logic circuits run from 12 volt car batteries

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rshayes
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Re: logic circuits run from 12 volt car batteries

Post by rshayes »

You will also need a resistor in series with the base of the transistor to limit the base current. The collector current of the transistor will be determined by the resistance of the relay if the transistor is in saturation. The base current needed by the transistor to insure saturation is about 5 to 10 percent of the collector current.<p>The base voltage of the transistor is probably around .6 to .7 volts when the transistor is on. The voltage of the logic driving the transistor will be at the high logic level for the logic series being used. This will be above 3 volts for TTL or aboue 4 volts for CMOS. The series resistor should be equal to the difference between the logic level and the transistor base voltage divided by the desired base current. The logic gate should be chosen so that it can supply this current and still maintain the logic level voltage. This may mean using a gate meant as a buffer that is capable of higher than normal output current. If necessary, two or more gates can be paralleled. The CD4000 series can not source much current in general, but sometimes paralleling several sections of the CD4049 or CD4050 will supply enough current.
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