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Current limiting LED's in musical light show

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Alex M

Mon Aug 26 2013, 07:09AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi,

So I have some LED's being driven by an audrino that makes them react to music via USB, at the moment I am using regular 20mA LEDs and a small resistor to limit the current.

But I want to be able to use some power LED's instead but am unsure how I should go about limiting the current. I know constant current should be used with power LED's like this but how do I go about doing that when the LED's are reacting to the music? I tested limiting the power LED's current with a power resistor, but the resistor got quite hot after a while as expected.

Some LED driver IC's I have seen have a "PWM dimming" function, but I am not sure if that would be suitable since in this application the LED's are being driven in such a way that they will be off for a decent amount of time (like a strobe that reacts to the music).

I came across IRS25401PBF on farnell for under Â£2, but again I am not sure if its built in PWM function will work this way.

I did try this sort of design below but found that changing the supply voltage would also change the load current, so its not really constant current at all. Even adjusting the supply voltage slightly (a volt etc) will make the current change by a large margin.

I just cannot work out why it changes with supply voltage, since everywhere on the internet says it should stay constant

So if anyone has got any ideas as to how to current limit power LED's in musical light displays then I would appreciate it greatly since I am going mad. Maybe there are some IC's out there that can do the job? The LED's I intend to use are those 1watt types with currents of 400mA.

Thanks.

Sulaiman

Mon Aug 26 2013, 08:49AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For the circuit shown .. it should work well provided:
1) the supply voltage needs to be at least 1 Volt greater than the led voltage to allow for the base-emitter voltage of T2 + collector-emitter saturation voltage of T1
2) The current through R1 should be a little more than the current through RS divided by the current gain of T1, the votage across R1 is supply voltage -Vbe of T1 - Vbe of T2
e.g. Supply = 5V, led current = 350 mA,
T1 current gain = 35 at 350 mA, T1 Vbe = 900 mV at 350 mA, T2 Vbe = 0.7 V
RS = 0.7V/0.35A = 2 Ohms
T1 base current = 350mA/35 = 10 mA
R1 = (5v -0.9v -0.7v)/10mA = 3.4V/10mA = 340 Ohms ... use R1 = 330 Ohms
OR,
for your 20 mA led, assuming transistor gains of 100 and base-emitter voltages of 700 mV;
RS = 700mV/20mA = 35 Ohms .. use 36 Ohms
R1 = (5v - 0.7v - 0.7v)/0.2mA = 18 kOhm

If the supply is increased from 5v the led current should remain fairly constant
BUT
if the supply current is reduced from 5v the led current will reduce as just enough current flows throug R1 at 5v.
So design for the lowest voltage that is expected with the lowest transistor gain expected
and the led current will remain constant for higher voltages.

Due to Vbe of T1 changing with temperature (approx. -2mV/C) the current will reduce as ambient temperature increases, which is good as the led cooling will be worse at higher ambient temperature.

I would not bother with constant current circuits,
use a regulated supply and a series resistor .. more than adequate.
If you intend to use several leds per colour use them all in series with a sutiable supply voltage and a single transistor switch.
e.g. 3x 1W led in series = c10v, use 12V supply and a 5.6 Ohm 1W (minimum) resistor
if you use a darlington transistor then it may drop over 1v at 350 mA so maybe a 15v supply with a 10 Ohm 3W resistor would be more suitable.
If you use a FET choose one that will be fully turned on by the output voltage of your controller.

johnf

Mon Aug 26 2013, 09:57AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

What exactly was vbat current sources require infinte voltage to make them work and usually will start to work on properly on 12 -15 volt rails

Alex M

Thu Aug 29 2013, 08:35PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

@Sulaiman
Thanks. Will have another crack at it using your extensive advice and see how it goes.

But regarding not using constant current are you saying that it should be ok just to not use it? as I always thought high power LED's like this will thermal run away. I also tried using a MOSFET (logic level) too but found that the current could be adjusted by simply putting my hand close to the LED leads, no amount of ferrite beads could tame it.

Could I add some loss at the FET in order to prevent the high impedance of the gate from being turned on by noise pick-up? the source of the noise was 50hz. Normally I put a 10k from the gate to ground but don't really know how it would effect operation of this circuit.

@johnf I was using a regulated 5V supply for the LED's (not the same as the audrino which was using USB power but it all used a common ground).

Dr. Dark Current

Fri Aug 30 2013, 03:42PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Just a note, the transistor will produce the same amount of heat as the resistor, and will require a larger mounting space for the heat sink than using a resistor.

Alex M

Fri Aug 30 2013, 04:42PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Dr. Dark Current wrote ...

Just a note, the transistor will produce the same amount of heat as the resistor, and will require a larger mounting space for the heat sink than using a resistor.

I am aware of this but thanks. The main concern in this little project is keeping the LED's max current to a max peak value, a few watts being dissipated as heat won't hurt (famous last words).

Sulaiman

Fri Aug 30 2013, 09:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Powering a led from a constant voltage supply with no series resistance will lead to thermal runaway, BUT
you will be using some additional series resistance, with a couple of volts across the resistor
so when the led(s) heat up the voltage across the series resistor will rise, and with it the current, but the few tens of mV due to the negative temperature coeficient of the leds is small compared to a few volts, so the system will stbilise.
SO, don't use too low a voltage across the series resistor ... allow some inefficiency.

Of course a true constant current supply is better but not always worth the cost/complexity.

If you use a circuit like the one originally posted, with suitable component values as described, instead of connecting R1 to +supply connect it to your controller output,
use a darlington transistor for T2 to reduce the load on the controller.
A FET would be difficult to drive directly as the controller would have to supply the gate-source voltage + the 0.6v to 0.7v of T1 base-emitter voltage.

Erlend^SE

Fri Aug 30 2013, 10:06PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

What's the scale? maybe some LED-driver IC's could be used? Lots of I2C/SPI IC's. They do PWM and and some even do current regulation for you. You can even get them as LED pixels or chains if you look some around.

b.t.w. found out that 200 RGB LED's = quite a bit of current(and power). (20-50+ full updates per secound, 16 MHz AVR microcontroller, 600 channels), 2 pins.

Alex M

Wed Sept 04 2013, 05:13AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

@Sulaiman

I gave it another shot and had some more success this time, it still vary's a bit with supply voltage though by about 30mA going from 6-17v.

However as long as the supply voltage was constant I could change the load impedance and the current would stay the same (not accounting for any thermal related changes, but that is a while other thing I need to look into).

I would have uploaded a photo but I keep getting a blank screen...

@Erlend^SE I am not sure what is meant by scale? its just an audrino controlling some LED's via PWM (this guys project

skip to 0:45 but with bigger LED's). These LED's are 400mA each so a bit much for many LED driver IC's out there.

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