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Subject: Re: Control Of Relays Thru NPN & PNP Trans.

Date: 11/16/01 at 7:21 PM
Posted by: Ray Page
E-mail: rpage@raylar.com
Message Posted:

In Reply to: Control Of Relays Thru NPN & PNP Trans. posted by Landon D. on 11/15/01 at 9:56 AM:

Your circuit is fine except for one critical component---a kickback-supressing diode. The coil is really a big inductor, and one trait of an inductor is that they "want" to keep the same amount of current flowing through themselves. It is analogous to a capacitor "wanting" to keep the same voltage across itself. Everthing is fine when the transistor turns on. The current in the coil ramps up until it is equal to the voltage across it (12VDC) divided by the coil's resistance. The problem occurs when the transistor turns off. The coil is going to try to maintain the same current by sending the voltage of the terminal connected to the collector as high as needed to make the current flow. That voltage level is reached when it gets to the Vce breakdown voltage of the transistor (in your circuit).

The solution is to put a diode (like 1n4148) across the coil with the anode connected to the collector side (the diode points to the +12VDC supply). When the transistor turns off, the diode will prevent the voltage at the collect from getting more than .7V above the 12V supply in order to satisfy the coil, thus protecting the transistor.

Hope that helps!


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