Temperature Compensated Constant Current Source

[radnusmar]

I need to make a LED glow with constant current and voltage at wide range of temperatures, say 0-35 degrees.

Since the above circuit is going to be used for sensing, I need high precision constant current and voltage.

Ampere Rating = 25mA
Supply Voltage = 5v

What sort of circuit, I have to design? Is there any IC to serve this purpose?

Thanks in Advance

 

[chuckey]

The voltage across an LED changes with temperature. It is relatively easy to get a constant current, but are you sure that the light output is absolutely linked to the current? i.e. for a constant current over the temperature range that the light output will also be constant?
For a very stable current, drive the LED from a NPN transistors collector. Put a current metering resistor in the emitter circuit to earth. A good value would be 82 ohms (82 X .025 = ~ 1V). Now use a Op-amp with a bandgap reference (or a pot across the +5V) on its positive input, via a 10K, and return the negative input to the 82 ohms via 10K. The op amp will try to make the volt drop across the 82 ohm the same as the positive input, so the ability to adjust this must be designed in (10 turn pot?). It would be a good idea to put a temporary resistor in series with the LED ( say 180 ohms) tp protect the LED against current overload during setting up.
Frank

 

[radnusmar]

Could you explain about connecting the OPAMP to the NPN Transistor?

I couldnt understand that part...Could you help me out with a rough schematic?

Thanks for you reply,

With regards,

Ram

 

[glowitz]

There are two simpler solutions compared to using an op-amp. These are as follows:

1) Use a variable voltage regulator, such as an LM317 series. By placing a single resistor between the voltage output and the variable sense input, you get a constant current source based on the reference voltage, which is typically 1.25 volts. Therefore, if you want 25ma, then R=V/I or 1.25V/25ma = 50 ohms. This resistor only needs to dissipate 30 mW - so using an 1/8 watt or 1/4 watt resistor is fine. For more information on this configuration, search for LM317 current source -- or just go here: https://www.eleccircuit.com/current-source-with-lm317/

2) An elegant method that uses only a single component (in addition to the LED) is the relatively more obscure Current Regulator Diode (CRD) such as a CLD20B from Micro Semi. This particular one regulates at 20ma but you can find other brands and sizes for a range of currents. For more information, search for Current Regulator Diode. Here's another one with a target of 25mA constant current: **broken link removed**

3) Now, to a point raised earlier, do you know that the LED brightness at a given constant current is the same over temperature? I know that LED brightness changes with junction temperature (related to current/voltage) but not sure if this is held constant, whether the brightness also remains constant vs. temperature. This would require a little more research.