Constant-current controller for an inductive load
A constant-current controller that supplies a constant current to an inductive load. This controller comprises an electric control circuit module. The electric control circuit module comprises a primary switch and a secondary switch. During a time interval in which the primary switch is closed (t on ), the secondary switch is open and the voltage across the inductive load is equal to the source voltage (V s ). At time t on until the end of a time interval (T), zero volts appears across the inductive load. During this interval, current continues to flow as supplied by the energy stored in the inductance. The periodic current in the inductive load becomes constant with a sufficiently large PWM switching frequency and is dependent upon the parameters of the control circuit and the duration of t on .
1. A constant-current controller operable to supply a constant current to an inductive load, said constant-current controller comprising:
a) a switching circuit comprising:
1) a source voltage;
2) a primary switch;
3) a secondary switch; and
4) a current transformer comprising two primary windings for sensing a current of said inductive load and a secondary winding,
wherein said two primary windings are connected in series with both said primary switch and said secondary switch, and said secondary switch is disposed between said two primary windings;
wherein, at t 0 , when said primary switch is closed and said secondary switch is open, a first voltage across said inductive load and a circuit resistance is equal to said source voltage;
wherein, a time interval between t on and T, when said primary switch is open and said secondary switch is closed, said current continues to flow to said inductive load as supplied by energy stored in said inductive load,
wherein a periodic current in said inductive load is dependent upon a time duration between said t 0 and said t on , and
wherein said constant-current controller operates as a pulse-width modulation controller to cause said periodic current in said inductive load to become constant through increasing a pulse-width modulation frequency to a frequency that has a switching period which is less than a circuit time constant.
2. The constant-current controller in accordance with claim 1 , wherein a boundary current and a peak current approach the same constant value as said pulse-width modulation frequency increases.
3. The constant-current controller in accordance with claim 1 , wherein said inductive load is selected from a group consisting of a solenoid, a DC motor and a magnetic actuator.
4. The constant-current controller in accordance with claim 1 ,
wherein said secondary winding is connected to a rectifier, said rectifier connected to a burden resistor and a low-pass filter.
5. The constant-current controller in accordance with claim 4 , wherein said switching circuit further comprises:
4) a timer integrated circuit configured to establish the time interval of said periodic current in said inductive load, wherein said timer integrated circuit receives a signal through an input to initiate said time interval.
6. The constant-current controller in accordance with claim 1 , wherein said inductive load is configured as having a multiple-filar winding.
7. The constant-current controller in accordance with claim 1 , wherein said primary switch is a MOSFET and said secondary switch is a free-wheeling diode.
8. A method of providing a constant-current to an inductive load, the method comprising the steps of:
a) sending an electric current to a switching circuit having a primary switch, a secondary switch, and a current transformer comprising two primary windings for sensing said electric current of said inductive load, and a secondary winding, wherein said two primary windings are connected in series with both said primary switch and said secondary switch, and said secondary switch is disposed between said two primary windings;
b) sending said electric current through said inductive load
and said primary switch at t 0 in which said primary switch is closed and said secondary switch is open, causing a voltage across said inductive load to be substantially equal to a source voltage;
c) continuing said electric current through said inductive load
and said primary switch until t on during which said primary switch is closed and said secondary switch is open;
d) sending said electric current through said inductive load during a time interval between said t on and T during which said secondary switch is closed and said primary switch is open, causing said voltage across said inductive load to equal 0,
wherein between said t on and said T, said electric current continues to flow as supplied by energy stored in said inductive load, wherein a periodic current in said inductive load is dependent upon a duration of time between said t 0 and said t on ; and
e) causing said periodic current in said inductive load to become constant through increasing a pulse-width modulated frequency to a frequency that has a switching period which is less than a circuit time constant.
9. The method in accordance with claim 8 further comprising the step of:
f) causing a boundary current and a peak current to approach the same value as said pulse-width modulated frequency increases.