In most cases the designer can select a Metal-Oxide Varistor (MOV) that meets the desired voltage ratings from standard catalog models. Occasionally the standard catalog models do not fit the requirements either due to voltage ratings or energy/current ratings. When this happens, two options are available: varistors can be arranged in series or parallel to make up the desired ratings, or the factory can be asked to produce a “special” to meet the unique application requirement.
Guidelines for series and parallel operation of varistors
Series Operation of Varistors
Varistors are applied in series for one of two reasons:
- Provide voltage ratings in excess of those available
- Provide a voltage rating between the standard model voltages
As a side benefit, higher energy ratings can be achieved with series connected varistors over an equivalent single device. For instance, assume the application calls for a lead mounted varistor with an VRMS rating of 375VAC and having a ITM peak current capability of 6000A. The ITM requirement fixes the varistor size. Examining the LA series voltage ratings near 375VAC, only 320V and 420V units are available. The 320V is too low and the 420V unit (V420LA40B) results in too high a clamp voltage (VC of 1060V at 100A).
For a V130LA20B and a V250LA40B in series, the maximum rated voltage is now the sum of the voltages, or 380V.The clamping voltage, VC, is now the sum of the individual varistor clamping volt ages, or 945V at 100A.The peak current capability is still 6500A, but the energy rating is now the sum of the individual energy ratings, or 200J. In summary, varistors can be connected in series providing they have identical peak current ratings (ITM), i.e., same disc diameter. The composite V-I characteristic, energy rating, and maximum clamp voltages are all determined by summing the respective characteristics and/or ratings of the individual varistors.
Parallel Operation of Varistors
Application requirements may necessitate higher peak currents and energy dissipation than the high energy series of varistors can supply individually. When this occurs, the logical alternative is to examine the possibility of paralleling varistors.
Fortunately, all Littelfuse Varistors have a property at high current levels that makes paralleling feasible. This property is the varistor’s series-resistance that is prominent during the “up-turn region” of the V-I characteristic. This up-turn is due to the inherent linear resistance component of the varistor characteristic. It acts as a series balancing, or ballasting, impedance to force a degree of sharing that is not possible at lower current levels.
At a clamp voltage of 600V, the difference in current between a maximum specified sample unit and a hypothetical 20% lower bound sample would be more than 20 to 1. Thus, there is almost no current sharing and only a single varistor carries the current. Of course, at low current levels in the range of 10A -100A, this may well be acceptable. At high current levels exceeding 1000A, the up-turn region is reached, and current sharing improves markedly.
For instance, at a clamp voltage of 900V, the respective varistor currents are 2500A and 6000A, respectively. While far from ideal sharing, this illustration shows the feasibility of paralleling to achieve higher currents and energy than achievable with a single model varistor. Practically, varistors must be matched by means of high current pulse tests to make parallel operation feasible.
Pulse testing should be in the range of over 1kA, using an 8/20µs, or similar pulse. Peak volt ages must he read and recorded. High current characteristics could then be extrapolated in the range of 100A - 10,000A. This is done by using the measured data points to plot curves parallel to the data sheet curves.
With this technique current sharing can be considerable improved from the near worst-case conditions of the hypothetical example given below:
Tech Tip: Varistors can be paralleled, but good current sharing is only possible if the devices are matched over the total range of the voltage-current characteristic. In applications requiring paralleling, DigiKey should be consulted.
Related Topics
Source - Littelfuse: Varistor Design Examples
Metal Oxide Varistor (MOV) Specifications & Selection Guide
Click here for DigiKey Varistors, MOVs Selection
Applicable Part Numbers
F3035-ND
V250LA40BPX2855-ND
V250LA40BPX10-ND
V250LA40B-ND
V250LA40BP
V250LA40BPX2855
V250LA40BPX10
V250LA40B
F3031-ND
2156-V130LA20B-ND
V130LA20BPX10-ND
V130LA20BPX1347-ND
V130LA20BPX2855-ND
V130LA20BP
V130LA20B
V130LA20BPX10
V130LA20BPX1347
V130LA20BPX2855