Capacitors in coil signal outputs

[TheMonkey]

i asked a similar question quite awhile ago in the MSefi forum, but i think it started to get confused with some other info.

the coil selection section in the MM that discusses LS1 & LS2 coils suggest putting a .001uF cap between pins B & C.

the confusion with previous post i think came from one of Bruce's posts about driving coils directly with microsquirt through VB921, he suggests using a 0.001uF cap; but the direct control is a different setup.

sooo.... couple Q's:

1) Bruce- have you tested LS2 coils with and without the caps? It just seems hard to believe that these OEM coils would not have put such a simple compenent inside the unit itself.

2) What capacitor would you suggest between pins B & C on LS2 coils? A partnumber from digikey would be great, because whenever i search for a cap from them, there are so many different kinds to choose from, i end up in a panic.

how did you come up with sizing it at 0.001uF in the manual? i understand their importance, and what they do, just don't know how to size them.

thx.

[TheMonkey]

actually, the coil section suggests caps between 100pF, and .001uF.

digikey has PN P3889-ND http://search.digikey.com/scripts/DkSea ... e=P3889-ND

actually looks very easy to solder into wires by connectors.

would this work?

[TheMonkey]

or... what's the difference between polypropelene caps and ceramic caps? manual suggests polypropelene.

i can drive to radio shack and get these, if ceramic is okay:

http://www.radioshack.com/product/index ... Id=2062361

[Bruce Bowling]

1) Bruce- have you tested LS2 coils with and without the caps? It just seems hard to believe that these OEM coils would not have put such a simple compenent inside the unit itself.

The capacitor is there in order to clean up the switching signal. On my test setup, scoping the 5V switching showed some high frequency hash. The capacitor is there as a bypass cap, shunting any high frequency signals to ground.

I cannot say if there is internal capacitance or not, there probably is. But I saw enough of a reduction in switching noise to recommend them. In fact this should probably be on the sequencer PCB itself, right now there is no added capacitance on the ignition output signals. There is series 1K resistors on the output, adding the 0.001uf cap is a low-pass filter with a 3dB point of 159KHz. This gives a risetime delay of roughly 6 microseconds, really not bad w.r.t the rest of the system.

But the capacitor is optional, as they say "your mileage may vary". I personally would scope the trigger signal and see if it is required. One thing to remember is that MicroSquirt's logic-level drive output has no buffering, unlike the sequencer which has a 74HCT541 driver. This part may yield better results, and the caps are not required. Again, test for your own application.

The signal here is 5V switched, you can use ceramic caps, it really does not matter if the tolerance wanders a bit, its still effective.

- Bruce

[cmonref]

Thanks -- that clarifies the same question I have had.

One further question: On the connection diagrams for LS-1 and LS-2 coils, it shows the ground for the 5v signal from the Sequencer to be grounded "at the Sequencer." All of the grounds in the Red connector are high-power grounds that go to the engine block. Am I correct to conclude that "at the Sequencer" means to use Green-20, the sensor ground for the 5v signal that controls the coils?

Brian

[TheMonkey]

Thanks -- that clarifies the same question I have had.

One further question: On the connection diagrams for LS-1 and LS-2 coils, it shows the ground for the 5v signal from the Sequencer to be grounded "at the Sequencer." All of the grounds in the Red connector are high-power grounds that go to the engine block. Am I correct to conclude that "at the Sequencer" means to use Green-20, the sensor ground for the 5v signal that controls the coils?

Brian

Brian- I brought them back to the 'sensor ground', shared with the sensors. But now you have me wondering if I'm introducing noise to the sensors. I used 20g wire for the signal wires. i'll share whatever info i come up with as i probe for what kind of noise shows up on these signals.

Also- be very cautious about wiring the connectors into coils. From some of the research I did, it seems that the order of wires in connector are reversed for LS1 & LS2. This is shown in the manual by looking at the clip placement, but it's still labeled A through D the same. I'm gonna send a note to Lance that perhaps this should be made very clear in the MM.

Scott.

[cmonref]

I caught your report about the LS-1/LS-2 diagram discrepancies. I used the LS-2 diagram and compared it with the factory wiring loom, and the LS-2 diagram is correct.

I'm following your reports with great interest. I'm preping for a Sequencer, and the April meet in Atlanta will see me with a pile of questions. Might you be at the meet?

In reading about grounding, the Sequencer wiring list (in the descriptions for wires 21-23) says to ground Green wires 18-23 to the engine grounds. But 18 is "Not Used", 19 is dedicated to the serial connection, and 20 is for sensors. Only 21-23 are hi power and should go to ground, as I understand Bruce's treatise on grounding with uS. Another Lance tidbit.

Brian

[TheMonkey]

Hey Brian-

I'd really like to go to the megameet in Atlanta, but it's gonna be kinda tough for me. Maybe next year.

On the grounding, I just used the Green side harness as provided (had serial wired in already, a signal ground, and 3 heavy ground wires). I didn't check pin location, but I would think it is as described in the manual.

For the red side, I beefed up the grounding to be 3 heavy gauge wires also. I suppose depends a bit on how much current your injectors draw, but if there's anything that dad tought me..... make sure your grounds are overbuilt. Sounds like RED Ampseal pin 3, 5, 7, 9, 18, 28, 29 are all the same - I used 3 of them.

Scott.

[TheMonkey]

thought i would share a look at capacitors on my ignition.

my ignition system:

8 each D585 coils aka LS2 truck coils, all main power leads collect to main heavy gauge wire through an MSD 8830 capacitor (26 Kufd) connected directly to battery lugs. signal wires have 1000 pF capacitors installed at the pigtail of each coil, and grounded at signal ground wire from SEQ.

here is a pic of engine running, with red trace on one of the signal wires from SEQ to coil. blue trace is main power where coil power wires collect together just prior to capacitor. pic shows 3 ignition events, and one signal pulse:



i pulled the MSD 88330 cap out, left the small caps on signal wires (same scale):



and then i pulled small caps off signal wires, so there are no caps on ignition system here:



my conclusion is that the MSD 8830 cap is very effective, but it's not clear that the small caps on signal wires are doing much for me. interestingly, the 8830 capacitor on the coil power leads cleaned up the signal wires more than the 1000pF caps directly on the signal wires.

[Bruce Bowling]

You may not see noise on the logic level coil drive, partly because your setup may not have noise, and part because of the bandwidth of the scope may not pick it up. The value of 1000pf is small and its effect will be on noise approaching and including the RF region. Good thing here is that the capacitor is not affecting the rise/fall time of the trigger signal.

I am a total proponent of using large caps on the battery supply, it just fixes all sorts of noise issues. Audio people have known this for years and only now are stiffening caps coming out on OEM setups, the cost is worth it. And you can get 1 farad capacitors on Ebay for next to nothing. The only downside is the surprise that people get when they disconnect the vehicle battery only to find 70+ joules of energy still left on the bus. I just kick the starter to suck out the charge. Same goes when you first connect the cap, the surge current can be startling. I have a circuit that manages the capacitor so that it slowly charges up then connects to the mains which eliminates this problem (can post if people want).

- Bruce