The Versatile LM317

In a previous video, I demonstrated how the LM317 works on the inside. Ultimately, we ended up making a larger version with a mosfet and an op amp. While this was a good introduction to the LM317 and linear regulators in general, there is still a lot of applications that the LM317 can be put into that extend beyond the standard variable power supply that we made, with some being more useful than others. So, in this video we will explore the various other applications of the LM317. Let’s begin.

If you haven’t watched the previous video, I recommend you do that before watching this one because it gives information on how the LM317 itself works and is made. Anyways, let’s start going over the various applications of the LM317.

The first applcation is very similar to what the LM317 is typially used for, and it involves a very similar IC: the LM337. The LM337 is nearly identical to the 317, except that it regulates negative voltages as opposed to the LM317’s positive voltages. Giving an example, we can use a center tapped transformer. Ignoring the center pin for a second, we can connect the transformer to a rectifier, which will give us a DC voltage. The center tapped pin is our ground, and therefore, will split our full voltage into halves, one positive and one negative. After adding two capacitors, we have our two rails. Using the positive and negative voltages and the LM317 and LM337, we can design a simple dual rail power supply. The datasheets for both regulators provide variable configurations that we can simply copy to regulate both rails. Dual rail power supplies are especially useful for audio and op amp circuits. Now, while I won’t be speaking much more about the LM337 throughout the rest of this video, just keep in mind that it can do the same things as the LM317, just in the negative voltage range.

The next item on the list is to deal with the minimum voltage of the LM317. If you remember back to the previous video, you will remember that the minimum voltage output from the LM317 is 1.25 volts because of the 1.25 volt offset inside. Well we can go all the way down to 0 volts using negative voltages. Looking at the typical schematic, we can see that the resistor divider is connected to ground at the bottom, and that will allow us to go no lower than 0 volts. However, if we replace ground in this case with -1.25 volts, we are able to bypass the voltage offset and make the LM317 regulate down to 0 volts. You can generate this negative voltage by using a LM337 for example. So, yes you can output 0 volts with a little work concerning negative voltages, although it can be quite tricky. The inverse is true for the LM337, just feed it a positive voltage instead. If you find all of this work excessive, you can choose another regulator such as the LT3080. It can go down to 0 volts without any negative input.

Now for an interesting application of the LM317: its ability to be a current source. Now typically, the LM317 is used as a voltage source, but with the proper external components, you can turn it into a current source. Before I explain how to make that current source, I will explain the difference between a voltage and a current source, since current sources are not nearly as common as voltage sources. You likely already know what a voltage source is, it is just a device that will maintain its output voltage however possible. They are quite common and you can find them in forms such as batteries or power supplies. A current source is basically the same except that it maintains its current as opposed to its voltage. Now current sources aren’t as common, but you can find them in ICs such as the LM334 or, in this case, a properly setup LM317. The constant current mode on a power supply is also a current source, it just limits its voltage to reach the desired current. By manipulating Ohm’s law we can turn the LM317 into a current source. Remember that with a higher voltage comes a higher current, and vice versa. So, if we can modify the output voltage so as to always output a fixed current, we will have made a current source. The datasheet recommends we do this by putting a resistor in series with the output and then connecting the other side to the adjust pin. This works, because the resistor will generate a voltage drop across itself, and the adjust pin will take that voltage drop to drive the output. The bigger the load, the lower the voltage, so that the current is maintained. In fact, the equation for this setup is the current equals 1.2 divided by the resistor. The the load is small on the output, such as a load of 100k ohms, the LM317 won’t be able to provide a high enough voltage to pass a constant current through it. You can get very small currents using this method, small enough that the fluke meter couldn’t measure it and I had to use the Brymen instead.

The next setup we can make use of is the slow startup configuration. Basically, in some configurations there may be a current in rush, possibly due to the charging of capacitors for example. And this current could cause other parts of the circuit to react in ways that we don’t want. Luckily, we can use just three more components to allow us to slowly ramp-up the output voltage on the LM317. And those three components are a 50 kOhm resistor, a PNP transistor, and a 25 uF capacitor. When we first start the circuit, the capacitor is completely discharged, so the PNP transistor will be ON and pulling the adjust pin to 0 volts. Meanwhile, the capacitor will be charging, and as it rises, it will cause the transistor to conduct less, and therefore cause the adjust and output pins to be higher. Finally, the capacitor will charge to the resistor divider’s voltage and shutoff the transistor. Once the power to the regulator is shutoff, or the regulator is adjusted to a lower voltage, the capacitor will discharge itself through the 50k resistor and the bottom potentiometer resistor. To have a faster turn-on time you can lower the capacitance. So if you are working with large output capacitors, you may want to look into using this circuit to prevent too large of a current inrush when you power on the device.

If you are curious on getting a deeper look into any of these applications, please have a look at the description, because I have included all of the schematics there. Overall, the LM317 is an extremely versatile IC, and is good for more than just the simple voltage regulation it is normally used for, and it does all of its jobs very well. I recommend you try some of these configurations to improve your own linear power supply or just learn how these circuits work. If you enjoyed this video please consider subscribing so that you can see the other videos that I create. Have a good one.