Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

I thought it would be useful to measure the performance of the two MOV types I've used:

The ROV14-180M is 14 mm diameter, about the same diameter as a dime. The V18ZA40P is 20mm diameter, slightly smaller than a quarter, as seen below.

One more ratings point—note the maximum power rating in joules (J). One joule is one watt--second, so in theory the ROV14-180M will dissipate 4.7 watts for one second and the V18ZA40P 80 watts for one second. More usefully for clamping short duration transients, the main purpose of the MOV, during a single 100 microsecond period these two devices are theoretically capable of dissipating 47KW and 800KW respectively.  In practice, other limitations may prevent these figures from being  realized, but the concept is clear—the MOV's are capable of clamping very large voltage spikes, provided they are of sufficiently short duration. The continuous power ratings are much more modest, of course, 100 mw for the ROV18-180M and 1 watt for the V18ZA40P.

This ability can be contrasted with  the much lower clamping ability of, for example, a Zener diode rated at the same continuous power as shown in the figure below, extracted from ON Semiconductor Application Note AN-784, http://www.onsemi.com/pub_link/Collateral/AN784-D.PDF .

The reason for this difference can be seen in the photograph of the two MOVs; note the leads are attached to a bulk chunk of material, which is then given an insulating epoxy coating. Unlike the Zener diode, the MOV has no small semiconductor junction with tiny  wires joining the die to the diode's leads. Hence, there is sufficient thermal mass to handle short duration overloads thousands of times greater than the continuous duty rating.

Before collecting data in the X-Y mode, I looked at the voltage and current as a function of time, with an ROV14-180M MOV. Channel 1 (black) shows the voltage across the MOV and Channel 2 (red) the current through the MOV. Channel 2 is shown as 500mVΩ. This is Tektronix's way of saying mA when a current probe is used. Hence, Channel 2's scale is 500 mA/division.

Several things can be seen in this oscilloscope image. First, the MOV clamps the positive going waveform at 26.8V, and the maximum current, disregarding the lead in spike, is around 800 mA. The negative voltage peak is -26V and the MOV is driven into conduction, with a peak current of 1.1 ampere. However, there's sufficient asymmetry in the BOP 100-1M current limiting to cause the voltage and current waveforms for positive and negative excursions to appear quite different. To verify this asymmetry as being in the BOP 100-1M's clipping limits and not the MOV, I reversed the MOV in the test circuit and found no perceptible change in the current or voltage waveforms.

The transition from non-conducting to conducting is reasonably sharp, and symmetrical. Low resistance mode starts to be seen at ±20 V and it's in full effect by ±22 or 23 V. The BOP 100-1M's maximum current limit peaks at +1.4A and -1.25A, with both likely being short spikes before the BOP 100-1M's current limiting circuit can take effect. Consequently, data for currents exceeding ±1A should be regarded with some caution.
 

The V18ZA40P also shows a steeper V-I curve in the high conduction range, indicating lower dynamic resistance than found for the ROV14-180M. This is not unexpected, given the relative power difference between the two parts.