Heber and any with interest Thanks for your feedback on the PTC /MOV application. Help me understand.
My sense of the problem is one of power dissipation curves that must overlay properly. The MOV has a joule rating that if exceeded will cause failure while the PTC needs to dissipate enough energy to engage yet not so quickly that the suppression of surge is not diminished. Isn't it a relationship of surge energy < PTC trip energy < MOV joule rating, curves plotted with respect to time. I imagine the real difficulty here is the long delay time of the PTC relative to the surge event and the MOV clamping. I was hoping that with some limiting impedance in the PTC itself, one could balance the curves so that they are concentric (for lack of a better term). That is the PTC could trip after clamping the surge and before allowing MOV damage. Am I close to understanding the challenge? I mentioned on a posting that I had inherited a design where the engineer had attempted to make this relation ship work. He had a Murata ceramic PTC (82 ohms impedance) in series with the 120 hot and then a MOV in parallel. As you said this combo does shut down the MOV but not until the MOV is damaged. In this design the MOVs shorted but did not create a catastrophic failure. I realize that preventing the MOV failure is a desirable goal. But isn't there some value in preventing the nasty flash over of a MOV failure? Lastly - what do you know about the acceptance of MOVs in Europe and in that light how would you deal with Surge? I have heard some contentions in particular when placed line to ground. What would you do: 1) Use robust front end filter and limiting impedances (resistors, caps and inductors) with no MOVs? 2) Put Gas tubes in series with MOVs. (this sounds like a mess) 3) Fuse each MOV. line to line and line to ground. This way the fuses could prevent a short failure from becoming dangerous and then the fuse could be removed for specific applications that do not allow MOVs to ground. 4) Your new suggestion here????? I am leaning towards #1 or # 3 but have no experience on selecting the right MOV/fuse combo. Any feedback would be appreciated Chris Wells Sr Des Engineer Cutler-Hammer [email protected] ------------- Original Text From: C=US/A=INTERNET/DDA=ID/hfarnswo(a)msmail.physio-control.com, on 5/19/97 5:12 PM: CWells wrote: >>>I understand your concern in regards to the clearing time. However the PTCs are basically thermally driven and the so is the MOV failure. I believe one could balance the two with the proper sizing of the MOV and the series impedance of the PTC. Wrong! A careful analysis will show that for any series combination of MOV and PTC thermistor, there is a voltage just above the MOV threshold where the MOV is still in a fairly high resistance mode while the (unheated) thermistor is a low resistance. Since current is essentially equal through the pair, heating will be largely confined to the MOV. True, the thermistor will shut the circuit down, but not until the MOV is damaged. >>>I plan to talk to the various vendors of MOVs and PTCs. If anyone has more experience in this area I would appreciate your input. Chris, I have extensive experience, including one patent (#5.379.022) assigned to a previous employer. I'd love to hook up with someone interested in a solution to this problem. Interested parties should contact me by email ([email protected]) or at: Heber Farnsworth 8029 137th Ave. SE Snohomish, WA 98290
