VDR OCELIT® High Performance Silicon Carbide Varistors

Series 820 SB for High Performance

Calculation of Varistor Data for Type 820 SB Assemblies

For applications with high pulse energy, the 820 SB series with energy absorption of 1.9 kJ is most suitable.

If necessary the discs can be connected in series or parallel.

There are no fixed types for the 820 SB series; a suitable disc type is determined for each assembly.

Our Applications Engineers would normally deal with assembly layout, however the procedure will be explained here, first for DC voltage, then for AC voltage.

 

Varistor Connected to DC Voltage

Similar to varistors with lower performance, calculation is based on pulse energy. The overvoltage is usually caused by inductance. Calculation is as follows:

E = ½. L . IL2

IL is the current caused by inductance. If E is less than 1.9 kJ, in the case of higher energy, several discs must be paralleled. In parallel assemblies the pulse energy is never distributed regularly between the individual discs. Therefore reduced mean energy absorption of 1.5 kJ per disc and reduced power dissipation of 15 W instead of 19 W must be expected.

The second aspect is power dissipation. Power dissipation Pi arises from energy pulses E at intervals t.

Pi = E / t

Total power dissipation P of one disc must not exceed 19 W or 15 W. This is composed of two parts:

- the Pi value calculated above of the pulse load

- the continuous power dissipation value Pd caused by the existing operating voltage.

Normally 30% to 50% is used for continuous power dissipation Pd and the rest for the pulse load Pi.

The data of the individual disc are calculated from the continuous power dissipation Pd as follows:

Imax = Pd / Umax

Umax is the max. operating voltage (the tolerance must be considered). Imax is the maximum current through the disc. By specifying Umax and Imax the data for the disc is now determined.

A tolerance range of – 50% is normally permissible for I.

Imin = ½ .Imax

When ordering, the following data must be given, for example:

Single disc 820 SB, (200 V=, 20...40 mA)

If the voltage lies above that of the obtainable range specified in the U / I diagram it is possible to assemble discs in series.

The effectiveness of the assembly can be determined with the aid of the U / I diagram. First plot the values of the varistor disc, (e.g. 200V=, 20...40mA) and trace a line parallel to the given curves through the "high ohmic" final point of the tolerance range (200V, 20mA). This line shows the maximum possible voltage with specified current ("worst case").

In the case of overvoltage, IL current flows through the varistor. If several discs are paralleled, IL must be divided by the number of discs. This current is plotted in the diagram and the maximum limiting voltage can be seen from the previously plotted curve.

If complete columns are ordered, the type of assembly must be specified. For large combinations with series and paralleled connections, it is usually better to combine the parallel varistor discs with one column and to assemble entire columns in series. The voltage in one column is then lower thus reducing the danger of sparkovers in unfavourable ambient conditions such as dust, dampness etc.

Varistor Connected to AC Voltage

In principle, the calculation of varistor assembly for connection to AC voltage is the same as for DC voltage. Only the differences will now be explained.

Pulse energy is calculated from the maximum energy at the peak alternating current.

E = ½ L.îL² = L.ILeff²

ILeff is the effective value, îLthe peak value of the load current IL.

The varistor discs can be measured with AC voltage. When measuring the current through the varistor, it must be noted that this is not sinusoidal. A normal amperemeter reacts to the mean value of rectified AC and is calibrated for sinusoidal currents in effective values. When such an instrumentis used, a measuring error therefore occurs. The real power dissipation is higher. When calculating the current for a single disc, an additional factor Kp must be considered. For the exponents 2 to 8 of OCELIT varistors, Kp lies between 1.08 and 1.2 Max. value of Kp is placed at 1.2. The following formula is applicable:

Pd = Umax.Imax.Kp  ~  Umax.Imax.1.2

Imax = Pd  /  (Umax . Kp)  ~  Pd  /  (Umax . 1.2)

Umax is the maximum value of operating voltage (effective value). A tolerance of –50 % is again permitted for I. An order could thus be designated as follows:

Single disc 820 SB (240 V~, 15...30 mA)

To check the effectiveness of the assembly, the U / I diagram is required again. However only AC voltage values are given for the disc, therefore that conversion to DC values is necessary.

Calculation is based on the measured values, in the above example 240 V~/15mA. In this case current value at the lower tolerance limit is of interest to calculate the max. limiting voltage which could appear. It must be considered that this current specification is not an effective value.

Now it should be determined what current flows when the varistor is connected to DC voltage U= within the following specification:

U= = U~

In the above example U== 240 V=

Thus DC current is I= Ki.I~

Ki factor is explained in the following section.

Ki Factor

Ki is a factor depending on the exponent to be gathered from the Ki-factor diagram. Since the exponent is at first unknown, calculation is based on a mean value of e.g. 5.

The correctness of this assumption is later checked within the diagram.

The exponent for every characteristic curve is shown in the U / I diagram. If necessary a subsequent calculation must be made with the corrected values.

 

 

Thus a characteristic point (U=, I=) has been established. For the above example the pair of values 240 V~ / 7 mA with an exponent of approx. 5 is calculated from 240 V= / 15 mA.

The required curve in the U / I diagram is obtained by parallel displacement of the neighbour curves at this characteristic point. The limiting voltage at a given current can now be read in the diagram.