits I build.
 
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I'm with you Thunderchild. I hardly ever use pin 7 for discharge. I much prefer to drive the cap from pin 3. It's more instinctive and gives you a lot more options for duty cycle...

And it's lower parts count, just one resistor and one cap for a 50:50 duty cycle osc which is most of the 555 circuits I build.

Precisely, well I think with 555 timers either they are being used in signal circuits or the will be used to drive a power transistor for power control.

I was always anoyed that I couldn't have 50:50 duty cycle as default and I think it is a grave error for datasheet writers to ignor the power of using the output pin to supply the RC network instead
 
well as you need a min 1 K resistor it can have limiations and like I said the result will always be biased by the powersupply being always present, using the output means you have less componenets and more control
 
And the energy efficiency! Since most of us use 7555 (cmos) these days driving from the output pin is a much higher energy efficiency, especially for any duty cycle nearing 50:50.
 
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