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Old 14th September 2006, 07:08 PM   #31
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That snubbing cap ... at C4 is a great idea = stabilizes the voltage reference.
Yes, but actually , it's a bit more interesting than that. To be pernickety, it [i]doesn't[/] stabilise the voltage reference but plays impedance tricks that the internal error amp likes.

The reference voltage is based on holding an accurate 1.25v across the upper resistor. This is multiplied by (R2/R1) to get an output voltage of 1.25*(1+(R2/R1)). What C4 does is reduce 'noise gain': to AC signals the voltage gain tends towards (0/R1) - since the cap looks like a very low impedance at HF. This is where the much better ripple rejection shown in the 317/338 datasheets comes from. So, big hint: make Z of C4 much smaller than R2 at twice your AC Mains frequency; 100uF can be better than 10uF.

Better yet, if you can replace R2 with somethig like a zener diode*, you'll get lower noise all the way down to DC - because the dynamic impedance of the zener will much lower than the required value of R2 (just 1-5ohms insteead of 1-2Kohms, say). So it mimics the action of a truly enormous cap at C4 without the potential for damage, or cost. Use a zener in series with a normal diode (anode to 0v), and you get nearly-perfect temperature compensation for free, too


*Output voltage will of course be (Vz+1.25)v
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Old 14th September 2006, 08:29 PM   #32
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Default great report ...

" ... Better yet, if you can replace R2 with somethig like a zener diode ... Use a zener in series with a normal diode (anode to 0v), and you get nearly-perfect temperature compensation for free, too ..."

This is also a neat trick to get closer to a particular desired Zener diode voltage ... each series "regular" silicon diode adds 0.6 Volts to a Zener's value ... so a 3.8 Volt Zener in series with two silicon diodes = 5.0 Volts with (as you say) thermal stabization ! ... and a well regulated 5.0 it will be.

(a trick on cheap solar cell power supplies to fix regulated output at 10.2 V. for a nine volt metal halyde battery. ... 9.0 volt Zener plus two diodes = no overcharging. On 12 volt lead acid batteries = 12 v. Zener + two diodes = 13.2 = no boiling of the battery.)

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