| ??? 08/05/09 12:52 Modified: 08/05/09 12:58 Read: times |
#168176 - That's too late! Responding to: ???'s previous message |
Anurag said:
But my code manages the relay status, as first activity in boot-up sequence. That's too late! If you have exceeded the maximum current of your mains fuse for some reason, for instance because an inductive load generates a high inrush current, then your micro will not have enough time to turn off the loads before the mains fuse crashes again! You will be caught in an endless loop. The only remedy then is to disconnect some of the loads by hand from mains before resetting the mains fuse. No, Anurag, a latching relay is no good idea, if you can reset them only in the boot-up sequence! Triacs are good for phase angle control and switching applications if the load is very well known. Then you usually use a MOC30xx series opto-diac to drive a power triac. But in any case you urgently need a well-matched snubber for the opto-diac, a well-matched snubber for the triac, a well-matched snubber across the (inductive) load, usually LC filter elements to suppress the switching noise, a well-matched fast acting fuse, eventually an inrush current control and a heat sink which usually has contact to mains voltage, if you take an inexpensive triac! For certain loads you need zero-crossing switching, for other loads you must not use zero-crossing but random phase switching. So, for a universal hosehold switching box triacs aren't a cheap and foolproof solution. By the way, you will not save much supply current either, because the opto-diac needs about 20mA to turn on. Take also care that during power-on reset no relay is unintentionally driven!!! This will not work with your ULN2003 solution. You must turn on a relay only if the corresponding port pin goes low. More, if your micro does not provide an asynchronous reset, you must additionally gate the relay driving section by the reset signal of a good reset controller. Also: It's no good idea to drive a 5V relay by a 9V supply!!! I guess you suffer from lot heat dissipation in your voltage regulators? Then you should think about supplying your relays (and the micro?) by a switcher, e.g. LM2674. But as relays not really need a regulated supply voltage you could also use a stiff mains transformer or even an electronic transformer built for halogen lighting applications. These are cheap and would help you to save huge power! Of course, you would need 12V relays then. Good luck, Kai |
