A while ago I posted here with a question about current sensing in a circuit to limit the output of an antique generator. Here's an idea for a comparator with hysteresis to cut off field current when generator output exceeds 10 amps.
36R is an estimate, and the value of the 470K hysteresis resistor is just a guess. I haven't built this yet. Also, I wonder if it would need a cap from the collector to base of the PNP transistor.
I compressed your circuit slightly. Your drawing is hard to understand, what's B+, which is in 0R1's current path, and what's "field 0", which is driven by the powerful MOSFET drain? Are we to assume a magnetic-field coil goes between these two?
Your circuit is rather tortured. The hysteresis part may be OK, but 0R1 with two diodes and pot plus 36R are troublesome. When you get the right circuit there won't be any guessing. One thing to keep in mind is diode voltage drops don't equal transistor drops (although they may match diode-connected transistor drops).
Also, you mentioned a voltage divider "connected to the base of the NPN transistor," but we don't see it in your drawing. Plus, it's not clear how a voltage divider on the NPN could be helpful. A resistor across the NPN base-emitter would establish a current in the PNP that might be helpful in setting the PNP Vbe voltage at some threshold current. Perhaps that's what you had in mind.
The 100 K resistor and the 0.1 uF cap are for hysteresis, to keep heat down in the mosfet; I'm putting this circuit into a small, flat enclosure without fins so it can be installed in a motorcycle's battery
compartment. The string of 1N4148 diodes are there so the regulator will have a temperature coefficient matching the charging characteristic of the battery. Another reason to keep heat down in the circuit. Similarly, I used a schottky for the blocking diode, which has to conduct all the charging current.
Generator configuration: One of the armature brushes is grounded. The other brush is wired to the A terminal on the outside of the generator and is also connected to
one end of the field coils. The other end of the field is connected to
the F terminal on the outside of the generator. It's called a floating
field or "type A" arrangement, common on the old GM vehicles and Harleys.
* *
Okay, first of all, I made a boo-boo in the diagram -- the current sense resistor is 0.01 ohms, not 0.1 ohms. In answer to your questions, B+ is the battery's positive terminal. The O next to the word field simply represents the connection point on the circuit board for the wire going to the field terminal of the generator. The NPN transistor and mosfet in the latest post is the very same NPN transistor and mosfet in the "voltage regulator" diagram. I didn't want to redraw the whole thing this time, so I just drew the two circuit elements the limiter mainly affects. Also, what I lazily referred to as a "divider" in my latest post isn't strictly speaking a divider. But that's neither here nor there. The 10K resistor is a pot with the wiper connected to ground. With the wiper at the midpoint, the 36 ohm resistor would have about
0.1 volt across it. So with the nominal ten amps through the current sense resistor, the pnp transistor will turn on. The pot is for adjusting it exactly. Hysteresis so the mosfet is always on or off and doesn't generate heat. I'll use diode-connected transistors instead of diodes. Well, that's enough for now.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.