LM317T Variable Voltage Regulator

The LM317T is a adjustable 3 terminal positive voltage regulator capable of supplying in excess of 1.5 amps over an output range of 1.25 to 37 volts. The device also has built in current limiting and thermal shutdown which makes it essentially blow-out proof.

Output voltage is set by two resistors R1 and R2 connected as shown below. The voltage across R1 is a constant 1.25 volts and the adjustment terminal current is less than 100uA. The output voltage can be closely approximated from Vout=1.25 * (1+(R2/R1)) which ignores the adjustment terminal current ``but will be close if the current through R1 and R2 is many times greater. A minimum load of about 10mA is required, so the value for R1 can be selected to drop 1.25 volts at 10mA or 120 ohms. Something less than 120 ohms can be used to insure the minimum current is greater than 10mA. The example below shows a LM317 used as 13.6 volt regulator. The 988 ohm resistor for R2 can be obtained with a standard 910 and 75 ohm in series.

When power is shut off to the regulator the output voltage should fall faster than the input. In case it doesn't, a diode can be connected across the input/output terminals to protect the regulator from possible reverse voltages. A 1uF tantalum or 25uF electrolytic capacitor across the output improves transient response and a small 0.1uF tantalum capacitor is recommended across the input if the regulator is located an appreciable distance from the power supply filter. The power transformer should be large enough so that the regulator input voltage remains 3 volts above the output at full load, or 16.6 volts for a 13.6 volt output.

LM317 Data Sheet


LM317T Voltage Regulator with Pass Transistor

The LM317T output current can be increased by using an additional power transistor to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In the figure below, the pass transistor will start conducting when the LM317 current reaches about 1 amp, due to the voltage drop across the 0.7 ohm resistor. Current limiting occurs at about 2 amps for the LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and produce a 700 millivolt drop across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amps. The input voltage will need to be about 5.5 volts greater than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or greater filter capacitor is needed. The power transformer should be large enough so that the peak input voltage to the regulator remains 5.5 volts above the output at full load, or 17.5 volts for a 12 volt output. This allows for a 3 volt drop across the regulator, plus a 1.5 volt drop across the series resistor (0.7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will reduce the input requirements, but not much.

High Current Regulated Supply

The high current regulator below uses an additional winding or a separate transformer to supply power for the LM317 regulator so that the pass transistors can operate closer to saturation and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055 pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm) equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used that is a few volts higher than the output voltage. The LM317 will provide over 1 amp of current to drive the bases of the pass transistors and assumming a gain of 10 the combination should deliver 15 amps or more. The LM317 always operates with a voltage difference of 1.2 between the output terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base. The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904 base when the output is 5 volts. For 13 volt operation, the 1K resistor could be adjusted to around 3.6K. The regulator has no output short circuit protection so the output probably should be fused.

Simple Adjustable Voltage Source

A simple but less efficient methode of controlling a DC voltage is to use a voltage divider and transistor emitter follower configuration. The figure below illustrates using a 1K pot to set the base voltage of a medium power NPN transistor. The collector of the NPN feeds the base of a larger PNP power transistor which supplies most of the current to the load. The output voltage will be about 0.7 volts below the voltage of the wiper of the 1K pot so the output can be adjusted from 0 to the full supply voltage minus 0.7 volts. Using two transistors provides a current gain of around 1000 or more so that only a couple milliamps of current is drawn from the voltage divider to supply a couple amps of current at the output. Note that this circuit is much less efficient than the 555 timer dimmer circuit using a variabe duty cycle switching approach. In the figure below, the 25 watt/ 12 volt lamp draws about 2 amps at 12 volts and 1 amp at 3 volts so that the power lost when the lamp is dim is around (12-3 volts * 1 amp) = 9 watts. A fairly large heat sink is required to prevent the PNP power transistor from overheating. The power consumed by the lamp will be only (3 volts * 1 amp) = 3 watts which gives us an efficiency factor of only 25% when the lamp is dimmed. The advantage of the circuit is simplicity, and also that it doesn't generate any RF interference as a switching regulator does. The circuit can be used as a voltage regulator if the input voltage remains constant, but it will not compensate for changes at the input as the LM317 does.

2 Cell Lithium Ion Charger

This circuit was build to charge a couple series Lithium cells (3.6 volts each, 1 Amp Hour capacity) installed in a portable transistor radio.

The charger operates by supplying a short current pulse through a series resistor and then monitoring the battery voltage to determine if another pulse is required. The current can be adjusted by changing the series resistor or adjusting the input voltage. When the battery is low, the current pulses are spaced close together so that a somewhat constant current is present. As the batteries reach full charge, the pulses are spaced farther apart and the full charge condition is indicated by the LED blinking at a slower rate.

A TL431, band gap voltage reference (2.5 volts) is used on pin 6 of the comparator so the comparator output will switch low, triggering the 555 timer when the voltage at pin 7 is less than 2.5 volts. The 555 output turns on the 2 transistors and the batteries charge for about 30 milliseconds. When the charge pulse ends, the battery voltage is measured and divided down by the combination 20K, 8.2K and 620 ohm resistors so when the battery voltage reaches 8.2 volts, the input at pin 7 of the comparator will rise slightly above 2.5 volts and the circuit will stop charging.

The circuit could be used to charge other types of batteries such as Ni-Cad, NiMh or lead acid, but the shut-off voltage will need to be adjusted by changing the 8.2K and 620 ohm resistors so that the input to the comparator remains at 2.5 volts when the terminal battery voltage is reached.

For example, to charge a 6 volt lead acid battery to a limit of 7 volts, the current through the 20K resistor will be (7-2.5)/ 20K = 225 microamps. This means the combination of the other 2 resistors (8.2K and 620) must be R=E/I = 2.5/ 225 uA = 11,111 ohms. But this is not a standard value, so you could use a 10K in series with a 1.1K, or some other values that total 11.11K

Be careful not to overcharge the batteries. I would recommend using a large capacitor in place of the battery to test the circuit and verify it shuts off at the correct voltage.


One or Two Cell Li-Ion Battery Charger

Another charger idea is to use a regulated power supply set for the full charge battery voltage and a resistor to limit current. It doesn't supply constant current and requires about 30% more charge time, or around 4 hours. A constant current charger might reduce that to 3 hours but would require more parts.

A charge current indicator LED could be added as shown in the lower left of the drawing. The LED turns off when the charge current is less than about 35mA and the voltage drop on the 18 ohm resistor is around 600mV or less. A test run required 260 minutes for the LED to go out which should indicate about 85% full capacity, but not sure. More information can be found at:

Lithium Ion Article at Battery University.com

Voltage    Capacity    Charge time       Capacity with
                                        full saturation
3.8         60%        120 Min           65%
3.9         70%        135 Min           76%
4.0         75%        150 Min           82%
4.1         80%        165 Min           87%
4.2         85%        180 Min           100%

Circuit Details:

When the battery is low, the voltage at the reference pin of the TL431 will be less than 2.5 volts, causing the TL431 to switch off, increasing the transistor base voltage and charge current. Current is limited to around 300 mA by the 18 ohm resistor (2 cell setup). As the battery approaches full charge, the TL431 reference pin approaches 2.5 volts, increasing the TL431 current and reducing the transistor base voltage and charge current. Using 2 cells (8.2 volts, 1000 mAH), the current drops from 300mA to about 100mA as the charge reaches 75% of capacity in 200 minutes. Another hour is needed to bring the charge to 85% Note, the value of 4.1 rather than 4.2 was chosen for a little more margin, and less stress on the battery at full charge. From the data above, it looks like only 5% of capacity is lost. The diode prevents a reverse voltage across the e/b transistor junction in the event the power supply connections are shorted while the battery is still connected. The 220 ohm resistor was selected for about 20mA base current. The minimum transistor gain is 30, so 20mA should produce at least 600mA. The open circuit output voltage is set with a voltage divider for either 4.1 or 8.2 volts. Two jumpers are used to select the desired voltage and current limit.

For example, to charge a single Li-ion cell to 4.1 volts, the current through the 10K resistor will be
(4.1-2.5)/ 10K = 160 microamps. The series combination of the other 2 resistors should total 2.5 / 160 uA = 15625 ohms. A 15K in series with 620 might be used, and the 620 adjusted to compensate for the 15K being slightly more or less. I ended up 15K and 750 since the 15K was a little low.

In the 2-cell (8.2 volt) case, two additional resistors are added in parallel with the 15625 (using the jumper) to increase the output voltage from 4.1 to 8.2. I ended up with 5.6K in series with 430 ohms. The 430 can be adjusted to get it just right.

A second jumper is used (across the 12 ohm resistor) to maintain approximately the same charge current with either single or double cell operation. Both jumpers are installed for 8.2 volt operation and removed for 4.1 volt operation. Note: The circuit board picture shows two 5 watt 12 ohm resistors. One of the resistors is out of tolerance and is actually 17 ohms.

Caution: Be careful not to set the jumpers for 8.2 volt operation while connected to a single cell (4.1 volt) battery. Use a DMM to verify the open circuit voltage is what you want before connecting the battery.

Telephone In-Use Indicator


Telephone In-Use Relay Controller


Astable Multivibrator