10 watt Led Driver

[bryan]

All;

Have a 3-10watt leds that i want to bench for a test( forward voltage 10-12v, max current 900ma). I don't have any dedicated drivers for them I do have a 35vdc 1.5 amp switched mode power supply handy. For testing can I hook up the LEDS in series and use this power supply with a LM317 as a constant current regulator.

The LM317 has a 3v drop which should bring the output of the power supply down to 32 volts which should be fine for the forward voltage of the LEDS. The current can be regulated by the LM317

Any reason why this would not work at least for testing purposes.

 

[ronsimpson]

So the LED voltage for three parts is 30 to 36 volts. At 30V the LM317 will work as a constant current source. At 36V it will not.

 

[ChrisP58]

Ditto to what Ron said. You also need to anticipate the power that the 317 will dissipate. If the LED's are using 30 volts, the power in the 317 will be about 4.5 watts. If the LED voltage is less than 10V ea, the 317 will make even more heat.

 

[bryan]

So the LED voltage for three parts is 30 to 36 volts. At 30V the LM317 will work as a constant current source. At 36V it will not.

Ron, I don't understand what you mean, why the 317 would regulate current at 30v, but not at 36v?

 

[ronsimpson]

Worse case; the LEDs may need 36 volts. The power supply is 35 volts. The LM317 will not work in like this.
Likely the LEDs need 33 volts the the power supply is 35 volts. The LM317 can not regulate in like this.
Lowest possibility; the LEDs need 30V and the supply is 35V. The LM317 will work. (hot)

 

[bryan]

Thanks Ron, looks like I need to rethink the redesign.

Does this look feasible, I think the TL431 is only rated to 100ma though?

View attachment 66838

 

[ronsimpson]

think the TL431 is only rated to 100ma though?

View attachment 66838

The poor little TL431 may need to short out the 1A from the LM317. 1A and 30 volts = very hot!

You don't need the TL431. There is no need to regulate the voltage on a LED. LEDs are like Zener diodes. They pull little to no current until they hit their voltage. In your case 10-12 volts each. Then they pull current to keep the voltage down. Just like a zener.

Pull out the TL431 and resistors and replace them with a 33V +/-1V Zener diode and the LM317 will deliver 0.9A to the load.

 

[bryan]

Thank Ron, so with a 30v zener I can be assured the 317 won't allow more than 900ma because the zener regulates the voltage? Concerned because if the leds draw more than 30-32v then the 317 won't regulate the current as you posted earlier. Power supply is 35v - the 3volt drop from the 317, or have I got it all wrong.

 

[mvs sarma]

perhaps voltage regulation is not needed and the set of LEDs would drop Vf*n across themselves.
the LM317 would only waste power as Vdiff * Iload as regulated by itself being a current regulator.
IMHO,

if a voltage regulation is foreseen like 48V battery and 30V drop across LEDs
then another front end lm317 could be placed as voltage regulator just to share the difference
input say 48, 1st stage out could be 36, and second stage would pump a predefined current.

 

[ronsimpson]

If batteries are used you don't want to wast energy in heat. We normally use PWM, (switching power supply) to regulate the current with out power loss.

I have built boost supplies; input 24V @ 1.2A battery, output 30 to 32V @ 900mA. Constant current.
In the case you mentioned; 48V @ 590mA buck switching supply to 30 to 32V @ 900mA constant current.

 

[mvs sarma]

i always regard pwm suppplies for the led lighting jobs. As the discussion was around managingwith LM317, i was expressing myself taking an example of 48V.

Yes. we need not waste power using linear regulation, either constant current and votage in tandem.

 

[Pulak]

I bought a 5 watt led module and the dealer said the supply voltage is 12 volt. By connecting to 12v supply using 10ohms resistor and 12v 1w zener across and it display more than specified ... i.e meter reads 80 ma at 12.5v giving 10 watts of power for 5 watt led. Please help..

 

[mvs sarma]

while calculating wattage using current measured, you need to measure voltage across led and measure the wattage.
linear power supply with current dropping series resistors waste power across the resistor.
It would better to study the way the leds (if more than one)are configured.
possible that the board has two leds in series and two or three such chains in parallel.
Please study and provide a re-engineered schematic for meaningful discussion.
It should not be difficult.

 

[alec_t]

80 ma at 12.5v giving 10 watts of power

Check your maths .

 

[Pulak]

Thanks alec_t and mvs sarma a lot.
I am sending this picture.
Well I checked it again and it show 73.4 milliampere and 12.79 volt
So please guide me using LM317 to make this module perfectly.
NB: I bought a 5 watt led module and the dealer said the supply voltage is 12 volt.
So what should be the exact voltage and current for this module!

 

[alec_t]

LEDs don't have an "exact voltage" as part of their spec. They have a forward voltage (Vf) which can vary depending on manufacturing tolerance. The datasheet for the LED will generally show a typical Vf. A LED module may include a series current-limiting resistor and if so the module can be specified as suitable for a given voltage.

 

[mvs sarma]

The led could match to the datasheet linked.

https://www.ledheadlight.net/specs/SL-WMMZ05-DA.pdf

As per datasheet , with If of 350mA, the forward voltage Vf approximates to 13 to 14V
It means with a 12V supply you need to boost it while controlling the current to a max of 350mA. Then, definitely you can't use linear controller like lm317.
but you may try switching regulators or led driver chips. let us plan and design one.

 

[Pulak]

Dear mvs sarma, I have read the datasheet. At this range the led module might get hot. And it may need heat sink. It has reverse voltage of 20 volt and current of 10 micro amp. It is the exact one I bought, but there is no model number written anywhere on the module. By then how should I go and check the nominal voltage and current as I dont have specific measuring instruments.

 

[mvs sarma]

the seller can see his stock or cartoon and indicate model number. Reverse current is a not a question for us as we take precaution. such 5 watt etc do need heat sinks. nothing strange about it.
i shall soon indicate a switcher based driver to you.

 

[Pulak]

thanks mvs sarma. waiting for reply.