commgrp@silver.bacs.indiana.edu (08/05/88)
In article <1320005@hpfclm.HP.COM> myers@hpfclm.HP.COM (Bob Myers) writes: >>> as the ripple gets smaller the 'conducting-time' (forward bias) >>> of the rectifying diodes gets smaller. If the power to the >>> load stays the same then the current during foward bias gets bigger. >>While this will work, there are better ways to handle the inrush current >>than overly-overrating your diodes... >>Two common means of taking care of this problem: ...thermistor... power >> resistor [shorted by relay] >I think you've misunderstood; the problem is not the inrush >current at powerup (although that is something to watch) but the >current drawn on each half-cycle of the AC afterward.... Hence >the need for bigger diodes, >-- >Henry Spencer at U of Toronto Zoology Henry is correct! Those current spikes heat the diodes and capacitors, causing failure if the components are not (apparently) overrated. Also, the huge pulses can saturate the transformer core, and can introduce 60 Hz + harmonics hum into the regulator output by groundloop effects, etc. An easy way to limit repetitive inrush current is to place a small air-core inductance (say, 5 turns of #12 wire wrapped around a broom handle, for a 20-amp supply) between rectifier and filter capacitor. -- Frank reid@gold.bacs.indiana.edu "Damn it, Hal, open the pod bay doors!"