Accompanying slides to my video blog. Constant current sources implemented with LM317 and LM7805. See video: https://www.youtube.com/watch?v=zmrEXBHdSRo See video blog: https://www.youtube.com/channel/UCtUcgFWvhZbknS5LxvfbMrw

1. #6 Constant Current
Sources
Exploring the LM317 and LM7805 as a Precision
Constant Current source

2. What is a Current Source?
Provides a set and stable current
Should be Independent of Load or Voltage
Many types and implementations exist
Passive sources (resistors, Zener diodes)
Active Sources (linear regulators)
With and without negative feedback
Commonly used components in current sources
Opamps

3. LM317 as Constant Current Source
Basic Current output formula: Iout = Vref/Riset
LM317 uses a Vref of 1.25V which it always tries to maintain between Vout and
ADJ. Using this principle we can ensure that a constant voltage is maintained
across a fixed resistor.
Using Ohm’s Law (I = V/R) we can
determine the Current that flows.
Examples using different Resistors:
1.25/125 = 0.01A
1.25/250 = 0.005A
1.25/250 = 0.0025A

4. Measured Current on Breadboard
LM317 using a 250Ω Resistor (0.1%) as a 5mA current source
Exactly as calculated! Without adjustment/calibration!

5. Resistor 125Ω 0.1% precision (Vishay PTF56)
Measured result: 9.96mA (-0.4% error)
Resistor 250Ω 0.1% precision (Vishay PTF56)
Measured result: 5.00mA (0% error)
Resistor 500Ω 0.1% precision (Vishay PTF56)
Measured result: 2.53mA (+1.2% error)
Notes: Powered by 6V battery supply (4x 1.5V AA). Measured using a UNI-T UT61B Multimeter which has
a DC Current accuracy of ±1.2%+3 in the Range of 40mA
So the Multimeter’s inaccuracy can be attributed to the error in measured results
Measured results of the LM317 as a current source

6. LM7805 as a constant current source
Basic formula: Iout = (Vout/Riset) + Io Bias current
Vout is normally close to 5V, varies slightly according to Load and input voltage
I Bias current is varies according to Vin
I Bias was measured a 2.8mA
If we want 10mA we can calculate using the formula:
0.01 = (4.98/Riset) + 0.0028
Riset = 4.98/0.0072 = 691.66Ω
Riset is Implemented as:
270Ω 1% + 500Ω 10-turn trimmer
This is a more complex and less stable setup as it does vary slightly due to load

7. LM7805 using a 270Ω and 500Ω
10-turn trimmer to output a set current
of 10mA
To confirm the results, we can plug in
the measured component values into the formula:
Iout = (Vout/Riset) + Io Bias current
Where Riset = 717Ω, Io bias = 2.8mA, Vout = 4.98
Iout = (4.98/717) + 0.0028 = 0.009745A or 9.75mA (-2.5% error)
Multimeters accuracy values of ±1.0%+2, ±1.2%+3 and ±0.8%+10 for Resistance, Current and Voltage
when added together can account for the error in the calculated values vs the measured value.
Measured current on a breadboard

8. Conclusion
It may seem logical to use trimmers everywhere, however they can also drift over
time and usually incur a worse Temperature coefficient that fixed type parts.
These two techniques explained, are cheap and easy ways to implement constant
current sources.
Many other constant current circuits exist that have their own challenges:
https://en.wikipedia.org/wiki/Current_source
I hope you learnt some basic techniques that you can apply in your own circuits.
Good Luck!

9. Further links and references
Current Sources tutorial: https://www.youtube.com/watch?v=lnU5d-KBMLg
Bob Pease at National Semiconductor shows several examples of constant
current sources (old video): https://www.youtube.com/watch?v=Sv21wD3FRmY