# Capacitor decoupling

#### k1ng 1337

Joined Sep 11, 2020
701
Good Day,

While playing with some oscillators I discovered I can create an AC signal by decoupling the output of a 555 with a series capacitor and with the scope I made some interesting observations with questions to follow..

a) The amplitude of the crest and trough was a ratio of the duty cycle and at around 45% became a mirror image and the average voltage read zero; I understand the 555 astable puts out significant ripple and my question is.. how "much" of the DC signal is "converted" into AC as well as losses?

b) Concerning a perfect square wave at 50% duty: if it were to instantly transition from ON to OFF would the intrinsic associated turn on/off times of the components involved then become the dictating parameters of the signal since electrons are unable to stop instantly? Provided of course there is enough time before they reverse direction.

c) How much current is available from a series capacitor? Even with small capacitors I was able to drive a fair load with a single transistor.

Other insight into the subject is much appreciated

Cheers

#### jpanhalt

Joined Jan 18, 2008
11,087
Good Day,
a) The amplitude of the crest and trough was a ratio of the duty cycle and at around 45% became a mirror image and the average voltage read zero; I understand the 555 astable puts out significant ripple and my question is.. how "much" of the DC signal is "converted" into AC as well as losses?
What do you mean by "ripple." Where did you learn that? Are you referring to coupling with the power supply

b) Concerning a perfect square wave at 50% duty: if it were to instantly transition from ON to OFF would the intrinsic associated turn on/off times of the components involved then become the dictating parameters of the signal since electrons are unable to stop instantly? Provided of course there is enough time before they reverse direction.
You seem to be asking for the practical outcome of an impossible input scenario. If the input can go on and off "instantly" then why can't other components do that too?

In reality, all circuits and components have capacitance and inductance. They also usually have resistance. For example, if you look at the model of a crystal, you will see all three listed. You may have seen such modifiers as "distributed" or "parasitic." Capacitance, inductance, and resistance are why circuits react more slowly or differently than one might expect from perfect components. PCB layout can be critical for high speed circuits.

c) How much current is available from a series capacitor? Even with small capacitors I was able to drive a fair load with a single transistor.

Other insight into the subject is much appreciated

Cheers
Capacitors have a (ripple) current rating in their datasheets.

#### Deleted member 115935

Joined Dec 31, 1969
0
if I understand you correctly
what you have is a square wave out of the 555,
which you are routing through a capacitor , into some sort of load.

What you should be getting is more like this , as you have created a high pass filter with the C you have and the R of the load.

https://www.electronics-tutorials.ws/filter/filter_3.html

Could it be that you have the capacitance to ground,
which would be low pass filter of sorts,

https://www.daenotes.com/electronics/digital-electronics/rc-low-pass-filter-circuit

which as is shown in fig 3 is a fairly ok sine wave.

Whats the aim of the work your doing ?

#### k1ng 1337

Joined Sep 11, 2020
701
What do you mean by "ripple." Where did you learn that? Are you referring to coupling with the power supply

You seem to be asking for the practical outcome of an impossible input scenario. If the input can go on and off "instantly" then why can't other components do that too?

In reality, all circuits and components have capacitance and inductance. They also usually have resistance. For example, if you look at the model of a crystal, you will see all three listed. You may have seen such modifiers as "distributed" or "parasitic." Capacitance, inductance, and resistance are why circuits react more slowly or differently than one might expect from perfect components. PCB layout can be critical for high speed circuits.

Capacitors have a (ripple) current rating in their datasheets.
Perhaps the term is bouncing not ripple, I'm referring to how in a real 555 circuit the output doesn't actually go from high to low but rather an infinitesimal numbers of voltage transitions gradually reducing

I noticed in a simulation app that certain components due to their position and function such as a capacitor had ac waveforms.. I'm investigating the possibility of extracting these sine / triangle waves as input for another circuit

if I understand you correctly
what you have is a square wave out of the 555,
which you are routing through a capacitor , into some sort of load.

What you should be getting is more like this , as you have created a high pass filter with the C you have and the R of the load.

https://www.electronics-tutorials.ws/filter/filter_3.html

Could it be that you have the capacitance to ground,
which would be low pass filter of sorts,

https://www.daenotes.com/electronics/digital-electronics/rc-low-pass-filter-circuit

which as is shown in fig 3 is a fairly ok sine wave.

Whats the aim of the work your doing ?
I'm trying to create oscillators from scratch, this topic was inspired by the analysis of a RC phase shift oscillator

#### jpanhalt

Joined Jan 18, 2008
11,087
Perhaps the term is bouncing not ripple, I'm referring to how in a real 555 circuit the output doesn't actually go from high to low but rather an infinitesimal numbers of voltage transitions gradually reducing

I noticed in a simulation app that certain components due to their position and function such as a capacitor had ac waveforms.. I'm investigating the possibility of extracting these sine / triangle waves as input for another circuit

I'm trying to create oscillators from scratch, this topic was inspired by the analysis of a RC phase shift oscillator
Are you sure that is not an artifact of either the screen of your monitor or oscilloscope or the simulation software. The 555 is like a Schmitt inverter. There is nothing in it that will cause a stepwise rise or fall.

#### k1ng 1337

Joined Sep 11, 2020
701
Are you sure that is not an artifact of either the screen of your monitor or oscilloscope or the simulation software. The 555 is like a Schmitt inverter. There is nothing in it that will cause a stepwise rise or fall.
I think scope sampling and resolution is a large factor.. are you telling me that when the 555 goes from HIGH to LOW the output drops to zero and stays at zero?

If there is any capacitance and inductance in the line would we not observe oscillations until the energy is ultimately dissipated?

#### Tonyr1084

Joined Sep 24, 2015
7,454

#### atferrari

Joined Jan 6, 2004
4,699
I use to ask this and I believe people hates me for that.

#### k1ng 1337

Joined Sep 11, 2020
701
For the question about square wave: remove the LED

And for the AC question, remove the LED and insert a capacitor before R3

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#### jpanhalt

Joined Jan 18, 2008
11,087
I think scope sampling and resolution is a large factor.. are you telling me that when the 555 goes from HIGH to LOW the output drops to zero and stays at zero?

If there is any capacitance and inductance in the line would we not observe oscillations until the energy is ultimately dissipated?
Where did you get that impression? I said the staircasing you described was an artifact. I said nothing about an DC offset or "staying at zero."

As someone once said, "There are liars, damn liars, and oscilloscopes."

#### SamR

Joined Mar 19, 2019
4,811
an infinitesimal numbers of voltage transitions gradually reducing
Is your power supply battery dying?

#### Audioguru again

Joined Oct 21, 2019
6,155
The 555 was designed using an antique TTL circuit. Then its output was not rail-to-rail and it produced a "shoot-through" power supply current pulse of about 400mA (!) each time the output switched. Your little battery cannot produce the 400mA pulses without the two supply capacitors mentioned in the 555 datasheet (0.1uF parallel with at least 1uF).

Many 'scopes nowadays are digital and produce a staircase instead of a diagonal line.