| ??? 12/09/06 03:58 Modified: 12/09/06 04:09 Read: times Msg Score: +1 +1 Informative |
#129200 - Decay time of solenoid is irrelevant Responding to: ???'s previous message |
Henry said:
The solenoid has a decay time of 300ns. What counts is the turn-off time of MOSFET, not the decay time of solenoid! What is the free wheel diode for, at all? The solenoid, carrying a certain current, stores magnetic energy. When suddenly turning-off this current, theory tells, that the solenoid tries to maintain the current. So it "eats" a bit of the magnetic field energy and generates a voltage in order to maintain the current, according to Lenz's rule. Here, the change rate of current is what defines the height of inductive kick and the associated change rate of induced voltage. So, when the transistor is very fast in turning-off the solenoid's current, then you need a very fast free wheel diode, too. Usually, solenoids are driven by rather slow transistors, like BC337 and such stuff. These transistors need several 100nsec or even microseconds to turn-off the current. So, rather slow free wheel diodes like 1N4002 can be used with them, which are similar slow. But when you take a very fast transistor to turn-off the current through the solenoid, then you also need a very fast free wheel diode. So, you see, it's not always wise to use a fast transistor for driving a solenoid. Kai |
| Topic | Author | Date |
| Inductive loads and diode protection | 01/01/70 00:00 | |
| BAV70 in SOT23 | 01/01/70 00:00 | |
| Decay time of solenoid is irrelevant | 01/01/70 00:00 | |
| current consideration | 01/01/70 00:00 | |
Exactly! | 01/01/70 00:00 |