At what level does the ripple become acceptable?
MikeW said:At what level does the ripple become acceptable?
MikeW said:Could somebody explain to me why you need such big caps for the power supplies?
MikeW said:If you a drawing 2 amps from a 40 volt supply and have 20,000 uf. caps or 40,000 uf. caps, is there an audible difference? After reading this forum for the last couple of weeks there seems to be more problems from in-rush current than from ripple. Is it possible they are using to much capacitence?
jwb said:Inrush is caused by two things: empty capacitors and torroidal transformers. You can have zero capacitance and a torroidal transformer will still draw 50amps when you apply mains power. But yes, more capacitance means more inrush.
Steve Eddy said:Mmmmm. What have toroids to do with it?
jwb said:If you have a large toroid, the iron core can hold a magnetic field after the mains power is disconnected. If you next connect the mains power in the opposite phase (over which you have no control, so it's a good chance), there will be a massive rush of current trying to reverse the field in the core. This can often cause intermittent blown fuses at turn-on. I always use a resistor between the mains and the primary to reduce this current inrush.
haldor said:Another issue that effects capacitor requirements is related to the design of the amp. Some amps designs are more susceptical to ripple on the mains than others.
For example, I built a pair of Slone's 250W MOSFET amps running in Class B and got virtually no audible hum with 40mF total capacitance (20 mF per rail voltage). The Zen amp at less than 1/10 the power needs at least as much if not more filter capacitance.
If you have a large toroid, the iron core can hold a magnetic field after the mains power is disconnected. If you next connect the mains power in the opposite phase (over which you have no control, so it's a good chance), there will be a massive rush of current trying to reverse the field in the core.
Other folks have stated that 1 - 2J per 10W of power is a good rule of thumb -- 0.5*C*V-squared, where V is the voltage across the capacitor and C is the capacitor.
A transformer doesn´t have stored energy in it after you disconnect the ac from it.
When you apply ac to the primary, the toroid has no energy stored in it, so it "reacts" with a great transient till it is charged. A transformer reacts in the opposite way as a capacitor. It kicks back at what you give it.
Usually a 500 VA or more Toroid can drop a 16 A security fuse by itself with no caps. The inrush current is 50 or amps but only for a few msec
Steve Eddy said:
Well inrush current isn't a problem as it's transient and only occurs during the initial power up.
However the companion to ripple voltage is ripple current. And as you increase the amount of capacitance, while you reduce ripple voltage, you increase ripple current.
And that can have consequences with regard to raidated magetic fields from the power transformer seeing as the magnitude of the magnetic field around a current carrying conductor (such as the windings in your power transformer) is proportional to the amount of current flowing through it.
So with regard to ripple voltage, its consequences depend on the power supply rejection of the amplifier topology, and with regard to ripple current, its consequences depend on how susceptible your circuit layout is to magnetic field interference.