# Constant current dimmer for LED

Discussion in 'The Projects Forum' started by embpic, Jul 14, 2014.

1. ### embpic Thread Starter Member

May 29, 2013
187
3
I have doubt that what is constant current dimmability???
Means i want to dim the LED but the keeping the constant current then what mechanism i have to follow.
So which are the way to LED dimmer with constant current.

2. ### urb-nurd Member

Jul 9, 2014
269
3
Im not too sure, but given R is constant, you cannot vary V without changing I/ vary I without changing V

3. ### Maxfooo New Member

Aug 24, 2013
17
2
My best guess (a simplified version) would be using a current driver with a mosfet or bjt and then adjusting a resistor at the collector. If the LED were in series before that resistor, you would be adjusting the voltage at the cathode of the LED but still inducing the same current via the transistor set-up.

This, however, seems like a very non-ideal approach to dimming an LED because you may have to play in the voltage break down region of the LED for high brightness, so I doubt my way would be correct.

4. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,784
973
Many LED drivers simply adjust the current level to achieve dimming.
The will typically have a dimming input that accepts a 0-10V DC or 10V PWM or simple resistor/pot attachment. The signal on that dimming input adjusts the output current to provide dimming.

example...
10V on dimming input = 100% current (1A)
5V on dimming input = 50% current (500mA)
etc...

5. ### crutschow Expert

Mar 14, 2008
13,466
3,353
Constant current just means that the impedance of the source is high so that variations in the LED voltage don't affect the LED current, not that you don't ever change the current. To dim the LED you reduce the constant current value.

LEDs need a current source drive which can be a resistor in series with a voltage (which is less than ideal) or a more complex constant current circuit.

6. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,434
1,624
As far as I know the color of a LED will change with different currents, especially white LEDs. If one wants to dim a LED and keep the color the same you use a pulse width modulation (PWM) of the max (100% on time) current.

This is sometimes referred to a constant current and sometimes as PWM.

Here is one driver to do that. It is for large voltage sources (I used it with a 10V to 28 supply) and a transistor to burn off the excess voltage. It accepts the PWM signal directly, and the pulse level and R1 sets the current.

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May 29, 2013
187
3