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2 NPN gates, wired from the same ressistor, not working as expected?

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settra

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Hello forum. i have the following schemantic:
Untitled.jpg

as you can see, the transistor gates both receiv current through the same resistor. i have a problem, and this schemantic is a simplification of the problem i have... is this idea correct? will this circuit work, even when both switches are pressed?

becouse some times when both are pressed, one transistor seems to be openin-closing all the time.. (Always the same one)

thanks !
 

alec_t

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The base-emitter junctions of the two transistors will not be identical, therefore for a given base voltage they will pass different currents.
Don't be cheap: give each transistor its own base resistor :D.
 

ericgibbs

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hi settra,
With such a low Base driving voltage the circuit will malfunction as your experiencing.
What are you trying to do with this configuration.?

Eric
 

settra

Member
both transistors are driving relays !! they are connected in the GND path of the relays.

the Vcc of the relays is also switched from two other switches (not shown above). if one relay does not receive the Vcc, and i am trying to operate both, then 100% of the times, the other relay will open-close
 

alec_t

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Most Helpful Member
If R1 is 1k, the maximum current through it would be about (1V-0.7V)/1k = 0.3mA. If both transistors drew the same current (unlikely) and had a current gain of 100, the maximum collector current in each would be 0.15mA x 100 = 15mA; barely enough to operate even a sensitive relay.
 

crutschow

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You should give each transistor it's own base resistor with a value such the the base current is about 1/10th of the collector current so the transistors will switch well into saturation when on.
 

ChrisP58

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I suspect that part of the problem is that the two base-emitter junctions have slightly different Vbe values.

This matters because the Vbe voltage is two things. It is the threshold at which the transistor starts to conduct, but it's also the level where the junction acts as a zener diode.

So, the transistor with the slightly lower Vbe is clamping the voltage at a level to low for the second transistor to turn on.

As has been said, each transistor needs it's own resistor.
 

settra

Member
yiep. i added different resistors in each base and for now it seems to be working stable ! i dont know what i was thinking using just one.... :)
thanks for your help !
 
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