Probably stupid LM317L questions, anyway here it goes...

[dmachado]

I am build an overtank lumminaire for my aquarium, and it will take some 360 leds, as follows:

Power supply 32V 1,5A

6 strings 10 leds each, at 98-100 mA each - 6 LM317L in current regulation
30 strings 10 leds each, at 19-20 mA each - 30 LM317L in current regulation

So, stupid question nr. 1:

Can a commom resistor set the current for a number of LM317Ls, i.e., for the ones regulating current at 20 mA, one single resistor...?

...and, stupid question nr. 2:

Can the capacitor shown here https://www.electro-tech-online.com/attachments/lm317ireg-jpg.30908/ be a single one for the hole circuit, or should it be one per led string/LM317L?

Thanks for all the kind help you can provide!

Best regards,

Daniel

 

[colin55]

The LEDs can be placed in series with a single resistor. We use 80 white LEDs in series with a resistor for 240v. Use 4 strings and 4 resistors. No other parts.

 

[dmachado]

I am using DC for future integration with an Arduino, fade the leds etc, so AC current is not really an option... Thanks anyway.

 

[Boncuk]

Hi dmachado,

for a constant current of 100mA I won't use an LM317L since it can supply just 100mA.

An LM317T can supply a constant current up to 1.5A.

Supposing your calculation is correct you won't even have to use a heatsink when choosing an LM317T instead of an LM317L.

With a power supply of 32V/1.5A you have a lot of freedom to play with different LED strings.

Just keep in mind not to exceed a total forward voltage of +Vin(-3V)=Vf(LED)Xn+Vreserve

where +Vin is the supply voltage ,
Vf(LED) is LED forward voltage and
n is the number of LEDs in a chain and
Vreserve is excessive voltage (over the voltage demand of one chain)


E.g. +Vin=32V, Vf(LED)=3.5V: (32V-3V)=(3.5V*8)+1.0V

Math is not my favorite stuff. All you need is to calculate for the maximum number of LEDs in a chain to have a little leftover of voltage (Vreserve, in that case 1.0V).

Hope I got it across.

Regards

Boncuk

 

[dmachado]

Boncuk, thanks for the reply.

Given the max 100mA for the LM317L I will probably set them at 98mA, the difference is not noticeable.

My main issue is to use a resistor for each LM317, or if one single resistor can set the current for a number of LM317s. Also the one capacitor for the hole setup or one capacitor per installed LM317...

regards,

Daniel

 

[Boncuk]

Hi Daniel,

each LM317L must be treated as one device with it's current feedback resistor, so you need a current feedback resistor for each regulator.

The reference resistor (Rf) is calculated by: Rf(Ω)=Vref(V)/Iref(A),

where Rf is the feedback resistor value(Ω), Vref is the IC reference voltage (nominal 1.25V) and Iref(A) is the desired constant current.

For 0.098A constant current the formula is Rf=1.25V/0.098V; Rf=~12.76Ω

Vref might vary individually from 1.2 to 1.3V, nominal Rref is 1.25V.

for tight tolerance (98mA) it is advisable to measure the actual current flow and adjust Rref accordingly.

Regards

Boncuk

 

[dmachado]

Ok, thanks for clearing this matter, 36 circuits it is... I'll go get more solder....

Regards,

Daniel

 

[Boncuk]

Ok, thanks for clearing this matter, 36 circuits it is... I'll go get more solder....

Regards,

Daniel

OK, get yourself some Chinese from Chenzen. They'll be happy to solder for you instead of being pushed to suicide by their bosses.

 

[mneary]

If you plan to control them with Arduino, you don't really need LM317s. In the example below, I assumed the Arduino is running at 5V. Use one circuit for each string. If your Arduino is running on 3.3V, change R1 to 680 ohms. I have not tried this on the bench, it may require 18R for 100mA.

(Note: Some sharp eyes will notice that I'm using a gain of much more than 10. That's because the transistor isn't saturated.)