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What makes choke input chokes special?

Depanatoru

Member
2006-07-06 10:27 am
And I said there is no need for anything as the voltage across the choke is not reversing to negative ... maybe some people should refresh their knowledge about simple filters . This is not SMPS switching .
 
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Really? Remember that in the tube heydays LC filtering was common especially with mercury filled rectifiers (80, 866 etc.). I've never seen an additional grounded diode there to »discharge« the choke.
There is no off-cycle in a full wave rectifier. Current comes from one phase or the other. But some circuits still use a ground diode to cover the dead time.
 
Yes, that is what I used at that time, a bridge, and that gave me more hum. [In my current 300B I have a valve after the bridge, two halves parallel. Works great, very soft commutation.]
For a choke input filter, having a valve diode between the bridge and the choke does not change the hard commutation of the bridge diodes - the valve diode is effectively just adding to the series resistance of the choke.
There is no off-cycle in a full wave rectifier. Current comes from one phase or the other. But some circuits still use a ground diode to cover the dead time.
I don't recall seeing any choke input filter schematics in amps showing a 'ground diode' - can you link to a schematic or amp ?
 
Mostly they are designed to not buzz so much when used as an input choke.

They may be designed as swinging chokes where they will have more inductance with a light load and have less inductance with a heavy load.
The point of the swinging choke is to just satisfy the minimum inductance number that is required for light loading.
Hammond had several swinging chokes in their product line at one time.
They would have been used in the PS for Class B amps.
 
Chokes for such duty need to have flux density headroom for the accounted AC voltage swing across. They will also radiate more EMF.

Due to the AC voltage they will as well vibrate more.

Swinging choke is one that saturates easily from Idc. Here is what happens.

1. Idc increases due to load requirements.
2. Choke flux density increases, reaches BH knee, chokes saturates.
3. Permeability decreases
4. Inductance decreases.
5. But because L vs I decreases linearly and critical current is linear, the power supply remains in choke input. Swinging chokes are advantageous for, for example, class AB amplifiers. I'm using an in-house swinging choke input for my LM1875 amplifier.