At 08:55 02/06/97 +0200, you wrote: >---------- >> From: Wayne Bennett >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Optoisolators or MOV's >> Date: jeudi 29 mai 1997 23:47 >> >> I'm monitoring five external lines for change in voltage level with the >74's >> ADC but have to consider the effects of induced voltage from lightning >> strikes. I was thinking of using either MOV's or optoisolators but I'm >not >> very familar with either. >> >> A couple of points I need to consider is that I need precise mesurements >> from the ADC so devices that affect the reading are not desirable and the >> amount of induced voltage and current from a nearby lightning strike (in >the >> magnitude of thousands of volts). > >There is two point of view : >1) No destruction >2) Always precise mesurements > >I think MOV (or tranzorb or zener) are really good solution for protection >but really poor for precise mesurement because they only limit the voltage. >The solution for precise mesurement are filter because they limit the >variation of the voltage in the time. So I place always a tranzorb to >protect and limit the voltage and then a filter to have a precise >mesurement under induced voltage. > >Optoisolators are really poor for induced voltage because of the internal >capacitor between the input and the output. They are only good for >isolation of low and middle frequence or low voltage but induced voltage >are some times 4000V with a rise time of 50nS. > >Didier The other problem with using opto-isolators without MOVs is flashover. When E-fields in excess of 1kV/m can be generated along a 10m wire the eensie weensie gap across the pins of an opto-isolator doesn't count for much. I would go with MOVs to ground every time. If your equipment is at risk from direct strikes (someone I met designed a golf course watering system which got one every few years) you need to think about grounding. If your network offers a low resistance to ground it might take a significant fraction of the strike. Hence it is better to install a local earth (metal plate and thick copper strap) to provide a lower resistance path to ground than your network. This won't protect the station which gets hit necessarily, but it should minimise the damage to the surrouding stations. Just my thoughts Keith. ========================================================== Keith Dowsett "Variables won't; constants aren't." E-mail: kdowsett@rpms.ac.uk WWW: http://kd.rpms.ac.uk/index.htm