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

[TheMonkey]

DIY EFI seems to be a war that i am determined to win; one battle at a time.

my current battle seems to be RF interference ( noise ). if i want to hang a capacitor to smooth the power bus, are there any type of guides to what kind, what size capacitors to use?

seems that the big capacitors, like MSD 8830 is an 'electrolytic capacitor' and is big, spendy, and requires polarity. i will be putting one of these onto my ignition system.

also, are there other things to battle noise? should noisy items be wired directly to battery rather than share a power lug with anything sensitive to noise (ECU, radio). ground loops are another issue that i'm paying attention to, but my question is more about the power side.

for other items, like fuel pump, how do i size it? i have 2 items that are causing noise. the fuel pump has about 0.1v AC signal with consistent frequency, and the LC-1 o2 sensor has about the same votage with stranger frequency and some whacky spikes with much higher voltage. when i remove just the heater ground from the o2 sensor (leave the controller powered), the noise goes away.

how much noise is acceptable, and if'n a capacitor can get rid of it, how do i know what type/size to put on?

thanks for any help.

Fuel Pump only:


o2 Sensor only:

[arlo#1]

Here is a link to a post of mine on sizing fuel pumps viewtopic.php?t=23026&highlight= I would like to smooth out the voltage on my car as well. I will be watching this thread and if I find anything I will respond with the info. Has anyone tried the voltage stabilizers off of ebay? they look cheep and maybe good?

[TheMonkey]

Arlo-

thx for reply. i'm currently thinking that fuel pump sizing and sizing capacitors for voltage fluctations are 2 different issues. maybe related, but i'm not sure how.

i'm trying to just smooth my power bus. currently, i have conducted noise coming from ignition system (lotta noise), o2 sensor (lotta noise), alternator (little noise), fuel pump (some load fluctuation, no spikey noise).

for ignition, i am going to put the MSD 8830 capacitor on to hopefully eliminate this (i'll report back with before/after pics): http://img176.imageshack.us/img176/8805 ... werfd5.png

for o2 sensor, innovate has issued an RMA, they said this is abnormal, hopefully will be eliminated with repair: http://img136.imageshack.us/img136/3200 ... erywf1.png

alternator has some spikes, but probably within normal range and can hopefully be smoothed with a capacitor, but what size capacitor?: http://img220.imageshack.us/img220/1517/altlumpsaa1.png

fuel pump as seen above, but i don't think can be improved, as there are no spikes like seen in the other items: http://img136.imageshack.us/img136/3070 ... nlyar9.png

i think the only capacitor i need to size right now is for the alternator to get rid of the spikes between the lumps, but i don't know how to size it.

importantly though, i'm wondering if there is a workable method to take inputs like frequency, voltage spikes, current, or whatever, and determine what capacitor could be used? when i try to hit google with it, i generally get puked on with tons of hits telling me 1farad per 1000 watts of audio power.... seems that there is a big market for expensive capacitors for audio setups. so much that it's muddying any attempts i make at finding info for my application. hmmppfffh.

[arlo#1]

This is were good test equipment come in handy I have a scope now and man it is handy. Firts off we need to remember to run good conections everywear and have proper wire. I have found that most sensors need rf cable. The second weird thing I found is with a electric fuel pump I can get a ac volt reading of 38 on one peticular motorcycle with the key shut off I tried it out side thinking it was the lights but it still did it. I have no way to explain it. As for the size of the pump it self it is important to size acording to your engine demands. but the capicitor is seprate. I want a capictor to smooth out the power coming out of my alternator and if I get any bad conections I have seen spikes as high as 17 volts but I think that is from the key switch. I would just like to smoth out the power to make it eiser to tune and so It will stay tuned.
I also looked into my lc-1 and couldnt make it give off rfi like you were asking I wanted to try another radio or two but haven had the time.

[Carl Gianonatti]

Im taking an electronics class now and dont have to book with me for the formulas. I really dont think you need it. Well from your scope you should be able to find out the frequency of the noise. Then off of that a simple rc circuit should work, a low pass filter. From a little bit of radio controll helicopters experiance they use tatlum capacitors right across the legs on the electric motor to reduce the line noise. Well you could try a 25v(just make sure the voltage is aleast 16v nothing lower) 100micro farad cap, or larger, and hook up positive to positive and negative to negative that should be a simple fix. I believe that should work. depending on the resistance of the wire the cut off should be around 1000hz. hook it up and see what happens to you scope reading. hope this helps some!

[TheMonkey]

i think i'll try RF wire on my TPS if the blips are still there after some of these efforts. i'll probe the sensor directly and see what's in there i guess.

...The second weird thing I found is with a electric fuel pump I can get a ac volt reading of 38 on one peticular motorcycle with the key shut off I tried it out side thinking it was the lights but it still did it. I have no way to explain it...

was your 38v AC reading from your scope or voltmeter? i have an inexpensive voltmeter (non-RMS) that will read bogus AC in a DC system. if i put my cheap digital voltmeter directly onto a known good stand-alone battery (set to AC), it reads 27v AC, and with motor running, it reads 32v AC which is not right.... need either scope it or use a true-rms voltmeter to read AC on DC system.

that's encouraging that your LC-1 is not screaming into your radio like mine. i've sent it in, i just hope they repair it. it seems to operate okay, but it is LOADED with RFI.

[arlo#1]

i think i'll try RF wire on my TPS if the blips are still there after some of these efforts. i'll probe the sensor directly and see what's in there i guess.

...The second weird thing I found is with a electric fuel pump I can get a ac volt reading of 38 on one peticular motorcycle with the key shut off I tried it out side thinking it was the lights but it still did it. I have no way to explain it...

was your 38v AC reading from your scope or voltmeter? i have an inexpensive voltmeter (non-RMS) that will read bogus AC in a DC system. if i put my cheap digital voltmeter directly onto a known good stand-alone battery (set to AC), it reads 27v AC, and with motor running, it reads 32v AC which is not right.... need either scope it or use a true-rms voltmeter to read AC on DC system.

that's encouraging that your LC-1 is not screaming into your radio like mine. i've sent it in, i just hope they repair it. it seems to operate okay, but it is LOADED with RFI.

Yeh it was mesured with a cheep multi meter so It shows that good test equement and the knoledge of how to use it is very important.

[TheMonkey]

thx Carl.

the alt scope show was at idle without any load. the frequency was just over 1kHz at idle, and could go up to 9kHz full tilt. i wonder if the AC spikes on the alt increase with current load on system?

i have a capacitor out of an old HEI distributor i'll put on for a goof to compare with above scope shot from idle.

soo.... with a changing frequency due to engine speed, does RF become tougher to battle? capacitors are sized according to frequency?

[Carl Gianonatti]

A changing frequncy doesnt matter because everything above the cutoff frequncy of the low pass rc circuit is basicaly shorted out. take a look at this web site it should be able to explain it better than I can.
http://en.wikipedia.org/wiki/Low-pass_filter

[DarrowConley]

Any updates on this.

I am having what I think is noise issues. On my setup all of the grounds go to one spot and from the same connection back to the negative side of the battery.

I am thinking about putting diodes on the negative legs of the sensors to eliminate any feedback from the other sensors. I am thinking this will help isolate the senor and hopefully take out the noise.

Anyone have some thoughts on this?

Darrow

[arlo#1]

Any updates on this.

I am having what I think is noise issues. On my setup all of the grounds go to one spot and from the same connection back to the negative side of the battery.

I am thinking about putting diodes on the negative legs of the sensors to eliminate any feedback from the other sensors. I am thinking this will help isolate the senor and hopefully take out the noise.

Anyone have some thoughts on this?

Darrow

I am thinking diodes will not work for you most diodes have a certian resistance in their conductive polarity and that will be added to the resistance of the sensor you are using so the readings the megasquirt sees as well as in megatune.

[arlo#1]

Any luck with this I just bought a 6800 uf 25v cap to wire in right before ms and see if it smooths out my voltage in ms. I have got my volt spikes down to 15.5 and mesured my injector opening time very precicely so I dont get weird air fuel spikes when the voltage spikes happen as bad.

[vinister]

You guys may want to check out some car-pc power supplies. Carnetix makes many which are simply DC to DC, filtered and isolated power supplies. The output voltage will stay at 12Volts with any input from 5-18 volts DC, and they should filter out any RF coming in on the power lines.

I have one in my car, powering my PC, and I have thought about hooking the MS up through it, but I don't seem to have any noise or grounding issues with my setup as-is. The other advantage is they have a seperate 5V DC output, ideal for running extra sensors and external circuits.

The other option is to use an RF choke on the power lines. Its essentially just a big chunk of ferrite with a hole in the middle that you run the power/ground wires through. You can loop the wire through it many times for increased filtering. The idea is that any AC signal or spike creates an electromagnetic field around the wire. The electro-magnetic field is absorbed by the ferrite material, thus removing or at least seriously attenuating the signal in the wire.

As for capacitors, just play around with different values. I am in an electronics program, and sure there are formulas to calculate anything, but chances are you will have measurement errors which will lead to calculation errors, so the calculated cap value won't be the most optimal anyways. Much easier and faster to just grab a sampling of caps, and try them while observing the signal on the scope. Remember that a filter uses RC, resistance and capacitance, in calculating the cutoff frequency.

[arlo#1]

I just simply wanted to smooth out the voltage but still let ms see a current value becaue the injectors are hooked to a different power source I want ms to compensate for the voltage changes still I just came home from installing my cap to get my computer to data log it idling. Let you know tonight!

[TheMonkey]

Arlo-

I found some great solutions to smoothing out voltage. I was most interested in nice voltage going into MS & LC-1.

IGNITION SYSTEM
I put the MSD 8830 cap on to the ignition system. this is with MSD 6AL box, but I had similar results on my LS2 D585 coil packs. here is pictures:
BEFORE: http://img210.imageshack.us/img210/447/ ... werkz2.png
AFTER: http://img212.imageshack.us/img212/4186/4sparkslc8.png

ALTERNATOR:
I found some spikes from the alternator brushes. This was newly rebuilt alternator. As mentioned above, I used an old canister capacitor from a distributor and had good results. capacitor size is 0.25 mf. pics:
BEFORE: http://img220.imageshack.us/img220/1517/altlumpsaa1.png
AFTER: http://img153.imageshack.us/img153/991/altwcap2lk5.png

POWER INTO ECU / O2 BOX:
I picked up 2 stereo style filters from crutchfield for these power sources. here is a link to the filters. http://www.crutchfield.com/p_003S25A/Am ... ml?tp=2653

the filters work great. here is a pic showing the power going into the filter (blue), and the power coming out to MS (red): http://img254.imageshack.us/img254/1920 ... er2oi6.png

The power bus going into my MS is pure butter now. but, in testing the sequencer, i have found that i need to be careful with RFI. i had the MAP sensor wire near a power line and that created some weirdness in some IGN signals, but was fixed with re-routing the wires.

[arlo#1]

Here is a couple screen shots of the voltage during start up and idleing before and after I installed a $5 6800 uf 25 volt capacitor.

graph.png

graph 1.png

[arlo#1]

Clearly the second one (graph 1) is the original and not as good I was hoping they would show up without having to open them but If anyone wants to see a datalog before and after I will post it I am very very impressed with this it changes as the voltage changes but got rid of my 15.5 pluss spikes (and lows) which were causing me some greif for sure. Again this costed me 5 bucks and about 20 minutes of work. You just hook the positive of the cap to the positive power wire and negative to the grounds from megasquirt as close as you can to the Megasquirt ecu as you can. I made sure it is the power wire that is only on when the key is on.

[arlo#1]

I just looked and knoticed how much this cleaned up all the signals from my sensors. You can even see in the picture the tps is a strait line not bouncing between -1 and 0 the mat and coolent are alot smoother now too.

[TT350chevelle]

Very interesting and something I should do, where did you buy that cap?

Brad J.

[arlo#1]

Very interesting and something I should do, where did you buy that cap?

Brad J.

I got my from a local electic parts store called Quales. It is about 1.5 inches long and 3/4 inch dia. Just make sure it is a high enough voltage I got a 25 to cover spikes from my ignigtion switch being old and loose and incase I acidently leave the battery charger on while I turn on the key or I am boosting it.