F5 Turbo Builders Thread

[Bfpca]

Originally Posted by BigE

I modelled this on LTspice years ago.

If I remember rightly, the resistance was chosen to limit the spike in the inrush current on startup, given the size of the cap banks. It is higher than usual.

The delay was chosen to limit the spike during switchover.

Initially, the caps charge *very* quickly. For the switchover spike to be under 15 amps , the voltage level at the cap bank needed to be, as you say, near 10% of the rail voltage. If it was say 11 or 12% , the spike would be well over 20 amps, and could take out a fast blow fuse.

It takes a lot more time to fill the cap bank near the end that it does at the start. All that waiting is to fill the last couple of percent. And still, a very close rated fast blow fuse will open after being stressed a few times. I've switched them to slow blow to avoid that nuisance.

With a class A amplifier you need to account for the high bias current that is also flowing through the series thermistor resistance. This lowers the maximum cap bank voltage during the initial thermistor limited charge period and makes the current draw much higher when the thermistor is switched out. So you need to use a lower value thermistor in order to get the charge up. The reason that the current is higher than expected when the thermistor is bypassed is that the energy stored goes up with the square of the capacitor voltage. So double the voltage=4 times the energy stored. 90 % voltage charge is storing 81% of the energy, leaving 19% to be topped up. Slow blow fuses seem to be very reliable when properly sized for this application.