problems with 2 555 timers sharing the same power supply

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Nimrod82

Joined: 13 Feb 2007
Posts: 3

13 Feb 2007 19:22 problems with 2 555 timers sharing the same power supply

Hi,

I'm working on an frequency adjustable sine wave oscillator, it comprises 2 555 timers, 1 8th order bessel LPF, a 5V voltage regulator and a 9V battery.

1 timer is used to generate a 280 Hz square wave with a 50% duty cycle, running this output through the LPF will give me a sine wave. The LPF takes in 5V as its power supply, and its cutoff frequency is determined by a clock(fclk=100*fo), that brings me to making the 2nd timer to generate a 30KHz square wave with a 50% duty cycle for fo=300 Hz.

so far I only have the timers on hand, when I use different DC sources and grounds for them, they do what they're suppose to do with a +-few% tolerances(280 Hz and 29KHz). When I use the same +25V DC source and ground for both of the timers, the frequency becomes very unstable(not what it is suppose to be), and the +25V DC source is limited to at around 8.8V no matter how much I turn the knob for the amplitude.

Is anyone familiar with this kind of problem? Thanks in advance!

Ave_Rapina

Joined: 06 Jan 2004
Posts: 121
Helped: 14
Location: Po

13 Feb 2007 20:35 Re: problems with 2 555 timers sharing the same power supply

Hello

You need to put capacitor to isolate the ripple created in the alimentation by the 555.
You can put for hte same alimentation 2 resistor and two caps for each one and minimize the influence in one regarding other by the supplie.
That problem is very constant when you use oscilators.

Regards

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Nimrod82

Joined: 13 Feb 2007
Posts: 3

13 Feb 2007 21:16 Re: problems with 2 555 timers sharing the same power supply

Hi Ave_Rapina =),

Thanks for your reply, I'm trying hard to interpret what you've said...

you said "You need to put capacitor to isolate the ripple created in the alimentation by the 555. "

put the first capacitor where? in parallel with the power supply to reduce the ripples? but its a DC source and shouldn't have ripples.

you also said "You can put for hte same alimentation 2 resistor and two caps for each one and minimize the influence in one regarding other by the supplie. "

connect 2 resistor and 2 capacitors for each timer, but on which pin?

Thanks again!

Ave_Rapina

Joined: 06 Jan 2004
Posts: 121
Helped: 14
Location: Po

13 Feb 2007 21:43 Re: problems with 2 555 timers sharing the same power supply

Hello

Sorry but today my english is not " working" well

Here is an circuit that examples what I say off course is just a explanaiton

Regards

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daviddlc

Joined: 11 Feb 2007
Posts: 196
Helped: 13

13 Feb 2007 22:08 Re: problems with 2 555 timers sharing the same power supply

You are using resistors (R1 and R2) for a voltage divider with a dynamic load
(555), this will give a problem.

You will need to replace this resistors with some other device (voltage regulator or diodes in series) to have the voltage divider.

For example if you want the load (R1 and R2) to have a voltage of 5 volts on them, then 7 Si diodes in series will do the job (7 * 0.7 = 4.9), this is one way of doing it. The other is to use a voltage regulator.

What is the voltage you want to have on the timers ?

Nimrod82

Joined: 13 Feb 2007
Posts: 3

13 Feb 2007 23:01 Re: problems with 2 555 timers sharing the same power supply

Ave_Rapina thanks for your quick replies Smile

Hi daviddlc,

the voltage through the timers shouldn't matter, as long as they are anywhere between 5<=V<=9, and the frequencies for both timers are stable while powered by the same source, keep in mind that I also have to power the MAX7401 LPF with a 5V source, but running the 9V through a 5V regulator should take care of that.

my main objective right now is to be able to power 3 ICs with 1 single power source without having things becoming unstable.

thanks again for the help! Smile

nguyennam

Joined: 06 Sep 2006
Posts: 459
Helped: 28

14 Feb 2007 6:07 Re: problems with 2 555 timers sharing the same power supply

Your problem is bad power supply. You will need a good power supply. In general, a voltage regulator with filtering electrolytic capacitors and decoupling ceramic capacitors can make a good power supply.

nguyennam

Azaruk

Joined: 12 Feb 2007
Posts: 37
Helped: 1

20 Feb 2007 8:36 problems with 2 555 timers sharing the same power supply

The 555 is very susceptible to false triggering due to transients in the circuit.

I suggest you put a 100nF capacitor across the supply pins (1 & Cool

of the 555, as close to the pins as possible.

Also, ensure that your power supply has adequate smoothing capacitors as well as 100nF caps to try and reduce transient noise.

What is the circuitry like on pin 3 of the 555. I have found it often helps to put a pull-up resistor of around 10k from pin 3 to the positive supply to keep things a bit more constrained.

Also - as mentioned already - the power supply must be adequately rated, adequaltely smoothed and adequately regulated.

Hope this helps.