How much capacitance - continued V

[Kenneth G. Gordon]

Now, up to this point, I have left an awful lot unsaid, and 
have ignored, or, at least, not mentioned, many other 
factors that should really be kept in mind when choosing 
the size of capacitors in any HV supply.

However, at this point, I think I can list some Rules-de-
Thumb-o when designing power supplies, and especially, 
when choosing the values of capacitors included.

1) In the interests of best regulation, keep ALL series 
resistances to an absolute minimum. (Duh!) Some of these 
are NOT obvious, by the way!

2) In "straight" power supplies (non-multiplying), avoid 
capacitor-input type filters unless the voltages involved are 
fairly low, and the current needs are low.

3) If you need to use a capacitor-only filter, then by all 
means use only and at least a voltage-doubler type. You 
may find that a tripler or quadrupler or even more might 
work as well or better.

4) If using a voltage multiplying type, make sure your total 
filter capacitance is at least 30 mfd.

5) If due to the design of your chosen power transformer, or 
the fact that it is "too good" exhibiting VERY low series 
resistance, the in-rush current is excessive (proven by 
blown diodes, fuses, damaged switches, heavy blinking of 
the house lights, or, in a worst case, blown breakers) by all 
means use some means of limiting it by either step-starting, 
or the use of properly chosen varistors in the input.

6) Keep in mind that although with everything else being 
equal and inrush current being kept to a safe value, 
capacitance values in a capacitor-only filter above a 
"certain value" will only increase the time it takes to achieve 
the full design voltage. This is a waste of money, at the 
very least. There IS a point of diminishing returns, after all.

7) It is my carefully considered opinion that in the Heathkit 
designed linear amps, the SB-220 and the SB-200 at least, 
total capacitance values (using MODERN, low-leakage 
caps) of from 2 to 5 times the original value is quite safe 
and will NOT cause excessive inrush current, but WILL 
significantly improve the dynamic regulation, reduce 
spurious emissions, and generally make the power supply 
and the amp run cooler.

One last thought: if we assume (and this is a very BIG 
assumption, and most likely not at all accurate) that the 
internal resistance of an uncharged original Heathkit 125 
MFD 450 VDC cap is about 1 ohm, then 6 of these in 
series would be 6 ohms.

At 2400 VDC, the current into 6 ohms is 400 amps. (We 
are ignoring the "time" factor here, obviously.)

We also know that the original Heathkit diodes were 500 
VDC at 1 amp types.

I seriously doubt that those would safely handle a surge of 
400 amps, even for one cycle.

Thus we have some assurance that the power transformer 
does have some means of limiting the current.

With 6 ea NEW capacitors, which have somewhere around 
10 times the internal resistance, that surge current would 
be 40 amps instead...and this would not depend on the 
value of capacitance, but only on the internal resistance.

This assumption may be incorrect, but it does illustrate the 
fact that we have to take many, many factors into account, 
not JUST the value of capacitance.

In my own case, I can positively state that my substitution 
of 330 MFD caps for the original 125 MFD caps in my SB-
200 did not change the switch-on effects for the worse one 
iota. In fact, everything appears to be BETTER than it was 
originally.

Ken Gordon W7EKB

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