Ok ,I have a wire with current I1 going thru it. As the voltage and I1 rise there is a transistor Q which turns on and it in turn stops the current from flowing. I1 and the voltage referenced is tied to the base of the transistor.
My question is:
When I1=.90amps Vbe = .63 volts
When I1=1.3 amps Vbe = .58 volts
When I1=1.5 amps Vbe=.38 volts
Why does the turn on voltage of the transistor keep dropping as the current increases?
(q1 is 2N3904)
Added picture upon request. Making it a bit more specific to my circuit
Now that you posted a circuit schematic,
From what you wrote and the schematic given,it looks like you using R3 as a current sense resistor, and the mosfet as a voltage controled current source, so if your current increases too much, you want the circuit to fold back and stop the current, this set up will not stop the current from flowing, as you noted in your description.
This circuit is acting as a feedback voltage regulator.
The reason the voltage is dropping at the base with increased current flow, is because Q1 is conducting at its Vbe value of 0.7v. , but what is happening as an increase of the voltage across R3 is sensed at the base, Q1 conducts dropping the gate voltage of Q2, which in turn drops the current I1 the amount needed to allow the Vbe of Q1 to get back to equilibrium.
Here is a experiment.
Remove Q1 from the circuit.
Then measure the voltage drop across R3 with normal current flow (I1);
then increasse the current flow, and measure the drop across R3.
It Volt drop should increase across R3 linearly.
Now place a diode across R3 and apply enough current through the circuit to get a substantial reading across R3. Then increase current (I1) and continue to monitor the volt drop across R3.
It should remain fairly stable at the diodes forward voltage.
Now take out the diode and place Q1 back into the circuit leaving the collector unhooked for the moment. Again check volt drop across R3 with increasing current flow.
It should act like the diode :
Now connect the collector into the circuit and see how the measurements look then with increrasing current flow.
If you want to stop the current flow completely, with a threshold current increase, that will take more design work.