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G4EBT > TECH 16.11.05 11:27l 81 Lines 2961 Bytes #999 (0) @ WW
BID : EC1217G4EBT
Read: GUEST DL1LCA OE7FMI
Subj: Re Capacitors (COG/NPO?)
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<7M3TJZ<UA6ADV<ON0AR<GB7FCR
Sent: 051116/1025Z @:GB7FCR.#16.GBR.EU #:12427 [Blackpool] FBB-7.03a $:EC1217G4
From: G4EBT@GB7FCR.#16.GBR.EU
To : TECH@WW
Clyde, ZS1CS wrote:-
> I am busy trying to convert a Norcal 20 to a Norcal 40 the conversion
> calls for a variety of COG and NPO capacitors.
>My query is! What is the difference???
There isn't any difference - they're one and the same thing.
>Is it voltage or stability or WHAT???
No - see below:
Some basic facts:
The dielectric of ceramic capacitors is (not surrpisngly) made of ceramic
materials. Dielectrics are the insulating material between the plates of
ceramic capacitors, and this material is chosen for its ability to permit
electrostatic attraction and repulsion to take place across it.
Ceramics offer material will have the property that energy required to
establish an electric field is recoverable in whole or in part, as
electric energy. In other words, good dielectric materials such as
ceramics, are poor conductors of electricity while being effective
supporters of electrostatic fields.
There are a number of ceramic materials and compositions that are used in
ceramic capacitors. These include "common" ceramic, ceramic COG (NPO),
ceramic X7R and ceramic Z5U.
Common ceramic capacitors comprise a solid body of high-temperature,
ceramic resistive material with bonded metal contacts. Ceramic COG (NPO)
is a high Q, low K temperature compensating type of dielectric with stable
electrical properties under varying voltage, temperature, frequency and
time.
It is suitable for circuits that require low loss, as well as timing and
tuning applications. The unofficial, colloquial temperature coefficient
designations for the capacitors are as follows:
The temperature coefficient is given as "P" for positive, "N" for
negative, followed by a 3-digit value of the temperature coefficient in
ppm/degree C. For example "N220", is -200 ppm/degree C, and "P100" is +100
ppm/degree C.
The one exception in this system is "NPO" where an "O" instead of "0" is
used, but quite a number of people use "NP0." In any event "NPO" ceramic
capacitors are stable with temperature.
Ceramic X7R (BX) has moderate K value and is temperature stable. It
shows moderate change in electrical properties under changing temperature,
voltage, and frequency. It is suitable for bypassing, coupling, and
frequency discrimination circuits applications.
X7R ceramic capacitors show no more than a 15% capacitance deviation over
the range -55-+125 degree C. Ceramic Z5U is a Class III dielectric that
exhibits a maximum capacitance change of +22 % - 56% over an operating
temperature range of +10 degree C to + 85 degree C in ceramic capacitors.
Hope that helps and reassures.
Best of luck with your Norcal rig!
Take care of yourself Clyde, QRP homebrewers are an endangered species:-)
73 - David, G4EBT @ GB7FCR
Eddystone User Group Member
G-QRP Club Member No: 1339
QTH: Cottingham, East Yorkshire.
Message timed: 09:53 on 2005-Nov-16
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