# Lm317 Adjustable Voltage Regulator Tutorial

So in a previous video I talked about fixed voltage regulators that take an unregulated voltage on the input and spit out a tightly regulated voltage on the output. For example, the popular 7805 is used to create a constant 5, volt supply in this video I'm going to show you how to use the lm317 adjustable voltage regulator to create any regulated voltage. You want all the way up to 37 volts.

You could use it as a power supply for a lot of projects, such as a fan speed controller. Furthermore, you can get a lm317. From anywhere that sells electronics, sometimes they're called lm317 T or lm317 8e. Once you've got it put a heat sink on it. So it doesn't melt your breadboard. The next thing to do is put a couple of capacitors on the input and output pins put a point 1 micro, farad ceramic capacitor on the input and a 1 microfarad electrolytic capacitor on the output, remember to use caps rated higher than the voltages that you're working with if you want to use more capacitance, that's, fine to these capacitors. Aren't always necessary, but putting them in their guarantees that the output voltage will be stable in almost any situation.

Next we set the output voltage using two resistors, labeled, r1 and r2. You can use this equation to figure out the output voltage. But to start out with I'll show you the quick and easy way to use a lm317 let's make our 1 510, ohms and let's use a 10 kilo, ohm, variable, resistor for r2 doing this gives us a constant voltage supply that we can adjust from 1.25 volts to 25 volts. But remember your input voltage needs to be at least 2 or 3 volts higher than the output voltage otherwise the output voltage is limited. For example, here I have a 12-volt lead acid battery, and it's charged up to 13 point 5 volts when I use it to power. My adjustable regulator I can get an output range from 1 point to 5 volts to 12 point 4 volts. When it's under a small load, the Headroom I need is called the dropout voltage.

And it will always be given in the data sheet. So anyway, this is a very. Common way to make an adjustable fan speed controller or test circuits at different operating voltages or see how many volts it takes to blow something all right now, let's say, you don't want to use a potentiometer. You just want to fix voltage using two resistors here's. The formula I showed you earlier. If you're curious about how the equation is derived read the lm317 data sheets from national semiconductor and Texas Instruments in order to simplify the equation, let's pick a value for r2 let's. Make it four point, seven kilohms.

And now the equation becomes much easier next. You can rearrange the equation like this. Now you can find out the value of r1 based on the output voltage that you want. For example, let's say, you wanted a nine volt output plugging that into the equation gives you r1 equals 782 ohms and r2 is still the four point seven kilo ohms. We chose so I, put a 680 ohm resistor in series with a hundred ohm resistor, which gave me 780 ohms. And that was close enough to give me nine. Volts finally, let's talk about wasted Heat.

The lm317 is a linear voltage regulator. And the efficiency equations are exactly the same as the 7805. S. The bigger the difference between the input voltage in the output voltage, the more heat gets produced. So let's say, you have your output set to six volts. So you can get more speed and torque out of a servo and let's power it from a nickel metal, hydride battery charge to eight point, four volts and let's say, the servo draws one amp, the heat generated in. The lm317 would be two point, four watts.

Now, if you were to power the lm317 from a higher voltage source like a lead acid battery charge to thirteen point, eight volts, all of a sudden things get a lot. Hotter seven point. Eight watts is a lot of heat that you'll have to dissipate. So you should try to keep your input and output voltages fairly close to each other.

If you expect to be drawing a significant amount of current and that's it check out my other videos to learn more about other electronic. Components.

Dated : 09-May-2022