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Capacitors - Dissipation Factor

Technical Bulletin No. 06

Whenever power (energy) in the form of voltage times current is applied to a capacitor, part of that total power is used or "lost" within the capacitor itself. The ratio of this "power loss" to the total power supplied is the "power factor" (PF) of the capacitor. This PF figure then is a measurement factor for rating the "inefficiency" of the power transfer capabilites of the capacitor.

For those capacitors where the PF figure is .1 ( 10%) or less, a ratio figure known as the "dissipation factor" (DF) is more commonly used. The reason for this usage of the DF figure is simply a conveni­ence that takes advantage of the fact that DF mea­surements on a capacitor are much simpler and easier to make on standard capacitance bridges than the determination of PF.

The relationships between PF and DF, and the fac­tors that are concerned in these figures are deline­ated in the following: (AC voltage applied)



And we see that the "dissipation factor" (OF) is the ratio of the series resistance to the reactance.

In the case of a capacitor. particularly in the low-frequency range (30K Hz and below). the XL term is extremely small compared to XC and can be ignor­ed for computation purposes. This is best illustrated by the following typical example of a metal­lized mylar dielectric capacitor:

Figure 1
Figure 1
Figure 1
Figure 1
Figure 2
Figure 2
Figure 2
Figure 2


And we see that the OF figure will vary with fre­quency, capacitance. and series resistance. In ad­dition, the DF figure will also vary with whatever environmental conditions cause C and R to change such as temperature, moisture. pressure, etc. Fig­ures 1 and 2 compare the DF vs. Temperature characteristics of some of the commonly used ca­pacitor dielectrics.These plots are average curves and should not be construed as specific or absolute values since special additives. fillers and special procedures can change the curves considerably for individual cases.

Mica and glass dielectric capacitors generally have OF values between .03 to 1 .0% OF over the full temperature range.

Ceramic dielectric units can be very stable or ex­tremely erratic depending on the dielectric con­stant (K) value of the ceramic mixture. NPO type units (low K values) will generally measure between .1 to .5% OF at room temperature, while the General Purpose type (high K values) generally read between 1.0 to 2.5% OF at room temperature. Most electrolytic type capacitors have PF values that exceed 10% and therefore the relation OF= PF is not valid. An exception to this could be the solid type tantalum line that will hold between 3.0 to 6.0% PF over the temperature range of -55°C to +85 °C. The accurate determination of the OF vs Frequency characteristic of capacitors, particularly at the upper frequency range, is highly influ­enced by testing equipment and procedures.

Except in a very few special cases where even a few ohms of series resistance becomes critical (ex­treme cases or discharge times). the value of the OF is of no real importance in the operation of an essentially DC circuit {for example; pure DC or a small AC ripple superimposed on a polarizing DC voltage).

The manufacturer can and does use the DF mea­surement as a quaility tool. Variations in OF above normal values for a particular line or lot of units would indicate possible loss of control on materials
or manufacturing procedures. By monitoring the OF measurements, possible troubles in the pro­duction line can be discovered quickly and correct· ive action instituted before the trouble reaches catastrophic proportions.

For the user who is faced with essentially AC or high frequency pulsing DC applications. the value 1 of the OF is of prime concern since the series resistance factor in OF is the heat producing element in these applications.

 
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