Need understanding on amperage

From the A21 schematic published by Parasound, each power buss to the output stage is fused to 8 amps. No amplifier fused for 8 amps will run 60 amps very long before vaporizing the fuse link.

From the Littlefuse data sheets for an 8 amp fast blow fuse, 60 amps blows the fuse in about 10 milliseconds. Each channel has 50,000 mfds of capacitance, and left and right channels have separate power supplies (so the full 100,000 mfds is not applied to a single 60 amp transient, only 50,000 mfds is, and those are further divided to a V+ cap at 25,000 and a V- cap at 25,000 mfd).

If you calculate the drop in voltage of the 25,000 mfd capacitor, I=C*dV/dt, so I=60 amps, C=25,000 mfd, and dt = 10 milliseconds (the time to blow the fuse), we find dV = 24 volts, or about 1/3 of the 80V supply voltage.

So if you shorted this amp, it would deliver 60 amps for the 10 milliseconds needed to burn the fuse, and have plenty of power supply voltage left in the capacitor.

It could potentially drive a 1 ohm load to 60V (or 3600 W) for 10 milliseconds. That's it, and it's limited by the fusing.

Dhl93449, your math looks right but your conclusion doesn't. At the end of 10mS the supply voltage is only 24V so the current would be 24 amps- only 576 watts. The fuse would go a little longer and the 3600 watt figure would go much shorter or am I missing something?

I think the change is voltage (dV) is 24V. So starting from 80V in DHl's equation means that the ending voltage after 10ms is 56V which is still 3136 Watts, right?

Atmasphere/Tonywise

Yes, the remaining voltage would be 56 volts, and the average power delivered would be between 3600 and 3136 Wts. And it may be somewhat less because we are not accounting for the voltage drop across the output stage transistors (it won't be zero, and will be dependant whether the transistors are being driven into saturation or not).

Guys, it was just a ballpark analysis. It also did not include the re-charge of the capacitor by the transformer or that perhaps both the V+ and V- capacitors might be discharged partially.

My point being is that the delivery of 60 amps to the load will be limited by the fuse characterisitcs primarily. Whether it is 10 milliseconds or 15 milliseconds is not that important.

BTW, I would hope that the fuse was sized to prevent the bipolar output transistors from going into secondary breakdown. If this happens, the power transistors fail and the entire output stage is toast before the fuse can protect them.

agree, Dhl93449 calculations showed the delta-V (or droop) across the power supply cap to be 24V meaning that the final voltage after 10mS would be 80V-24V=56V but at this point in time the fuse would have blown (per the Littlefuse data sheet) as the current draw from the wall would have peaked at 8A in a valliant effort to re-charge the drooping power supply caps. Thus, the output protection ckt should have kicked in ASAP to cut off the output signal from reaching the speaker binding posts & there should be pin-drop silence from the speakers! ;-) And, as Dhl93449 noted, the output stage is hopefully intact & there is no smoke curling out of the amp......