so how did you ever find the ESR of these caps at 10MHz??
I can't find that information for either kind of cap that I have.
Some other cap datasheets have a curve of esr vs. frequency but I've found nothing of the sort for these particular caps.
I had assumed that for the most part, my 1" by 1/4" by 6" buss bars would have very little inductance. Apparently this is not so, there is a 1V difference in the amplitude of the rails oscillation along the length of the BM+ bar.
However the real inductance is the 1oz copper clad of the capacitor board.
I tinned it heavily in an effort to increase its thickness, and the traces are all as wide as possible, 0.8" at the thinnest, >1" everywhere else.
The ripple at the far end of the cap board has an amplitude of only 0.4V when it is 5 volts at the mosfet. Thus adding more capacitance here would be of little use; the inductance of the traces would dominate. It needs to go elsewhere.
The connection between buss bar and cap board is solid; very little difference in ripple amplitude across the fresh solder joint.
I tried attaching a 100uF electrolytic I had lying around across the buss bars near the mosfet and it did wonders for reducing the ripple. The spare poly cap I tried was not as effective, likely due to its high ESR as you mentioned.
(spare = I ripped it off the driver board hehe)
I'm sad, I thought those had a lower ESR
What type of cap would you suggest? I am leaning towards ceramic.
My oscillations occur at about 1MHz and 15MHz.
Looking at HF caps, their resonance point is very small, such that no one cap could counter both the 1MHz and 15MHz noise. Since the 15MHz has a higher amplitude I would want to beat it first.
This cap
Digi-Key - 495-1024-1-ND (Manufacturer - B37986G1222J054)
has an impedance of 50mOhm in the MHz range, but apparently the leads have to be the correct length to get the impedance right for the resonant frequency to move. Seems reasonable but how would you design with this in mind? Apparently adding 2cm of lead shifts the resonant point from 100MHz to 10MHz.
Hmm yes, I knew you can get fake signals at high freqs but it didn't even cross my mind here. Since it has different magnitudes at various locations and since I can change it by adding capacitance I agree that it is real.
I want to return that epoxy but I lost the receipt. I should note that the bond was quite strong between the FR-4 and buss bars; I took off some copper clad when I pried them apart.
apowers: the solenoid (and thus the bendix) is disabled. However it would draw way less current than the starter itself. I know the starter draws 370A stalled. Once I had 2 mosfets (total 380A rated) and they still fried in <2s, and were cold afterward. No sign of overheating, which is an indication for overcurrent AFAIK.