I see various values in different designs of PSU for this resistor. What dictates this? Is there a formula?
And also sometimes one sees several resistors in parallel at this point. What is that for, sharing dissipation?
Thanks
And also sometimes one sees several resistors in parallel at this point. What is that for, sharing dissipation?
Thanks
The higher the resistance, the better the ripple filtering and the worse the DC voltage drop across it. The DC voltage drop is simply V = IR, where V is the DC voltage drop, I the DC current and R the resistance. The ripple approximately gets suppressed by a factor of 2 pi f R C, where f is the ripple frequency and C the capacitance of the second capacitor.
Yes.
You can of course design for a specific amount of ripple rejection, but in practice you are normally more limited by headroom (= the max. voltage drop over the resistor) or power dissipation (Vdrop x peak current draw).
In terms of the peak current draw a "reasonable worst case scenario" for a class A amp is twice the idle current (as that is where the amp leaves class A). In terms of dissipation, you should probably aim not to exceed half the rated dissipation of your resistors.
Thanks for the replies. I have had another Google and using the term 'filter' soon brings up some good reading. Some of it from this forum.!
Cheers
Cheers
This calculator will give you an idea.
Generally, you can make R smaller if you make C bigger.
RC Low-pass Filter Design Tool
Generally, you can make R smaller if you make C bigger.
RC Low-pass Filter Design Tool
Allowable voltage drop, and power dissipation are key elements as above.
Another consideration is capacitor ESR vs real R in the RC component; the bigger the difference between the two, the better the ultimate attenuation.
Trivial examples -
Say for a preamp, or phon stage, or similar element with a low current draw you can use R=10 ohms, and your reservoir caps have an ESR around 0.1 ohms. The maximum attenuation you will get from the RC element is 40dB - a really useful amount ahead of a regulator, or for many other purposes.
In a power amplifier, really large reservoir caps might achieve ESR=0.03 or so; perhaps two in parallel, so equiv ESR = 0.015 but you might only select R = 0.1R for reasons of I^2 R losses, and supply impedance (e.g. look at some of the PSUs in the Pass labs area of the solid0state forum.)
Then - yes, there is some filtering - but considerations of thermal dissipation/ supply impedance limit the R value; and the ultimate attenuation of the 'RC' element is limited, depending on exact implementation, to 16-20dB. Again - that might be helpful enough - esp if your chosen amp architecture has only moderate PSRR.
Basically - it depends on how you balance your priorities.
Another consideration is capacitor ESR vs real R in the RC component; the bigger the difference between the two, the better the ultimate attenuation.
Trivial examples -
Say for a preamp, or phon stage, or similar element with a low current draw you can use R=10 ohms, and your reservoir caps have an ESR around 0.1 ohms. The maximum attenuation you will get from the RC element is 40dB - a really useful amount ahead of a regulator, or for many other purposes.
In a power amplifier, really large reservoir caps might achieve ESR=0.03 or so; perhaps two in parallel, so equiv ESR = 0.015 but you might only select R = 0.1R for reasons of I^2 R losses, and supply impedance (e.g. look at some of the PSUs in the Pass labs area of the solid0state forum.)
Then - yes, there is some filtering - but considerations of thermal dissipation/ supply impedance limit the R value; and the ultimate attenuation of the 'RC' element is limited, depending on exact implementation, to 16-20dB. Again - that might be helpful enough - esp if your chosen amp architecture has only moderate PSRR.
Basically - it depends on how you balance your priorities.