> > > > > > On Sat, May 16, 2015 at 1:39 PM, Harold Hallikainen > > wrote: > > >=20 >=20 > > > > > Opto > > couplers are one way to get the isolation, but then you have to power t= he > > > LEDs with an isolated supply. Ideas? > > >=20 > I'll probably write this up in more detail sometime, but I had a recent > experience where about 25 VDC managed to cross >=20 > a barrier rated at 2000 + or VDC. >=20 > When the primary side catches fire so that the isolated IC is incinerated > and an arc forms across the broken primary circuit and then finds a nice > carbon char path into the primary side 'fun things can happen'. And did. >=20 This is a reason why an optocoupler by itself isn't a good way to isolate=20 between different voltages in safety critical applications.=20 If you design for Ex, you need to put one or two zener diodes and one fuse = to=20 limit voltage and current into the LED side of the OC in order to safely be= =20 able to maintin isolation between the solid material inside the OC. You als= o=20 need to limit energy through the output transistor in order to guarantee=20 meltdown. Instead of an optocoupler you can use discrete IR diodes and receivers with= a=20 long air distance between them. Any type of fault in one of the components= =20 can't break the safety provided by isolation barrier. /Ruben --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .