lm317 fixed voltage regulator that take an unregulated voltage on the input and supply a tightly regulated voltage on the output and Configuration of the LM317 circuits
LM317 Adjustable Voltage Regulator
|LM317 Voltage Regulator|
I talked about fixed voltage regulators that take an unregulated voltage on the input and supply a tightly regulated voltage on the output.
To create any regulated voltage you want all the way up to thirty seven volts, you can use it as a power supply for a lot of projects such as a fan speed controller, and you can get an LM317 from anywhere that sells electronics. Sometimes they are called LM317T or LM317AT, once you've got it put a heat sink on it.
Configuration of the LM317 circuits
You need to put a couple of capacitors on the input and output pins, Put a 0.1 microfarad ceramic capacitor on the input and a one microfarad electrolytic capacitor on the output, and “Remember to use capacitors rated higher than the voltages that you are working with”.
If you want to use more capacitance that is fine too. These capacitors are not always necessary, but putting them in there 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 an LM317.
Let's make R1 510 ohms and let's use a 10 k 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 two or three volts higher than the output voltage. Otherwise the output voltage is limited.
For example: I have a twelve volt lead acid battery and it is charged up to 13.5 volts, When I use it to power my adjustable regulator I can get an output range from 1.25 volts to 12.5 volts when it is under a small load.
The headroom I need is called the "dropout voltage" and it will always be given the datasheet.
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 up.
|Drop Voltage, lm317 datasheet|
Alright now let's say you don't want to use a potentiometer, you just want a fixed voltage using two resistors.
Here's the formula I showed you earlier. If you are curious about how the equation is derived, read the LM317 datasheets from National Semiconductor and Texas Instruments.
In order to simplify the equation let's pick a value for R2, let's make it 4.7 k ohms 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 9 volt output. Inserting that into the equation gives you R1 = 782 ohms and R2 is still the 4.7 k ohms we chose. So I put a 680 ohm resistor in series with a 100 ohm resistor which gave me 780 ohm and that was close enough to give me 9 volts.
Talk about wasted heatThe LM317 is a linear voltage regulator and the efficiency equations are exactly the same as the 7805's equations. The bigger the difference between the input voltages in the output voltage, the more heat gets produced. Let's say you have your output set to 6 volts so you can get more speed and torque out of a servo and let’s power it from a nickel metal hydride battery charged to 8.4 volts and let's say the servo draws one ampere.
The heat generated in the LM317 would be 2.4 watts now if you powered the LM317 from a higher voltage source like a lead acid battery charged to 13.8 volte all of a sudden things get a lot hotter, 7.8 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.