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It is desirable to limit the charging current to avoid overheating or damaging components. How do we go about choosing an appropriate value?
There are inherent current limits on a mains-connected power
supply. The power supply impedance comes in part from the internal
resistance of its components. The transformer also limits
current because all power is transferred magnetically through
the iron core. The core’s cross-section and material
dictates the maximum secondary current.
The purpose of the series charging resistor is to limit surge current, but not so much that charging time becomes excessive.
The charging time is governed by the RC time constant. We will assume the capacitor is sufficiently charged in 5RC time constants. Our self-imposed requirement is a charging time under 10 seconds, so let's begin with a goal of being fully charged in 5 seconds or less.
Since the power supply impedence is a significant part of the total series charging resistance, our choice depends on the implementation.
The Variac is capable of providing very large currents (6 amps or more), so a charging resistor is required to limit current. The calculations are:
5t = 5 seconds
T = 1 second
Based on this calculation, we started with a 50-ohm series resistor. Some experimentation found that a 25-ohm rated at 25W worked well, remained cool, and provided full charge in five seconds or less.
The isolation transformer in the fixed-voltage supply was
selected to be of the proper size to limit charging current.
See Isolation Transformer to
see how this was done. No current-limiting resistor is needed
when the transformer is of appropriate size.
Last update June 23, 2007 by Barry Hansen ©2007