> >George: Not to put too fine a point on it but ZNRs are large and slow moving >so they do have mechanical aspects but more to the point they do NOT have >capacitance, save lead capacitance (a few pf),
Not true: MOVs have a self-capacitance on the order of 1000pF, which is sometimes an advantage. The surge limiting components with a very low capacitance are spark gaps, but those have quite different characteristics. MOVs are bidirectional devices which are "soft limiting", so they are not as effective as transzorbs in limiting any incoming voltage spikes. On the other hand, MOVs are bulk-effect devices which are capable of absorbing much more surge energy than the semiconductor junctions in the transzorbs. The main function of MOVs is to "take a bullet" for the more sensitive (and valuable) components downstream. However, this also involves some cumulative damage to the crystal structure and eventually an MOV will either become ineffective or fail (failure could involve either an upward or a downward shift in the limiting voltage, ending in either an open or a short circuit). These failure modes can be an acceptable price to pay for the energy absorbing capabilities, so long as both the designer and the user are aware of the possibilities. The most effective protection strategies will combine two or more different types of device to exploit each of their best features. One example is to install a big MOV first in line, followed by a series impedance (usually a mains filter) and then a fast device such as a transzorb to clip any residual spikes. Another very simple tip is to install all of these supply protection components AFTER the switch, so they are not exposed to surges when equipment is not in use. 73 from Ian GM3SEK ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
