Constant Current Source

Thread Starter

Broncos

Joined Feb 2, 2012
33

JDT

Joined Feb 12, 2009
657
This is quite a high current even for a power LED. Usually they are wired in series strings running at 350mA or 700mA.

To reduce wasted power they are usually driven from a switching circuit. The current is controlled on a cycle-by-cycle basis.

See the attachment
 

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DickCappels

Joined Aug 21, 2008
5,933
To answer your question, the value of R3 determines the current. In operation, R3 is equal to the base-emitter forward voltage of Q1. Figure that voltage will be about 0.6 volts. The actual voltage depends on Q1 and R1.

R = 0.6/I = 0.6/3 = 2 ohms.
 

bertus

Joined Apr 5, 2008
20,099
Hello,

Do you have the specsheet / datasheet of the leds to use?
3 Amp is very unlikely for leds.
The highest current for high power leds I have seen is 1.4 Amp.

Bertus
 

Thread Starter

Broncos

Joined Feb 2, 2012
33
Hello,

Do you have the specsheet / datasheet of the leds to use?
3 Amp is very unlikely for leds.
The highest current for high power leds I have seen is 1.4 Amp.

Bertus
It's a Cree t6 led. I don't have easy acsess to finding data sheets right now on a mobile, but it should be a top google hit for the datasheet.
 

Thread Starter

Broncos

Joined Feb 2, 2012
33
To answer your question, the value of R3 determines the current. In operation, R3 is equal to the base-emitter forward voltage of Q1. Figure that voltage will be about 0.6 volts. The actual voltage depends on Q1 and R1.

R = 0.6/I = 0.6/3 = 2 ohms.
Doesn't that equal .2 ohms?
Is there a value of r1 or specific q1 that could help create lower tolerance for error? Like how does r1 change the voltage/ what value r1 should I use?
 

bertus

Joined Apr 5, 2008
20,099
Hello,

Did you read the datasheet?

Notes

Lumen Maintenance Projections

Cree currently recommends a maximum drive current of 2000 mA for XLamp XM-L white in designs seeking the ENERGY
STAR* 35,000 hour lifetime rating (≥ 94.1% luminous flux @ 6000 hours) or 25,000-hour lifetime rating (≥ 91.8%
luminous flux @ 6000 hours).
Please read the XLamp Long-Term Lumen Maintenance application note for more details on Cree’s lumen maintenance
testing and forecasting. Please read the XLamp Thermal Management application note for details on how thermal design,
ambient temperature, and drive current affect the LED junction temperature.

* These lifetime ratings are based on the current ENERGY STAR Solid State Lighting Luminaires V1.1 (December 12,
2008) and ENERGY STAR Integral LED Lamps V1.0 (December 3, 2009) lumen maintenance criteria.
The 3 Amp is the maximum that may be used.

Bertus
 

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Thread Starter

Broncos

Joined Feb 2, 2012
33
Hello,

Did you read the datasheet?



The 3 Amp is the maximum that may be used.

Bertus
I don't always want to drive it at 3 amps, but I want a mode where I can show off its muscle!
I was thinking using the circuit I linked to I would have a switch where I would lower the resistance down to where it would drive it at 3amps(or 2.8ish to be safe), normally I would drive it at 1 amp to 2.
 

Thread Starter

Broncos

Joined Feb 2, 2012
33
Showing off its muscle can very well mean showing how quick you can burn it out with LED's.
I just want a momentary mode where I can have the current near max, is driving it at 2.8-2.6 amps going to burn it out? or cut its life soo much from its advertised 25000 hour life at 2amps?

Really I'm just curious about building the circuit to drive it, its all I can think about the last few days, I just want to hear some positive feedback that the circuit will work, I can switch R3 resistors by connecting another resistor in parallel with a switch ( to change brightness), some theory, and how exactly to calculate the values I would need.
 

castley

Joined Jul 17, 2011
31
One transistor, a reference diode at the base and an emitter resistor should work.
The resistor vaue at the desired current should have a voltage accross it about
.65 V. lower than the reference diode connected between the base and the current output point.
 

DickCappels

Joined Aug 21, 2008
5,933
To answer Broncos, yes, you are correct, I left off the decimal point. The resistor should be 0.2 ohms. And yes, the exact current depends on Q1.

The circuit is not terribly stable, as the current will increase at about +0.33%/°C. But I think you will find the LED's output drifts about that much in the opposite direction.

This is not a particularly good regulator, but fine for an LED used for lighting. For laboratory instrumentation, you might want better. What I like about your original circuit is that the excess power is dissipated by a MOSFET, and they are pretty hard to kill with power (compared with a bipolar).

A note about operating current of the LED: Take note of the chart "Thermal Design" in the datasheet. Learn to calculate power dissipation and temperature rise (good things to understand for any electronic design) and then observe that chart.
 
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