Electronics experts Question?

Mo_Gearhead · 06-14-2007, 05:55 AM

I have an "antique" (circa 1950's) Sun Distributor Machine that I am restoring. Had it working but the old capacitor failed and I need a replacement. The item is marked: .3MFD 600 VDC.

I am seeing listings for "modern" capacitors, similar MFD, but ratings of only 200 VDC.

This old machine has a 110V AC plug that drives the rotating motor and also uses (2) D cell batteries. My question to the experts is: do you think I could use a 200 VDC rated capacitor?

I really don't understand where the large DC rating is being utilized?

id10t · 06-14-2007, 06:13 AM

Might want to track down Don Marks - he advertises in the 356 Registry magazine. He has an old Sun Dist. Machine to rebuilt 356/912 distributors on, he may be able to point you in the right direction for correct spare parts...

Other than that, no idea on the voltage. I'd think (just off hand, remember that i'm an idiot) that the DC vs. AC would be enough to keep it from working for ya...

Mo_Gearhead · 06-17-2007, 01:27 PM

Thanks for that lead.

Still wish someone could explain to me why the capacitor is rated to 600 VDC? It's the original (1950's era) capacitor.

I forgot to mention that there are 2 leads that connect to a car (12volt DC) battery, then the 110V AC plug for the drive motor and 2, D-cell batteries.

Cannot comprehend why the D.C. rating is so high?

Hugh R · 04:32 PM

Are you sure its MFD or mFD? I don't believe you would have a Mega (M) Farad capacitor, as opposed to a micro (m) farad capacitor. An ampere is the rate of electrical flow (current) of one coulomb per second, an alternate definition is that a farad is one ampere-second of charge at one volt.

One Farad=Couloumb/Volts.

So .3MFD=X/600 volts, or 300,000FD=X/600, where

X=18,000 Couloumbs per second, not likely, that's 18,000 amps per second of discharge. More likely is

.3mFD=X/600 volts, or 0.000003 FD=X/600 volts, where

X=0.00018 couloumbs per second.

To get X=0.00018 couloumbs per second, I'd think with a 200VDC capacitor you'd want a .9mFD, 200 VDC capacitor to get the same "capacitance". But I'm not an electrical engineer so take it for what its worth.

The capacitors in old time distributors are measured in micro farads. At MIT once, I worked on capacitors that were the size of trash cans and they measured in Farads.

rick-l · 06-17-2007, 06:33 PM

Does it hook to the primary of the coil? With a 40,000 volt spark the primary will see 400 volts.

Joeaksa · 06-17-2007, 06:55 PM

Where do you live? Have an old friend in Springfield who used to have one of those and might have some spares...

Joe

rick-l · 06-17-2007, 06:56 PM

If you don't want it to be authentic you can string 3 200 volt capacitors in series as long as they are each 3 times as big.

To really do it right you need three resistors, one each in parallel with each cap that provides about 10x the leakage current to ensure the voltage shares.

Mo_Gearhead · 06-18-2007, 05:51 AM

Hugh,
It is indeed "micro" and you have told me just what I needed. If I can't find the .3 / 600 perhaps I can find a .9 / 200.

Rick, No this is a distributor testing machine only (no coil) so no high voltage present. And this capacitor mounts under the "distributor mount" (the part of the machine that spins at high revolutions) so space is very limited and "balance" is critical.

Thanks ALL for your input , I believe I can figure this out and find a modern replacement.