VDR Silicon Carbide Varistors

Electrical Design Parameters

Selection Criteria

In the data tables provided for the different series, the varistors are grouped according to types which differ in disc diameter, energy absorption and power dissipation.

The selection of a varistor for a particular use is based on energy absorption. The mean value of the permissible absorption energy is specified for each series. If inductive loads have to be switched, the energy accumulated in the magnetic field of the inductance is calculated thus:

E =½ . L . IL2

In this case IL is the current arising from the inductance. A series must now be selected with the permissible absorption energy being higher than the value calculated.

Now check if the permissible power dissipation of this series is not exceeded.

Power dissipation PL is calculated from energy pulses E at intervals in time t:

PL = E / t

If the specified value is exceeded, proceed to the next larger series.

The selection of one type from a series is determined by the operating voltage. The maximum voltage specified must not be exceeded.

You can find the voltage at the varistor for a particular pulsed current within the curve of the relevant type of varistor.

Operating Voltage

The data tables shown for the different series specify the operating AC or DC voltage for each varistor type. The maximium operating voltage, which is also specified, is approx. 10 % higher. It must not be exceeded. These values apply to AC and DC voltage with a ripple factor of less than 5%. If the ripple factor is higher, calculation is based on peak voltage and not on the mean value.

The AC voltage data refers to the effective value. At ambient temperatures of over +70°C operating voltage must be reduced accordingly. Please refer to the thermal data section.

Power Dissipation

The maximum value for power dissipation applies to the operating temperature range specified (see thermal data section). Power dissipation must be reduced for higher temperatures.

Power dissipation is composed of two parts:

Pd is the power produced by leakage current, under existing operating voltage.

Pi is the power which arises through absorbed overvoltage pulses. In case of energy pulses E at intervals in time t, Pi mean power dissipation is therefore:

Pi = E / t

The following is therefore valid:

P = Pi + Pd = U . I + E / t

The temperature of the varistor element must not exceed particular limits. If power dissipation is constant the varistor surface and ambient air temperature determine heat abstraction and thus the temperature of the varistor element. At high ambient temperatures only a lower dissipation is therefore permissible. Please refer to the thermal data section.

Measuring Voltage and Measuring Current

Measuring voltage is used to characterise the type of varistor. It is normally measured with constant DC. Standard tolerance is ±20 %. Other tolerances can however be supplied on request. Measuring voltage and measuring current are specified in the data tables provided.

Unlike the former regulations a few types are measured with fixed measuring voltage. AC measuring is also possible. Alternating currents are measured with usual AC measuring instruments. The mean value of the rectified voltage is measured. The relevant effective value of a sinusoid current is indicated. If an effective value measuring instrument is used deviations occur because of the distorted curve of the current.

B-Value

The so-called B-value is part of the varistor designation and denotes the approx. voltage at the varistor at a current rate of 1 A. It is proportional to the thickness of the varistor disc and proportional to the disc surface.

Exponent

Similar to the B-value, the exponent is also a part of the varistor designation. While the B-value determines the position of the varistor curve in the U / I diagram, the exponent determines the slope: for silicon carbide varistors n lies between 2 and 7. High n values are achieved by varistors for high voltages, as shown in the data tables.