ray_moth said:A little more information would help. Will the filter be choke-input or capacitor input? What will be the output voltage and demand current on the filter? Will you be using a tube or SS rectifier? What value capacitors do you intend using?
Thanks, yes I will come up with values soon, I am still in the drawing process Didn't really think about all this yet, i'll draw it up and show here.
Geek said:Hi,
A rule of thumb... take operating voltage and divide that by operating mA and you have a choke value.
So if the circuit runs at 400V at 100mA, choke would be 4H (5H is close enough).
Cheers!
Awesome, sounds good to me - i'll still post here to see what you think ! Thanks
Hi Tweeker,
Yes, true enough. If you have a class AB amp, use the *minimum* expected current draw for calculations.
Cheers!
Tweeker said:Geek is giving a bear minimum there.
Yes, true enough. If you have a class AB amp, use the *minimum* expected current draw for calculations.
Cheers!
The purpose of the choke, on a cap input filter, is to work with the following filter capacitor to form a LC filter....
DC current capabilty should be based on MAX DC current draw, to keep the choke from saturating....give your self about 20% to 30% headroom with the max DC current..
The LC filter is a DOUBLE POLE filter....this means it attenuates at -40db/ decade.....
Your main objective is to smooth the ripple...in most cases it will be twice the mainis frequency...so a 60 Hz mains will create a 120Hz ripple in the B+ that needs to be attenuated to an acceptable level..depending on application....
Example....
You have a 400V cap input supply...with a 5v rms 120Hz ripple on the first cap.... You next have a 5H choke and a 32uF cap.... 1/[6.28* (LC)^.5 ] ....You find that this LC has a corner frequency at 12.5Hz ..... So the filter has -40dB/ decade ...meaning that at 125Hz you will be attenuated by -40dB .... This is fairly close to saying 120Hz.... SO since -40dB is the same as having a 1/100 attenuation in voltage amplitude.... So 5V/100 = 50mV rms ripple after this filter stage.... Actually you would need to include the load current and place a Resistive element in the analysis since loading plays a role in filter performance...as well as damping, since you would like your filters properly damped from a step response in the system....
Depending on the type of load, the filter can behave differently..such as a Resistive load vs a Constant Current load and whether or not the load has a negative impedance...
Chris
DC current capabilty should be based on MAX DC current draw, to keep the choke from saturating....give your self about 20% to 30% headroom with the max DC current..
The LC filter is a DOUBLE POLE filter....this means it attenuates at -40db/ decade.....
Your main objective is to smooth the ripple...in most cases it will be twice the mainis frequency...so a 60 Hz mains will create a 120Hz ripple in the B+ that needs to be attenuated to an acceptable level..depending on application....
Example....
You have a 400V cap input supply...with a 5v rms 120Hz ripple on the first cap.... You next have a 5H choke and a 32uF cap.... 1/[6.28* (LC)^.5 ] ....You find that this LC has a corner frequency at 12.5Hz ..... So the filter has -40dB/ decade ...meaning that at 125Hz you will be attenuated by -40dB .... This is fairly close to saying 120Hz.... SO since -40dB is the same as having a 1/100 attenuation in voltage amplitude.... So 5V/100 = 50mV rms ripple after this filter stage.... Actually you would need to include the load current and place a Resistive element in the analysis since loading plays a role in filter performance...as well as damping, since you would like your filters properly damped from a step response in the system....
Depending on the type of load, the filter can behave differently..such as a Resistive load vs a Constant Current load and whether or not the load has a negative impedance...
Chris
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