AIM:To study dc chopper circuit power circuit(Jone’s Chopper).
This unit consists of two SCR’s, two diodes and an LC commutation circuit to construct a Jones Chopper power circuit. Each device in the unit is mounted on an appropriate heat sink and is protected with an RC snubber circuit. Further each power device has a glass fuse in series with it.
All the components are independent and their connections are brought out to front panel. The cathode and gate of each SCR is brought out on to separate terminals for firing pulse connection. DC MCB and a fuse is provided in series with the DC input.
A field supply bridge rectifier is also provided for DC motor control application in medium and high rating power circuit.
The devices and components can also be used to build different chopper circuits.
DC chopper firing unit provided triggering pulses for this power circuit.
Fig.1 shows the circuit diagram of the Jone’s chopper .Thyristor T1 is the main power switch corresponding to S1, Thyristor T2 is the auxiliary thyristor. The circuit operates in the following modes.
Mode(i):It is assumed that prior to this mode T1 has been conducting and that the capacitor C1 is charged to Vin volts in the direction shown in fig.2.The gating of T2 by signal G2 starts this mode. Thyristor T1 is almost instantaneously turned off. The load current,assumed to be constant during this mode,now flows through C1 and T2.The capacitor C1 discharges from –Vin to +Vin,almost linearly.The output voltage V0 jumps to 2Vin at the start of this mode and falls linearly to zero.The duration of this mode is given by,
The mode ends when diode D1 is forward biased and is conducting.The capacitor is charged to +Vin at the end of mode.The turn off time for T1 is t1/2.
Mode(ii):The load current freewheels through diode D1 in this mode .The duration of the mode is (T-Ton-t1).
Mode(iii):This mode occurs when the main thyristor T1 is gated.An oscillatory circuit is formed with L1C1T1 and Ds.The capacitor voltage changes sinusoidally from +Vin to –Vin.The duration of this mode is given by,
The turn-off time for T2 is ts/2.