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I need a transistor lesson that I could not find on the internet:

I have a standard GND-PNP-Load-(-20Vcc) arrange:

0V-Transistor-Load--20V

(The two resistors attached to the base are to form a divider - ignore the values).

Im struggling because if I leave the base floating (for testing purposes), the transistor conducts.

I tried making every thinkable voltage divider between GND and -20V to feed the base. The voltage on the base never goes above -0.65V, where (I suppose) it would cutoff.

Was it a NPN transistor, leaving a small voltage (<0.6) on the base would cut it off. Not here.

  • It's a BC556.

  • The transistor does work well on the linear region

  • The datasheet says ICBO is -15nA. Very low value. Does this change anything ?

  • Those schematics come from a humble simulator that doesn't allow me to tag the components.

I'm clearly missing a polarity rule or something. Who can point it out ?

Edit:

The schematic above was a simplification. After no success following your suggestions, I started to think that a simplification was not a good idea (probably the "load" messes with the system).

The device on the collector is actually the control pin of the LM7905 IC, which sinks a maximum of 3mA.

I'm away from a PC, so I won't represent it. But you can replace the current source below with it.

Just hope this information doesn't add to the confusion...

My final objective is to vary this current, as best as possible, from 0 to 3mA using the transistor.*

Around -10V is the voltage on this pin when the current is 2.7mA. This voltage probably varies with the current

Non-Simplified Circuit

*Must be a transistor. A resistive divider doesn't span the minimum and maximum current. Also, a transistor worked very well on the positive side.

Lucas BS
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    What is the purpose of the 1 kohm resistor between base and -20 V? (Please add designators to your schematic so we can say things like "R2" instead of "the 1 kohm resistor between base and -20 V") – The Photon Aug 02 '19 at 17:58
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    If you connect the base directly to the emitter it should not conduct. If it does, it is broken. By the way: leaving the base floating will give erratic behavior. Just don't. – Oldfart Aug 02 '19 at 17:59
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    Double-check the pinout. Transistor manufacturers will use all possible combinations of leadouts, even though the usual one is very common. – TimWescott Aug 02 '19 at 18:06
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    A BJT isn’t a voltage comparator. It just suddenly doesn’t start conducting at a specific base emitter voltage. It’s a non linear but gradual thing hence it is usable as a signal amplifier. – Andy aka Aug 02 '19 at 18:23
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    Never leave inputs floating. I repeat, Never. Leave. Inputs. Floating. – Harry Svensson Aug 02 '19 at 19:33
  • When the base is open, you can look up $I_\text{CBO}$ to see what you can plan on regarding leakage current. It won't be zero and it will be orders of magnitude different over its entire temperature range of operation. – jonk Aug 02 '19 at 19:42
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    try 100 ohms for the top resistor and 1 meg for the bottom resistor, that should turn it off about as well as can be expected. – Jasen Слава Україні Aug 02 '19 at 21:51
  • I've changed the post to reflect your suggestios (none of them worked, unfortunately). I confirmed the transistor type (PNP), confirmed lead polarity (with multimeter), attempted to ground the base (connect it to the emitter), and to change the resistors (with fixed and potentiometers). – Lucas BS Aug 03 '19 at 00:45
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    What happened when you connected the emitter and base? Did it stop conducting as expected? – Kent Altobelli Aug 03 '19 at 02:25
  • I don't like the way your 2mA current source connects to GND, i. e., to the positive side of your power supply in your second schematic. I would connect it to the negative side of of the supply instead. – joribama Aug 03 '19 at 03:53
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    I'm also not sure why you have a current source as a load. If you're simply trying to convince yourself you can modulate the colector current with the base polarization you should simply have a resistor as a load as you had initially. – joribama Aug 03 '19 at 03:56
  • It did not stop conducting, Kent Altobelli. I removed the resistors to test. Same result. I replaced the transistor with another from the same batch. Same result. But I'd like to remind everyone that it DID work on the linear region (using potentiometers). Just doesn't cutoff – Lucas BS Aug 06 '19 at 19:29
  • Since no suggestions resolved the problem, I had to do something. In this case, releasing more detailed information about my circuit seemed like the right thing to do. But it remains in your hands to pick the schematic that allows a better understanding. – Lucas BS Aug 06 '19 at 19:33
  • I really believe that the -10V on the collector, after the resistor, has something to do with it. I updated the thread to add even more detail. – Lucas BS Aug 06 '19 at 19:34
  • It clearly HAS something to do with the "load". I just replaced the BC556 with a BC327. The exact same behavior. Reminding: in my project, the collector is connected to the control pin of the LM7905. Thank you all for the help so far. – Lucas BS Aug 08 '19 at 13:23

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