Min. current draw for choke input?

[Klimon]

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...

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!

[Johan Potgieter]

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.)

[EC8010]

Originally posted by Klimon
Is a choke input not suited for small current applications as fleapower set driverstages?

I'm afraid you've pretty well summed it up there. Choke input is better for higher current demands.

[DougL]

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

[Klimon]

Thanks for the replies!

@Johan: I call the ps 'fast' because it reaches the B+ voltage very fast (= around 0,2 milliseconds versus maybe 0,4 for a 'slower' ps)...

Simon

[eduard]

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

[Klimon]

Hallo Eduard,

Thanks for confirming that solution; I'll check it out - could be ideal

Zuidelijke groeten;

S.

[eduard]

Hallo Simon,
Soon i will be ofline longtime but i will surely like to know about the results when i am back ( 13 January) will leave 23 november. De groeten, Eduard

[Charles Hansen]

There is a book that clearly explains the answers to all of your questions regarding choke-input power supplies. Luckily, it is available online for free:

http://iweil.com/electronics/rectifi...plications.pdf

Grab it while you can. It unfortunately seems to have disappeared from the OnSemi web site, and is an indespensible reference.

[martin clark]

Charles - nice reference, thanks.

Martin
www.acoustica.org.uk