current-limitingtransistorsvoltage
I cannot understand how this current limiting circuit works.
Image source: UNLV Physics Department, Bill O'Donnell, Current Limiter Circuit
Here is where I get confused:
When Q2 is turned on, the voltage at the base of Q1 becomes close to 0 and Q1 stops conducting current. Therefore VR1 should be 0, which will turn off Q2 and turn on Q1. What am I missing here? If we put an emitter resistor for Q2, shouldn't that solve my issue?
I might be totally wrong here but my excuse is the question is very confusing. The circuit shown above is trying to pull the 2.52 V line down to ground through the 10R resistor and an NPN transistor. The original (pre-hacked circuit) you describe was it this? Forward and Reverse are mutually exclusive as are left and right. On the vehicles I've hacked into they usually have a spring loaded centre off action (joystick). Are you absolutely certain the pole of the switch went to ground (0v)?
You say when you touched R2 (140k) the remote worked sending a signal to the car. This sounds like you were acting as an input signal. By touching the input your body as an aerial picking up the local EM field, the 140K resistor being large enough to prevent the weak signal from being shorted out. That suggests to me that the control input needs to go HIGH rather than LOW and that the switch was actually connected to the positive rail. To test this hypothesis connect up the circuit below and let us know what happened.
Bipolar junction transistors (BJT) have 3 primary modes of operation:
In cases 2 and 3 there is current flowing from the collector to the emitter.
You can saturate the BJT by attaching a smaller load (i.e. higher resistance) than the linear limit allows. Think of a pipe with a valve. If there's something clogging the pipe upstream, at a certain point no matter how much you open the valve you're not going to get any more fluid flowing through the valve.
Yes, the electrons do literally move to generate the current, though this isn't directly used for most electrical engineering analysis.
For most EE analysis we don't consider the motions of charged particles directly, instead relying completely on the abstract concept of current. This is an approximation of reality, but it turns out to be remarkably good in most cases.
Ironically, in my day-to-day work with plasma physics we do consider the motion of charged particles instead of relying on abstract currents because the analysis does differ depending on what species are moving and where/how fast they are moving.
Best Answer
It balances around the voltage drop across R1.
Q2 doesn't just suddenly turn fully on. As current through the load increases, the voltage drop across R1 increases...once it gets high enough (around 0.7 V) Q2 STARTS to conduct. At low enough currents, this transistor is just barely conducting current.
Once Q2 starts conducting it starts reducing the base voltage on Q1.
It stays in equilibrium. As the current goes down, so does the R1 voltage drop, so Q2 conducts less, so Q1 lets more current through.