Lm317 Current Limit Circuit

Versatile solution for your power supply needs.

by Hugh Welis W6WTU

hen 1 get involved in projects, I never seem to have enough power supplies to go around. When one's available in my shack, it's usually the wrong voltage, or it lacks a feature 1 need for the project. Building this versatile supply solved my power supply problems.

Multiple Applications

This power supply provides a fixed 5-volt output suitable for TTL logic and a variable voltage output ranging from about 1.2 volts to 21 volt,%, which is suitable for almost anything else. In addition, variable current-limiting protects the power .supply components and the project.

For example, you can set the maximum desired current for charging a NiCd battery. You can also use it to tune up an RF power amplifier, since it's desirable to limit the maximum circuit current draw: to a sate value to protect the transistor.

You can measure current and voltage with one meter. Two meters may he more convenient tor simultaneous measurements, but this one is easily switched. If you buy all the parts new, the supply costs about S53,

When switched to current, the meter indi-catcs the total current drawn from both the fixed and variable circuits. In the voltage position, the meter indicates the voltage output only from the variable regulator.

The Circuit

The power supply was designed around the ever-popular LM317 and LM 340-5 (7805), three-terminal regulator ICs See Figure 1. In

Figure 2, a pictorial wiring diagram gives a conversion perspective from a schematic to f i the actual hardware wiring. Color-coded wires in the diagram indicate the circuit relationship between the schematic and pictorial diagrams. See the table for a separate component listing.

The operation of each regulator is conventional for three-lead devices, except for the current-limiting feature added to the LM317 circuit.

The LM317

Before describing the current-limiting feature of the supply, it is worthwhile reviewing

Photo A. Front panel of the supply*

Photo B. internal wiring; note the position of the L/Ê317P site of the only critical factor in assembly*

the voltage control characteristic of the LM317. The three terminals ol the LM317 regulator are input, OUTPUT? and adjust. At the lowest regulated output, the voltage differential between the input and OUTPUT terminals must not exceed 40 volts. In other words, the power transformer's output must not excccd 28 Vims, which at peak would produce 40 VDC across capacitor C6. Typically, an 18-26.5 volt transformer would be used with the LM317. When the output voltage is adjusted to the highest amount, the voltage differential from in put-to-output of the regulator is at the lowest possible value. At that time, the regulator must have an input-to-output differential fheadroom) of at least 3 -4 volts to remain in regulation, The third terminal, adj, of the LM3I7 is for controlling the input-output voltage differential to a value below the maximum regulated output voltage to a minimum of L2 volts. Regardless of the output voltage, approximately L2 volts (essentially constant) will be automatically developed between the apj and OUT terminals, if the adj terminal is

allowed to float (open circuit) along with the output terminal, the output voltage will rise to



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