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Re: [TCML] Terry Filter Resistors
Bart -
All "standard" tubular ceramic core, wire-wound power resistors are constructed with a single layer of bare (uninsulated) resistance wire. Hit one of your Ohmite "Terry filter" resistors with a hammer and you'll see what I mean.
Regards,
Herr Zapp
--- On Thu, 1/29/09, bartb <bartb@xxxxxxxxxxxxxxxx> wrote:
From: bartb <bartb@xxxxxxxxxxxxxxxx>
Subject: Re: [TCML] Terry Filter Resistors
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Date: Thursday, January 29, 2009, 7:27 PM
Hi Gary,
The resistor I'm measuring is the resistor that Terry Fritz called out
on the Terry filter (L100J1K0). It's 0.75"D x 6"L, 1000 ohms,
100W, 1000V.
Your right, it is a good sanity check, but remember your coil is a
single layer air core type. The answer is layers. For proof, I ran
multi-layer air core coil equations to find what wire size I would need,
turns, layers, etc. at the .75" x 6" dimensions in order to achieve
1000
ohms with 62.7mH.
If I use 39 AWG, 1460 turns with 4.23 close-wound layers at the
dimensions above, then I get 1000 ohms at 69mH. I used simple magnet
wire for these calculations.
The larger 145mH direct measurement value is possible with more layers,
but it requires a slightly larger wire size near 37 AWG. If I use 37
AWG, 950 turns, 10.33 layers, then 1000 ohms at 132mH.
Anyway, you get the point. I basically backward engineered the resistor
to see how easy it would be to achieve both the measured and the
calculated values. I would need to run a resonance test to be sure which
is right. I expect the lower 62.7mH value is close.
Layers make all this possible and is the "only" way it could be done.
If
it was a single layer, then "extremely" low inductances.
Take care,
Bart
Lau, Gary wrote:
> What are the dimensions of these resistors? My 6"x28" secondary
coil's inductance is only 89mH, so 145mH seems huge. Granted, the resistor
is wound with much finer wire with more turns, but it'll make an interesting
sanity check to see how many turns a secondary wound to the same dimensions as
your resistor would require to achieve 145 mH.
>
> Regards, Gary Lau
> MA, USA
>
>
>> -----Original Message-----
>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>> Behalf Of bartb
>> Sent: Wednesday, January 28, 2009 9:17 PM
>> To: Tesla Coil Mailing List
>> Subject: Re: [TCML] Terry Filter Resistors
>>
>> Hey Gary, Matt,
>>
>> Confirmed today at 145mH on todays reading (leads). Also measured 992
>> ohms. Measurements with 27XT.
>>
>> I think Matt is correct and it's logical. If I do the math (noting
that
>> the 27XT frequency is 1kHz):
>>
>> Z = 145mH x 6.28 x 1kHz = 910.6 ohms.
>>
>> XL = sqrt(992^2 - 910.6^2) = 394 ohms (note R-Z due to -sqrt).
>>
>> L = 396 / (6.28 x 1kHz) = 62.7mH
>>
>> Still significant, but then these are rather large wire wound
resistors,
>> so not overly surprising.
>>
>> Regards,
>> Bart
>>
>>
>> Gary wrote:
>>
>>> Please confirm "I just checked the resistor in question and
it measured 146mH at
>>>
>> 1kHz." 146 mH sounds about a million times too high for the
inductance of a
>> wirewound resistor.
>>
>>
>>> Matt wrote:
>>> I think part of the problem is that most LC meters don't
measure Inductance
>>>
>> directly. They actually measure the Z of the device and assume Z=XL,
and
>> therefore L=Z/2pif, which for most inductors is close enough. For a
wire-wound
>> resistor though, this is not a good approximation, since actually
Z=sqrt(R2+XL2),
>> and R is no longer negligible. You could use the L?shown by the meter
to calculate
>> what Z it actually read by Z=2pifL, then using the known resistance,
calculate a
>> corrected XL? from XL=sqrt(Z2-R2)and from that, determine the
corrected L by
>> L=XL/2pif.
>>
>>> Matt D.
>>>
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>>
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>
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