Definitely. Even a 0.1uf will weld the contacts or wear them out premature= ly.=20 Geo wrote: >Electron wrote: > >> Even the >> original digital odometer may have one, in fact the "peak current of 400= uA vs >> average current of 9uA" I measured may well be due to a cap in parallel = to the >> reed input which gets charged by a high value pull-up resistor (math sug= gests >> 330kohm), the pulse closes the reed only for few degrees of wheel rotati= on, >> so in the open state the capacitor is charged (albeit relatively slowly)= by >> the resistor, the input at the microcontroller raises (slowly, but as it= 's a >> schmitt trigger this doesn't cause problems) and the capacitor opposes a= low >> impedance to eventual high frequency transients. When the reed closes, t= he >> capacitor discharge (with relatively high current) and the digital pin g= oes >> low very quickly. > >I suggest being careful with the discharge current. I have replaced 100s=20 >of reed relays which have been cold welded in the closed position by a=20 >small excess current flow (they could be cleared by vibration). I think=20 >you need to work out the maximum frequency of the reed opening and=20 >closing (i.e. using max speed and wheel diameter) then choose a=20 >capacitor with an additional resistor in series with the reed to limit=20 >the current flow on closure. This resistor will also form a low-pass=20 >filter in conjunction with the capacitor which should help with your=20 >noise problem. > >George Smith > >--=20 >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .