Capacitors connected parallel with power source

Thread Starter


Joined Jun 19, 2009

I have a small circuit, 555 & 4017, it work good with a battry of 9V. But when using the lab power source from school, it doesn't work like it should be, the 4017 doesn't count in sequence anymore, but very random. When I connect a small capacitor parrallel with the power source, it work normal again, can anyone example that :confused:


Joined Aug 10, 2008
Some power supplies have bad regulation, so there could be some residual AC voltage getting through causing your circuit to see it as clock pulses.

By putting a capacitor across the supply will help further filter the frequencies getting through and give more of a DC value to your circuit.

A fresh strong battery has no AC signals like a linear supply, so that's why it worked with the battery.

But, a battery when it is depl;eting will give the same results due to it's internal impedances, that's why it is always good practice to put a capacitor across the battery for any circuit it powers. Whether the battery is strong or not...

Thread Starter


Joined Jun 19, 2009
I see, that clear a bit up :-D . Thank you.
And in what value range (voltage and capacitance) of capacitors should I looking for?


Joined Mar 24, 2008
100µF is probably overkill for this circuit, but if you are putting it on the power supply think nearer 1000µF.


Joined Dec 20, 2007
Read the datasheet for a 555. It produces supply current spikes of 400mA when its output switches so the datasheet recommends using two supply bypass capacitors.

Or a Cmos 555 could be used to drive the Cmos CD4017 and it does not produce such a high supply current spike.


Joined Jan 28, 2005
Keep in mind that a capacitor is an energy storage device. With an ideal capacitor, the voltage across it cannot change instantaneously. By providing this short term energy storage in the form of the voltage to which the capacitor has been charged, the power supply's output resistance is reduced. The supply's output resistance is normally quite low to dc loads but can increase significantly with ac loads such as the switching circuitry that you are powering with it.

Actual capacitors are non-ideal so they possess a small amount of resistance referred to as "effective series resistance" or ESR. A wise old engineer once told me that with every capacitor you get a free resistor.

Last edited: