Voltage regulator to maintain constant amps

[nheugel]

I am trying to design a circuit that has 20 mA acoss an LED, I had been advised I could use a voltage regulator to keep the current at 20 mA. However, I am not exactly sure as to how to set up the circuit. Would I set it up so that it would maintain a voltage and then run that voltage across a resistor to keep the current constant? or is there some other configuration I should be using?

 

[vk6kro]

Yes, the easy way would be to use a 3 terminal regulator to produce 5 volts then use a resistor to limit the current to 20 mA.

A suitable regulator would be a 7805. These are available very cheaply in most electronic component stores or on Internet.

The resistor will depend on the type of LED, but a red LED would drop about 1.5 volts.
So, this leaves 3.5 volts across the series resistor.

Using Ohms Law, the resistor has 3.5 volts across it and 20 mA ( 0.02 amps) flowing through it, so it has a value of (3.5 / 0.02) or 175 ohms.

You can't normally buy 175 ohm resistors, but you can buy 150 or 180 ohm resistors. 180 is closest, but you can check the current with a multimeter.

 

[Averagesupernova]

It IS possible to build a constant current source using a 7805, or about any 780X regulator. Pick your regulator, (7805 for sake of discussion), pick your current (.02 amps for sake of discussion). Put a 250 ohm resistor from the output pin to the ground pin except DON'T ground the ground pin. Instead, put the LED from the ground pin to ground. No matter what type of load or LEDs in series for instance that you put here, the current will always be .02 amps unless the load resistance gets so high that the regulator runs out of voltage headroom. Might want to watch capacitive loads though. Be careful there.
-
Advantage of such a circuit? I once needed something like this to run an LED array that drew around .25 amps with a developed voltage across the bank considerably higher than the standard 1.5 to 2 volts that a normal LED drops, like closer to 6. So in order to guarantee the same array brightness I used the above described scheme with a low dropout 5 volt regulator. This prevented the input voltage to the regulator which could vary between 11 and 14.5 volts from changing the LED intensity. Now arguably I could have just went with a low dropout 10 volt regulator and picked a suitable current limiting resistor, but variations in the LED array would have had much more of an effect on the brightness.

 

[schip666!]

Whoa...Kewel idea...
Never occurred to me that it would be so easy to turn a voltage reg into a current regulator...
thanks!

 

[sophiecentaur]

If you're only feeding a humble LED you only need to use a transistor in ce mode with an emitter resistor to define the current. Cheaper and you'd learn more about the way things work, I'd bet.

But a series resistor and a voltage source of a few volts higher than the LED operating voltage is all that's usually required