NOISE: How to size a capacitor, and what types?

[pit_celica]

So, you are saying that adding a 6800uf 25V capacitor between DB37 pin 28 (12V to MS) and DB37 pins 8-11 (ground) smoothed all the others sensors signals? How this is possible? How the voltage to MS can influence other sensors?

[arlo#1]

So, you are saying that adding a 6800uf 25V capacitor between DB37 pin 28 (12V to MS) and DB37 pins 8-11 (ground) smoothed all the others sensors signals? How this is possible? How the voltage to MS can influence other sensors?

To be clear the cap is outside of the megasquirt ecu it is to big to be fit inside. So I did hook it to the main positive wire going into ms and the main negative on my wiring harnes right at the 37 pin end of the harnes. I belive this worked for 2 reasons!
Reason #1 If the power going into megasquirt was spiking up and down then the 5 volt power supply inside the ms bord would have to try to smooth out the spikes that it recives from the 12 volt system to keep a smooth 5 volts which it probably did being as how much power is used from the 12 volt side of things
Reason #2 For those of you who use innovate tuning systems you will probley have read in their info all about ground offsets. In my car I have potential for noise to be picked up on the grounds and fed into the ecu as well if I get a sudden ground loss because of some oxidation my voltage may spike for a bit then drop and seeing as how all of the sensors use the same ground this noise on the ground from rfi or weak grounds can cause quite a mess in how the ecu multiplys the fuel equations.
My vr sensor is a sheilded cable but the ground is still common to all other senors and I have have synce loss problems for a while this cap also helped to elimate that by taking alot of external noise off the vr curcit!
I am not trying to make it look like I have a poor install with weak grounds and other problems I have spent alot of time running extra grounds and positives around my car. I have a hi amp alternator and alot of things this old car was not intended to use!
To top it all off I had to install two more 3300uf 6.3v capacitors on the five volt curcit at pins 5&9 on the serial port to make my bluetooth work smoothly I am realy becoming amazed in what a few cents spent on a capacitor can acomplish!

[arlo#1]

I have two large capacitors from an AC system compressor. They are rated at 3 and 5 farads. One is a motor start type and the other is a motor run type. Would these be useful for cleaning up the power coming into a Megasquirt? I noticed that they are substantially less expensive for their capacity than the ones marketed for audio systems. Is there a significant difference between this type of capacitor and the audio filter ones?

Julian

Three and Five farads is huge I have never herd of any that big what is the physical size of them? I am not an expert on capacitors but I figured the car audio type were a stack of smaller caps inside a bigger case. I am not sure if there would be any downside to the bigger caps I knoticed my 6600uf cap cuts the nasty spikes out which is what I wanted but the bigger the cap the smoother the voltage will be. What voltage are they rated at?

[psu_Crash]

I have two large capacitors from an AC system compressor. They are rated at 3 and 5 farads. One is a motor start type and the other is a motor run type. Would these be useful for cleaning up the power coming into a Megasquirt? I noticed that they are substantially less expensive for their capacity than the ones marketed for audio systems. Is there a significant difference between this type of capacitor and the audio filter ones?

Julian

Three and Five farads is huge I have never herd of any that big what is the physical size of them? I am not an expert on capacitors but I figured the car audio type were a stack of smaller caps inside a bigger case. I am not sure if there would be any downside to the bigger caps I knoticed my 6600uf cap cuts the nasty spikes out which is what I wanted but the bigger the cap the smoother the voltage will be. What voltage are they rated at?

Bigger the better. When choosing a capacitor for a filter on a DC power line use the biggest electrolytic (polarized) capacitor that you can find. Bigger will filter out spikes much better than a small cap. I use a 2.5 Farad cap directly on the power coming into MS. I also have 2 more of these caps connected in the car. One directly off the battery and one on the charge wire from the alternator. This helped to clean up my noisy signals greatly!

[arlo#1]

I didnt want to get to crazy with size because the ms still needs a current voltage to know for things like the injector opening time corection. I like how I have mine now works great.

[Heribert]

The AC motor start and run caps are definitely NOT 3 and 5 F ( Farad)
They are likely to be 3 and 5 µF, but rated at at least 250 VAC. Not
the ideal caps for filtering a noisy 12 VDC supply. Capacitance is much to low
and voltage rating not relevant for the application .

Get hold of good electrolytic caps with a voltage rating of minimum 16 VDC and the cpacitance
above 10000µf( ie 10 mF). Car audio type caps on several F are of course great but a bit pricey.
If you can get hold of a good quality inductor in the 10 mH range with very low DC resistance ( a few mOhms)
connect that in series with the MS supply lead and then the cap from the MS side of the inductor to a real good
grounding point for the MS.
Voila , a low pass filter , which will gobble up the majority of the conducted transients.

Heribert