>
>A thread which has been going on for a few days now deserves some added
>points.
>
>The original question was whether some caps advertised for sale were
>suitable for high
>voltage power supply duty in amplifiers. Whilst the topic may be "old
>hat" to most I
>feel it necessary to make a few comments in case you are not aware of some
>of the quirks
>of high voltage electrolytics, and especially for the benefit of some less
>experienced
>hams.
>
>The Nichicon LLK2W271MHBS and LGQ2W471MHSC types were among the capacitors
>discussed.
>Fortunately I have old Nichicon catalogs so I could look up the
>parameters. By old I
>mean going back to 1996.
>
>The LLK series is the LK type in the catalog, (discard the first L to arrive
>at the part type in the catalog) and is not listed in the 1998 or 1999
>catalog and, after
>checking with the local agent, I can tell you it is now obsolete. The
>(L)GQ series is
>still current and probably will remain so for some time.
>
>Merit W6NQ in a post erroneously informed us that the (L)LK series is
>rated at 105C, in
>fact it is a 85C rated part. The (L)LQ series is also only rated at 85C,
>however the
>(L)GQ
>series is a 105C part and here is where it gets interesting!!
>
>If you examine the ripple current rating of these 3 types you will notice
>that the 105C
>part has a substantially LOWER rating than the 85C parts!!
>
>Why is this? Well it is fairly simple to explain. All these caps for the
>450V rating
>have a case diametre of 35mm and vary from 40 to 50mm in height. They are
>all pc mount
>with snap-in lugs.
>
>Ripple current rating is a function of the esr of the capacitor and esr
>increases
>(generally in a linear fashion) with increasing temperature, like a
>resistor with a
>positive temperature coefficient. (At very low temperatures - below -20C
>the esr also
> increases, but this is due to a different mechanism, which is the
>electrolyte freezing).
>
>If you take the 3 capacitor types and examine the ripple current rating
>you will see that
>
>for 120 Hz ripple:
>
>LQ series 85C 270uF @ 450V Ir = 1,80A (maximum value is 470uF @
>450V)
>LK series 85C 270uF @ 450V Ir = 1,61A (maximum value available
>is/was
>390uF)
>GQ series 105C 270uF @ 450V Ir = 1,19A (maximum value is 470uF @
>450V)
>
>Taking just the LQ and the GQ series for 470uF @ 450V the data is:
>
>LQ Ir = 2,47A @ 85C
>GQ Ir = 1,74A @ 105C
>
>Which is a difference of about 30%.
>
>The ability of an electrolytic to sustain ripple current is primarily the
>external
>surface area available to dissipate the heat. Both these caps are
>identical size (35mm x
>50mm) so have exactly the same surface area. But the GQ is rated at a
>case temperature
>of 105C.
>
>I think that the trade off is in the expected temperature the case will
>achieve
>in a practical design. The bottom line is this - if you require high ripple
>current rating and high temperature operation then you need to increase
>the capacitor
>value by some 30 to 50% to arrive at the same ripple current capability of
>a 85C rated
>part.
>
>Hope this of some interest as I feel that spending $$ un-necessarily on
>105C rated
>capacitors to end up with less ripple current capabilty is a pointless fact.
>
? However, comparing a ripple-current rating for 105 deg C with a ripple
current rating for 85 degrees C is hardly valid. My guess is that 105
deg C capacitors are going to last longer in a typical anode supply.
- later, John
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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