| ??? 07/19/06 14:17 Read: times |
#120587 - Heaven, no! Responding to: ???'s previous message |
Per said:
Great idea with this faq, but 1N4148 is not to be used in this circuit. Choose a 1N4005 or something like it. Heaven, do you really expect in this circuit inductive kicks of 600V, which is what the 1N4005 can withstand?? Nooo! There's no such inductive kick, just becasue the free wheel diode is there. In the moment the transistor turns-off, the inductive mechanism of relay tries to maintain the relay current, which is 30mA here. So, the only inductive kick you will observe is, that an induced voltage is generated which turns-on the diode in such a way, that the 30mA are furtherly flowing through the relay. So, you will observe a voltage of about 12.8V at the anode of 1N4148 and 12V at the cathode. That's all! Also, this 30mA is flowing through the diode only for a short period, means as long as it takes for the magnetic field to collapse. So, there's even no problem at all with heat dissipation. The two design criteria for the free wheel diode are 'current' and 'reverse voltage', which is 30mA and 12V, just what a 1N4148 can easily withstand. There's absolutely no need for your 1A/600V diode! Of course, when you take a much stronger 12V relay, one which switches much more contacts at a time, then coil impedance can be much lower than 400 Ohm and a much higher relay current than 30mA can flow. In such a situation you can take a stronger free wheel diode, but only in terms of current, not reverse voltage. Means then you can use a 1N4002, or similar. But the cicruit I proposed here was a suggestion to Asok, and he used a 12V/400Ohm relay, where the 1N4148 is totally sufficient! In more sophisticated free wheel applications, not necessarily relay driving applications, also the turn-on time of free wheel diode can be critical! In any case, the free wheel diode must turn-on faster then the switching element needs to turn-off!!! In our application, we have a transistor which needs about 1µsec to turn-off. So, the 1N4148, switching in nanoseconds is fast enough. Also the 1N4002 is fast enough, although it needs a considerably longer time to turn-on! Yes, when exchanging the switching transistor by a much faster one, you can even run into trouble with the 1N4002! Kai |
