Current Limiter Circuit

[chinngap]

It is a project of Switched Reluctance Motor whereby I need to use some winding connections to build up winding current quickly and allow rapid collapse of winding current when the transistor is turned off. This is the controller to make the switched reluctance motor spins. However, the peak transistor current in this case cannot exceed 8A or the objective of this project will fail. Together with this question, I attach it with the winding connections. Thanks a lot......

 

[Optikon]

It is a project of Switched Reluctance Motor whereby I need to use some winding connections to build up winding current quickly and allow rapid collapse of winding current when the transistor is turned off. This is the controller to make the switched reluctance motor spins. However, the peak transistor current in this case cannot exceed 8A or the objective of this project will fail. Together with this question, I attach it with the winding connections. Thanks a lot......

Consider the usual way.. place sense resistor in emitter circuit. This should be of a small value. Allow this sense voltage to drive the base of a small signal transistor that has its collector (NPN) connected to the base of the power transistor. You may even want to place a voltage divider on the small signal trans. base. The idea is that when the sense voltage gets too high (too much current like 7.98 amps), the transistor begins to conduct and steals base current from the power transistor and so the power transistor begins to turn off thus limiting the current. Adjust the sense resistor and optional voltage divider to the proper levels for limiting to 8 amps.

 

[Oznog]

A simple resistor lowers the voltage as the load increases, rather than a sharp cutoff. This is not a common solution.

The transistor will do well for many jobs, but calculate how much heat may be produced. A lot of the voltage will be dropped on the transistor. It may also be inappropriate for AC circuits- you do not mention if this is AC or DC.

Since you mention this is a motor, you must consider how much current this will normally need to start. This may be higher than the acceptable limit for continuous operation, but the transistor solution has no time lag and may limit startup current preventing it from starting.

Ideally you'd have a microcontroller which senses current, probably through a shunt resistor or current transformer (AC), using a much higher threshhold for startup, and gradual slowdown limiting with PWM for DC or an SCR for AC.

 

[jornathan]

good idea

that is a good idea!

 

[Russlk]

Optikon's solution is good, this is the circuit:

 

[cokafor91]

current limiter

i have a similar project . the idea is a good one but in the image attached im having some difficulty understanding what the resistor connected to the base drive does, i.e does it have any effect on the current limiting circuit. pls any help?: thanks

 

[crutschow]

i have a similar project . the idea is a good one but in the image attached im having some difficulty understanding what the resistor connected to the base drive does, i.e does it have any effect on the current limiting circuit. pls any help?: thanks

The base resistor is the normal resistor that determines the "on" base current to the transistor driving the coil. You always need such a resistor to limit the base current. Its value will have some small effect on where the current limit starts but that's primarily determined by R3.