Today I'm going to explain what a constant current source is, and show you how to use
one to safely power LEDs and laser diodes. This video builds upon material I covered
in my voltage regulator tutorials so make sure you have watched those first.
Most of the time in electronics you will find yourself working with constant voltage sources
Wall warts, voltage regulators, and other types of power
supplies can all have small variations in voltage, but they are basically
constant voltage sources. So what is a constant current source? Well,
as the name implies, it's a circuit that creates a constant current regardless of what the
load is. This is the schematic symbol of a current source connected to a resistor. If
you have an ideal 1 amp constant current source it doesn't matter if you connect it to a 1
ohm resistor or a 100 ohm resistor, it will always shove 1 amp through the resistor. Don't
want to use a resistor? Put an LED in the circuit, and regardless of what color the
LED is, it will push 1 amp through the LED. This may or may not be a good thing, depending
on how much current your load can handle.
Certain devices, like high powered LEDs, are best used when they are fed a constant current,
because they will maintain the same brightness regardless of how much they heat up. You could
also use a constant current source to drive a string of LEDs in series. Here I have a
12 mA constant current source I built and it doesn't matter how many LEDs I put on it,
they all get 12mA. So how do we build this? Well, the easiest
way to build a constant current source is using an LM317. I'll put a link in the video
description section. Here's the circuit diagram - it's very simple.
You connect your power source to the input pin, and then you have a resistor between
the output and the adjustment pins on the LM317. A pair of 10uF capacitors will help
stabilize the current source during sudden load changes. The value of the resistor determines
what the constant current will be. In the example I showed you, I used a 100
ohm resistor, which gave me a theoretical current value of 12.5mA. Now let's see what
we are getting in real life! Here I'm pushing 12.6mA through an LED. If
I add another LED, I still get 12.6mA. Let's add a laser diode to switch things up. It
doesn't matter what the load is, the current stays the same. And the other cool thing is
that even if the input voltage increases to 24V, the current stays constant. The input
voltage is still important though. If we go from 12V down to 5V, we no longer have enough
voltage to turn on all the LEDs. You need to make sure that the input voltage is at
least 2 volts higher than what the load requires to overcome the dropout voltage issue. I explained
this in more detail in my video about voltage regulators.
Now how does all this magic work? Isn't the LM317 supposed to be an adjustable voltage
regulator? Well, it is! But we are using a special trick to make it behave like a constant
current source. Inside the LM317 is some special circuitry
that creates a feedback loop. This feedback loop is set up
to maintain a difference of 1.25 volts between the adjust pin and the output pin. If the
input voltage suddenly changes, it doesn't matter. The LM317 quickly adjusts the output
voltage to maintain a difference of 1.25 volts between the adjust pin and the output. If
the load changes, it doesn't matter, the LM317 reacts and maintains the 1.25 volt difference.
You can use this behavior to build an adjustable voltage source,
or you can use it to build an adjustable current source.
This equation is basically ohm's law. Volts divided by resistance = current, and since
you know the LM317 is giving you a 1.25 volt difference, by changing the resistor value,
you can choose the current. Now there is one more thing you should know
about. The LM317 is a linear voltage regulator, and our current is passing through a relatively
large resistance value. In other words, this design is not very efficient. I wouldn't recommend
using it for currents higher than 200mA, otherwise your components are going to get really hot.
If you want more current, search for switch mode constant current supply designs, or visit
Amazon for a premade solution. Thank you for watching, and check out my other
videos if you want to learn more about electronics!