Hey GoayGuy,
Thanks a lot, that does help clarify the situation in my mind. What was throwing me, is the unexpectedly large measured vs. marked value on a cap that's around 45 years old.
I was expecting it to meet spec, maybe exceed slightly - but not by 40%. I guess Cornell made some pretty nice caps in 1976 or whenever.
And yes, the actual capacity will be just fine in this circuit. But depending on the rating of the extant part, I may decide to swap the bridge rect out for a 35A or 50A part.. not sure what it has atm.
Thanks a lot, that does help clarify the situation in my mind. What was throwing me, is the unexpectedly large measured vs. marked value on a cap that's around 45 years old.
I was expecting it to meet spec, maybe exceed slightly - but not by 40%. I guess Cornell made some pretty nice caps in 1976 or whenever.
And yes, the actual capacity will be just fine in this circuit. But depending on the rating of the extant part, I may decide to swap the bridge rect out for a 35A or 50A part.. not sure what it has atm.
You're welcome.
You can always 'cheat' and stick a LITTLE choke between the rectifier 'bank' and the first capacitor. The classic “choke input” power supply uses a rather large choke, which in the end only passes the mean or average voltage to the capacitors. But a small choke (as in 20 millihenry or so) squashes the dI/dt (rate of current change, or pulse charging current), which is what is damaging to rectifier stacks. Don't forget to include both a 'reverse EMF diode' and a 10 Ω resistor … across the choke. Soaks up the flyback voltage, safely wicking it to the 5 watt resistor. Good practice.
Such a low choke value only slightly drops your B+ supply. Maybe 1 volt or 2. Not enough to design a work-around to.
In any case, there you are.
Good luck!
⋅-=≡ GoatGuy ✓ ≡=-⋅
You can always 'cheat' and stick a LITTLE choke between the rectifier 'bank' and the first capacitor. The classic “choke input” power supply uses a rather large choke, which in the end only passes the mean or average voltage to the capacitors. But a small choke (as in 20 millihenry or so) squashes the dI/dt (rate of current change, or pulse charging current), which is what is damaging to rectifier stacks. Don't forget to include both a 'reverse EMF diode' and a 10 Ω resistor … across the choke. Soaks up the flyback voltage, safely wicking it to the 5 watt resistor. Good practice.
Such a low choke value only slightly drops your B+ supply. Maybe 1 volt or 2. Not enough to design a work-around to.
In any case, there you are.
Good luck!
⋅-=≡ GoatGuy ✓ ≡=-⋅
I think you can measure the actual value of the smaller cap and use it to calculate the actual value of the larger cap.
I did a quick test using my radioshack meter:
Actual measurement : C1=2.679uF, C2=27.44uF, Ctotal in series=2.439uF
Calculated Ctotal=2.441uF, look like it's 99.9% accurate.
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