Capacitor Value/type for low frequency filter help

[willis9076]

Hello everyone,

I have a DC waveform that is oscillating from about 0.25V to 0.75V at about 2Hz.  I would like to use a capacitor to smooth it out so that it is almost flat but my knowledge of electronics is somewhat limited.  The idea is to attach the capacitor to the signal line and ground.  Can anyone recommend a capacitor value and type to accomplish this?  Or if i am even approaching this the right way?

Thanks!

[Zero999]

It depends on the source impedance of the supply and how much attenuation you desire, you'll never be able to gey 0V of AC i.e. 0.5VDC with no ripple.

[willis9076]

Thanks for your reply.

Actually, I'm not too sure about the source impedance or attenuation, but now that I think about it a little more what I really want to do it just lower the frequency of the wave form from about 2Hz to maybe something less than 1Hz.  It is okay to have ripple and I do not expect it to hold steady at 0.5v.  Is this possible with a Capacitor?  or capacitors and resistors?

Would this work if I cut the signal line and place a capacitor and resistor in between the two ends?

Thanks for the advice!

[Zero999]

Thanks for your reply.

Actually, I'm not too sure about the source impedance or attenuation, but now that I think about it a little more what I really want to do it just lower the frequency of the wave form from about 2Hz to maybe something less than 1Hz.  

You won't change the frequency, you'll just get 2Hz with a reduced amplitude.

It is okay to have ripple and I do not expect it to hold steady at 0.5v.  Is this possible with a Capacitor?  or capacitors and resistors?

Would this work if I cut the signal line and place a capacitor and resistor in between the two ends?

Thanks for the advice!

Why are you doing this? Can we please see the bigger picture.

[willis9076]

I am trying to "slow down" the output of my secondary O2 sensor of my car.  My car is throwing a P0420 code which is Low Catalytic converter efficiency.  How it detects this is by comparing the waveform of the primary O2 sensor with the waveform with the wave form of the secondary O2 sensor.  If the catalytic converter is working properly, then the switching activity of the secondary sensor will be lower and fluctuate less than than the primary.  If the catalytic converter is bad, then the waveform of the secondary sensor will mimic the one of the primary. 

I am trying to do this just to see if it can be done until I can replace the catalytic converter for my car.

[Mechatrommer]

a very large capacitor is needed to smoothen this low frequency signal (2hz)

...is by comparing the waveform of the primary O2 sensor with the waveform with the wave form of the secondary O2 sensor...

it needs waveform to function properly and now you want to kill it?

[willis9076]

For my car it only counts the number of times it crosses 0.45v.  It does not matter what the wave form looks like.  Its pretty ugly to begin with.  In order for it not to throw the code, the ratio of the secondary to the primary cross counts should be <0.75

Here is a link to what some of the waveforms look like.

http://autolabscopediagnostics.com/sensors.html

These are not my waveforms, but similar to what I have been getting with my oscilloscope.

Here is another example of some waveforms.  The o2 sensor data in the middle of the page.  My car sometimes looks like the one titled 2 Channel Oxygen Sensors, but not all the time.  It varies considerably with RPM, load, etc.

http://www.omitec.com/en/support/waveforms/

[Zero999]

For my car it only counts the number of times it crosses 0.45v. 

As I said above, a RC filter won't change the frequency, it'll just attenuate the waveform and more so at higher than lower frequencies.

To divide the frequency by an even number (i.e. to get 1Hz, 0.5Hz, ¹/₃Hz  you need a divide by n counter which will just generate a pulse or every nth pulse on the input. To divide by an odd number, it's much more complicated, you need a PLL or a microcontroller.

To be honest I think this is a waste of time, if you want to tweak the settings you should look into reprogramming the ECU rather than trying to fiddle with the input.

[Zad]

I would get the catalyst (or sensor) sorted instead of bodging the results. Catalysts are a LOT cheaper than they used to be, especially if you buy them online. A Cat that is breaking up may well be throttling the exhaust and reducing your engine power and efficiency anyway.

Other than that, as above, you can't change the frequency by filtering.

[willis9076]

Thanks everyone for the advice.  I will still try and replace my catalytic converter.  I thought I might just try this in the meantime.  I found some cheap catalytic converters online, but for some reason a lot of places will not ship to California.