| ??? 07/19/06 11:31 Modified: 07/19/06 11:36 Read: times |
#120567 - the case of fault Responding to: ???'s previous message |
hi,
Asok Sankar Rudra said:
Dear Members,
Can I drive a 12 Volts 400 Ohms relay (like LEONE SC5-S-DC12V OR GOODSKY RW-SH-112D)by a '51 microcontroller working at 5 Volts? I normally use 5 Volts 100 Ohms relays (GOODSKY GS-SH-205T-DS) with a BC557/8B and a 10 KOhms resistance connected between the port pin and the base. The collector goes to ground and the emitter to the relay coil, the other end of which is connected to the Vcc +5Volts,the usual IN4148 is across the coil.This unisolated arrangement works faultless even with inductive loads like fridge. When the same arrangement is used with 12Volts relay connected to 12v,the relay is put on only once by the microcontroller as desired, by clearing the port pin;but the relay cannot be put off by setting high the port pin. Indeed, think about next: even a port pin provides about 4.9V at high level output state nevertheless there is about 7V (12-4.9) between +12V power and port pin at high state. First of all, this voltage level is not safe for pin itself and may destroy it due over voltage conditions and CMOS "latchup" effect as well - read http://www.8052.com/forum/read.phtml?id=75866 Secondly, transistor will be kept open by this voltage anyway as long as difference above ~0.7V. Another point is "collector goes to ground". This is not ordinary connection of transistors. During such connection about 2,5V or so drops over collector-emitter when transistor opens. Regards, Oleg |
