MJ2955 78XX Increasing Regulator Current circuit and explanation

Notes:
Although the 78xx series of voltage regulators are available with different current outputs, you can boost
the available current output with this circuit. A power transistor is used to supply extra current to the load
the regulator, maintaining a constant voltage. Currents up to 650mA will flow through the regulator, above
this value and the power transistor will start to conduct, supplying the extra current to the load. This should
be on an adequate heat sink as it is likely to get rather hot. Suppose you use a 12v regulator, 7812. The
input voltage should be a few volts higher to allow for voltage drops. Assume 20 volts. Lets also assume
that the load will draw 5amps. The power dissipation in the transistor will be Vce * Ic or (20-12)*8=40watt.
It may keep you warm in the Winter, but you will need a large heatsink with good thermal dissipation.
If you want to increase the output current with a negative regulator, such as the 79xx series, then the circuit
is similar, but an NPN type power transistor is used instead.

Source ::
http://www.mitedu.freeserve.co.uk/Circuits/Power/boosti.htm

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Switchmode Constant Current Source

Operating a stepper motor using a fixed (constant) voltage supply results in poor torque at high speeds. In fact, stepper motors tend to stall at fairly low speeds under such conditions. Several approaches can be used to overcome this problem, one of which is to use a constant current supply in place of the more conventional constant voltage supply. A disadvantage of many constant current supplies is that simple circuits are inefficient but that doesnt apply to switchmode supplies such as the circuit shown here.

Basically, this circuit is a conventional switchmode regulator adapted for constant current output and is specially designed for stepper motor drivers - although it could be used for other applications as well. The circuit works as follows: IC1 (LM2575T) and its associated components (D1, L1, C1, etc) operate as a switchmode power supply. Normally, for constant voltage operation, the output is connected - either directly or via a resistive divider - back to the feedback input (pin 4) of IC1.

In this circuit, however, Q1 senses the current flowing through R1 and produces a corresponding voltage across R3. This voltage is then fed to pin 4 of IC1. As a result, the the circuit regulates the current into a load rather than the voltage across the load. Only one adjustment is needed: you have to adjust VR1 for optimum stepper motor performance over the desired speed range. The simplest way to do this is to measure the motor current at its rated voltage at zero stepping speed and then adjust VR1 for this current. The prototype worked well with a stepper motor rated at 80O per winding and a 12V nominal input voltage. Some components might have to be modified for motors having different characteristics.

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