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@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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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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