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Min. current draw for choke input?

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Klimon

Klimon

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Joined 2005
#1
Hello,

I have designed a choke input power supply for the driver stage of a little se amp. That's mainly because I have a second ht winding and choke input would come in handy to drop the voltage + gives a very fast ps (much faster than just 'hanging' it behind the output stage ps with a many K resistor and cap)... I am worrying about the minimum current draw however. There doesn't seem to be a definitive formula (one textbook says a rule of thumb is Imin = V / H) - the driver stage draws approx. 3mA; so with 240v and 60H we'll get around 4mA Imin... Not quite enough...:xeye:

Is there hope left for this configuration / did I forget something?
Is a choke input not suited for small current applications as fleapower set driverstages? 😕

Thanks!🙂
 
#2
The quick answer is: NO.

A formula I always used is from Schade, given in "Radio Designer's Handbook" by F. Langford-Smith, chapter 30.3, equation (1). That states that for full-wave amplification and low current:

L should be equal to/greater than RL/6.pi.f

This translates to L = RL/940 for a 50 Hz supply (or L greater than...). [RL = total load on power supply]

Your rule-of-thumb is thus quite close. In your case RL appears to be 80K, which would indicate a minimum L of 85H. Thus an LC filter is never really economical for low current applications and hardly practical, where the disadvantages of a C-input filter (e.g. high peak charging current etc.) is minimal, and large serie resistors are practical.

How do you define a "fast" ps? (There is not much difference, for the same ripple, between an LC and a CLC filter, or for that matter a CRC, unless you mean something not in evidence here.)
 
#4
Just a thought: You could burn a few mills of current in a suitable resistor or low wattage light bulbs.
As long as the power transformer is capable of providing the increased current, its a valid design option.

HTH

Doug
 
#6
use a bleeder

Hallo Simon,
Yes, the solution has allready been given. Just ad a bleeder that will '' eat '' some extra current that is needed to make the power supply work as a real choke input. If the transformer can deliver the extra current it is not a problem to let the bleeder draw the same current as the pre-amp itself. De groeten, Eduard
 
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