6.Coil impressed circuit
1.Relay drive by means of a transistor
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3. Snap action (Characteristic of relay with voltage rise and fall)
It is necessary for the relay coil not to impress voltage slowly but to impress the rated voltage in a short time and also to drop the voltage to zero in a short time.
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(No good) Without snap action | (Good) Snap action |
4. Schmitt circuit (Snap action circuit) (Wave shaping circuit)
When the input signal does not produce a snap action, ordinarily a Schmitt trigger circuit is used to produce safe snap action.
1.The common emitter resistor RE must have a sufficiently small value compared with the resistance of the relay coil.
2.Due to the relay coil current, the difference in the voltage between at point P when T2is conducting and at point P when T1is conducting creates hysteresis in the detection capability of Schmitt circuit, and care must be taken in setting the values.
3.When there is chattering in the input signal because of waveform oscillation, an CR time constant circuit should be inserted in the stage before the Schmitt trigger circuit. (However, the response speed drops.)
5. Avoid Darlington circuit connections. (High amplification)
Care must be taken in this circuit due to increase of VCESAT. It does not cause a failure immediately, but it may lead to troubles by using for a long period or by operating with many units.
（No good） Darlington connection | (Good) Emitter connection |
Due to excessive consumption of | Tr2 conducts completely. |
power, heat is generated. | Tr1 is sufficient for signal use. |
A strong Tr1 is necessary. |
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