[Amps] Resonant choke filter

[Will Matney]

Rich,
I'm running a regulator after the swinging choke for the bench supply. I 
was wanting to do this to try and clean up the ripple before the 
regulator had to handle it (plus I have a surplus swinging choke). If I 
hadn't wanted the regulator, I would have tried a straight resonant 
choke. I would think that the regulator would hold the choke output 
current some steadier  unless I'm not thinking correctly. Also, by what 
I've read, the chokes made today using interleaved lams still act 
somewhat like a resonant choke with a gap but the swing is not as large. 
I seen a couple of remarks that they could not saturate easily, but I 
think they can. You are correct though that without a constant load, the 
circuit can produce transients and spikes especially if the bleeder 
resistor opens or is not sized correctly. The circuit I'm using is a 
standard choke input with capacitance across the choke to resonate at 
120Hz. There is no capacitor to ground before the choke (capacitive 
input) which is very bad to produce high voltage spikes. Let me know if 
you have any ideas on this before I fire up my iron =)

Will

R. Measures wrote:

>
> On Aug 2, 2004, at 10:54 AM, Will Matney wrote:
>
>> Peter,
>> With looking through the books texts I have here, That is the way you 
>> design this filter using a swinging choke. With mine, I needed 400 
>> Vdc @ 80mA maximum. So Lcrit = E / Imax which is 400 / 80 = 5 Henrys. 
>> Now the choke is a 5-20 henry swinging choke. So to get the minimum 
>> current for the bleeder resistor to have the 20 Henry portion we take 
>> 400 / 20mA = 20 Henrys or 20H x 20mA = 400V. So for 20 mA at 500 
>> Volts, our bleeder resistor would be 400 / .02 = 20K ohms. Then to 
>> size the wattage, 400 x .02 = 8 watts. A 10 or 15 watt resistor would 
>> work here. As long as Lcrit is met, the swinging chokes inductance 
>> will swing and level out with the DC load without saturating.
>
>
> Will --  Swinging filter chokes work well with constant current loads 
> such as RTTY, FM, and AM linear amplifier service.  However, with a 
> varying current load, such as in a SSB linear amplifier, they do not 
> provide satisfactory transient regulation.  The only types of filter 
> that do are the capacitor-type and the resonant-choke-type.  Those who 
> doubt this should do a bench test with a DC oscilloscope since a meter 
> can not follow the transients.  Step up or down the load current and 
> the resultant V-transients are no less than amazing.  This is the 
> reason why Harris, Collins, Henry, Hughes, and other manufacturers of 
> SSB amplifiers use resonant-choke filters.
> end
>
>> If Lcrit is below this value, it will try to run as a capacitive 
>> input filter. Actually, I will be adding a page on filters to the 
>> transformer tutorial when I get to feeling like it as promised. I 
>> have some done, but nothing ready yet to publish it. Last, Tony, 
>> I0JX, has a page on this at:  http://www.qsl.net/i0jx/supply.html
>>
>> Will Matney
>>
>> peter.chadwick at Zarlink.Com wrote:
>>
>>>
>>>
>>> Is actual resonance what you want? The argument I've seen runs as 
>>> follows;
>>>
>>> In a choke input supply, there's a critical minimum value of 
>>> inductance for
>>> regulation. At low currents, this is very high, but if the choke is 
>>> tuned
>>> slightly HF of the ripple frequency, it looks like a very much bigger
>>> inductance. At resonance, it will look like a resistor, of course. 
>>> As the
>>> load current increases, the critical value of inductance decreases, 
>>> and the
>>> choke will start to lose inductance anyway.
>>>
>>>> From this, the choke needs to be tuned a tad HF at minimum load.  This
>>>
>>> certainly ahs worked for me. Incidentally, by using negative lead
>>> filtering, you can rectify the ripple across the choke for a low 
>>> current
>>> negative bias supply.....
>>>
>>> I tend to go for negative lead filtering anyway to ease the choke
>>> insulation requirements.
>>>
>>> 73
>>>
>>> Peter G3RZP
>>>
>>>
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>>
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>>
> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
>
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