| ??? 07/14/03 08:43 Read: times |
#50645 - RE: P89C51 relay interface Responding to: ???'s previous message |
Hallo Don,
I would offer you the following circuit: ![]() Sometimes it's desired, that relais switching is immune to power-up conflicts. For achieving this, relais driver are gated with reset signal. Use of MAX1232 allows a rather elegant way to do this, because it delivers not only RST signal, but also !RST. As you can see, circuit works with negative logic, means, low level is required for turning-on relais. That makes sense, because high level at output ports is just the reset level. Circuit works as follows: At power-up, Vcc is 0V. Up to 2V, relais can't be turned-on, because Vcc is not high enough. So, uncorrect signals at !RST or port outputs don't matter. When Vcc reaches about 2V, MAX1232 begins to work and forces at !RST pin low level. Output low voltage is <0.5V (guaranteed) and in combination with voltage drop over Schottky diodes 1N6263 of about 0.3V, BS170 is securely turned-off. Afterwards, mcu is reset and output ports show high level. This causes first BS170 to turn-on, which guarantees that second BS170 stays turned-off. If now, after ending of reset time, !RST is removed, means open-drain is turned-off, second BS170 stays turned-off, furtherly. Only when according port output of mcu is switched low, second BS170 will turn-on, because first BS170 is then turned-off. If power supply is switched-off, or when a brownout situation occurs, !RST is switched low, and both second BS170 are turned-off. Not shown in this simplified schematic is power supply decoupling. Connect at both second BS170 at least 100nF X7R directly to one terminal of relais and source of according BS170. This discharging loop should be kept as small as possible. It's best to add 47µF/25V (two ones, one at each second BS170). It could be advantegous, to connect some nF from gate of second BS170 to ground. But junction capacitance of 1N6263 is so small (<2pF) and gate source capacitance is so big (60pF), that there should not be any relevant cross coupling. Good luck, Kai |
| Topic | Author | Date |
| P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| Correction: collector not base | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
Thanks for the help | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 | |
| RE: P89C51 relay interface | 01/01/70 00:00 |




