Here is another must need circuit for the DIYer. We have seen the 7805 5v voltage regulator, but sometimes that is just not enough. The LM317 is an integrated circuit that can have an output voltage of anything from 1.25 to 37 volts. The above circuit though is limited to 1.2v to 25v. This circuit can handle a load of approximately 1 amp. If a heat sink is added, it can handle around 1.5 amps. In this circuit, the 240 ohm resistor is fairly universal, and if the 5k ohm variable resistor is changed, the output voltage can be regulated. The output voltage can be found by using the equation Vout=1.25V(1+R1/R2)+Iadj(R2). The input voltage needs to be at least 1.5v higher than the output voltage. The 0.1 microfarad capacitor evens out any ripples, and the 1 microfarad capacitor gives the power supply a better transient response.
Anyway, that was kind of techincal, but this device can be used in a number of applications. There are many times that different voltages are needed. This device can be set with constant resistors to make a constant power supply, or if a potentiometer is used, it can be used to make a variable power supply. A voltage calculator can be found at electronics-labs. It will calculate what output voltage will be supplied by the different input resistances.
The data sheet can be found at [Link]
3 Responses to “Variable Voltage Regulator”
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I believe you’ll find that the maximum output current depends on the difference between input and output voltage. For linear regulators, Iin is about the same as Iout, but Vin is higher than Vout. Thus, the power dissipated by LM317 is Iin * (Vout-Vin). The result is in Watts.
The maximum operating temperature of LM317 is 125C, and we’ll make ambient temperature 50C. This means the temperature rise due to power dissipation may be up to 75 degrees.
The datasheet I have gives the “Thermal Resistance, Junction-to-Ambient (No Heat Sink)” as 50 degC/W for the TO-220 case. That means permissible heat dissipation is 1.5W. So if you’re dropping from a 24V input to a 5V output, you’re limited to about 80mA (1.5W / 19V = 79.95mA).
With an infinite heatsink, you can use the figure for “Thermal Resistance, Junction-To-Case” of 4 degC/W, or 18.75W. Now, if you’re dropping from a 24V input to a 5V output, you’re limited to around 1A (18.75W / 19V = 986.8mA).
Now, if you have a 9V input and need a 5V output, the drop is 4V instead of 19V. That changes things quite a bit. With no heatsink, you can get 1.5W / 4V = 375mA and with an infinite heatsink you can get over 4A (except I believe in this case there is additional current-limiting circuitry in the LM317 which will give you a lower current limit than this).
Another use I’ve made of the LM317 is a constant-current regulator to drive LEDs. Basically, compared to the above diagram, replace R1 with a current control resistor, and R2 with the “load” (one or more LEDs). The current is 1.2/R1, so a 60 ohm resistor gives 20mA current. As long as the input voltage is about 3V higher than the voltage drop across the LEDs, you’ll run them at exactly the right current. When the input voltage is lower, the output voltage will decrease until the LEDs eventually go out. For more about this circuit, see my blog entry about it: http://emergent.unpythonic.net/index.cgi/projects/01111254927
Jeff, that is some good information. Thanks for posting it.
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