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In Reply to: RE: Question about recapping a Dynaco ST70 posted by Lee of Omaha on July 10, 2014 at 13:26:41
"The size of the capacitance in the power supply is only a start-up issue; after the first few seconds the value harldy matters to the rectifier."That's not true.
With a capacitor input filter every half cycle one diode "turns on" for a brief time.
The peak current is high, much higher than with a choke input filter, and that peak current (that happens twice each cycle) is what can not be exceeded.
BTW, for capacitor input filters the data sheets state the minimum DC resistance per plate. If you have more than the minimum DC resistance per plate (either as drop resistors or as DCR in the HT winding of the power transformer) you can use a larger value cap and still not exceed the max peak current limit.
As an aside, you will notice (on the data sheets) that there is no minimum DCR per plate requirement when a critical inductance choke input filter is used. That's because the current flows for the entire half cycle for each diode so there is not large peak, in fact the current through the power transformer secondary and the diodes is just a little over the load current.
Start-up is a different issue.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/10/14 07/10/14 07/10/14Follow Ups:
it's been while - C1 gets slammed(charged) 120 times per second (mains Freq. x2)
The size of the cap in uf determines the amount of current drawn from the secondary, required to charge C1. Which in turn, (ohms law) the primary sees that current as well.
This is why increasing the already huge C1 bank on SS amps can cause the fuse to blow.
I gave the simplified answer. As the capacitance goes up, the conduction angle goes down, and peak current goes up.
Rectifiers can take significant repetitive short-term overloads due to the electron cloud surrounding the cathode, so in most cases there should be no problem. If it does cause a rectifier to have a short life, an input choke can be inserted (low resistance, moderate inductance, enough to increase conduction angle).
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