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Transistor strange behavior

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igeorge

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The circuit in the attached is strange
In the top is the same like in the bottom, but the to circuit act normal switching on and off -see the values on the voltmeters
the one in the bottom, never switch off, and is contently on
The voltage on the base is changed as you can see the logic dots - red = 1 blue = 0
I need an explanation how to do it because i will switch the transistor from a logic gate
Thank you
 

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Simple question in return, please calculate the voltage required at the left side of R1 to switch the transistor off. Why can't it be driven by a TTL/CMOS logic gate?
 

If you use a transistor as a switch means using transistors to work as saturation(switch on = CE short) and cutoff(switch off = CE open). PNP transistor works as switch on when VEB > 0.7volt, if less then it will work as a switch off. Please, calculate again resistor to obtain the results as above.
 

Thank you to all for suggestions.
Can you indicate the value of the resistor when the load current is 10 milliamperes.
If i knew how to get it , probably this post never existed
Thank you for understanding
 

It has nothing to do with the resistor values; you can't control a 24V transistor input directly from a 0/5V logic gate (as FvM tried to tell you above).

It could work with a 22V Z-Diode in series with R1, and a logic inversion of U1 (either an AND instead of the NAND gate, or a 2nd NAND gate in series).
 

igeorge said:
The circuit in the attached is strange
In the top is the same like in the bottom, but the to circuit act normal switching on and off -see the values on the voltmeters
the one in the bottom, never switch off, and is contently on
The voltage on the base is changed as you can see the logic dots - red = 1 blue = 0
I need an explanation how to do it because i will switch the transistor from a logic gate
Thank you
Click to expand...
The transistor turns on when the emitter-base junction is forward biased. The emitter in both cases is fixed at +24v above ground. So when you drive the base of the transistor from a logic gate whose power supply is ground and +5v, the output of the logic gate is either 0v or +5v with respect to ground. Both of these voltages are much lower than the +24v on the emitter, so the emitter-base junction is still forward biased and the transistor remains on no matter what the state of the logic gate.
 

Usually, a level conversio circuit should be prepared for voltage variations. 24V must not necessary be exact, a good standard in automation is to have the circuit working at least with 15 to 30 V. In this case, it's necessary to place a second transistor as level converter. If you don't want to invert the logic polarity, a NPN transistor in common base circuit could be used, base biased to 5V. Or the more common solution, a NPN transistor in common emitter configuration which inverts the logic polarity. A high input voltage turns on both transistors.
 

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