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!
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!
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.)
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.)
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
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
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/rectifiers/rectifier applications.pdf
Grab it while you can. It unfortunately seems to have disappeared from the OnSemi web site, and is an indespensible reference.
http://iweil.com/electronics/rectifiers/rectifier applications.pdf
Grab it while you can. It unfortunately seems to have disappeared from the OnSemi web site, and is an indespensible reference.
I find the free PSU Designer program from:
http://www.duncanamps.com/software.html
really useful for this kind of thing.
http://www.duncanamps.com/software.html
really useful for this kind of thing.
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