| ??? 07/22/04 14:04 Read: times |
#74696 - Negative Logic... Responding to: ???'s previous message |
Hallo Kashyp,
it's an often overlooked fact, that '51 microcontrollers standardly work with 'negative logic'! Means, output is active, when providing logical low state. That's the reason, why invoking a reset will let the outputs toggle to logical high state! We have discussed a lot about 'negative logic', also named 'inverse logic', sometimes. Have a search here!! As you want to drive 6 relays, I would put inverters at according outputs of microcontroller, for instance 74HC04, followed by an ULN2003. At inputs of 74HC04 I would add 10kOhm pull-ups (resistor array saves board area!), because there are periods, when the outputs of microcontroller are driven only by very weak internal active pull-ups. An added 10kOhm pull-up resistor can prevent the situation, that noise injected via stray capacitance will eroneously 'fire' your relays... Sometimes, driving relays from regulated +5V power supply can present problems with enormeous heat dissipation across regulator. Then, it can be helpful to power the relays not from output side of +5V regulator, but from input side, where the big storage capacitor sits. A suited resistor in series to relay coil provides voltage dividing, so that finally about 6V drops across relay. Nevertheless, choose relays only needing a minimum of current to turn-on!!! Even if costs rise, there's nothing more annoying than a board that 'burns', only because old-fashioned, high current consuming relays were mounted... Kai |
