LED driver circuit with LM317T for parallel strings

polashd · Jan 22, 2011

I need a LED driver circuit for my aquarium.
- it should supply power to multiple (6 in my case) LED strings of which are connected parallel to each-other.
-each string contains 5 to 10 LEDs in series
As each LED differs in forward voltage, the required voltage for all the series strings can't be same.
-required current for each string should be 20mA
I want to make it using LM317T

I found lots of circuit web,
But I'm not sure whether these will work for me (multiple parallel strings simultaneously)!!

I'll be glad if anyone can help/suggest me.
I need help specific to my situation.
note: I'm not expert in electronics.

SABorn · Jan 22, 2011

Why dont you tell us how many leds each of what colour, and the ratings for each colour, as well as the voltage you want to operate at.

As it sounds like you have approched this wrongly, and could be a much simpler solution.

Pete.

polashd · Jan 22, 2011

clearing the fact

SABorn said:
Why dont you tell us how many leds each of what colour, and the ratings for each colour, as well as the voltage you want to operate at.

--
I may make it with different transformers ( option01 (transformer 01): 19v-20v (after filtering and regulating), option02 (transformer 02): 24v-25v (after filtering and regulating) with LM317T)
Whatever the option is, I'll make all parallel strings in such away that they will differ +/-1V from each-other.

MrAl · Jan 22, 2011

polashd said:
SABorn said:

Why dont you tell us how many leds each of what colour, and the ratings for each colour, as well as the voltage you want to operate at.

--
I may make it with different transformers ( option01 (transformer 01): 19v-20v (after filtering and regulating), option02 (transformer 02): 24v-25v (after filtering and regulating) with LM317T)
Whatever the option is, I'll make all parallel strings in such away that they will differ +/-1V from each-other.

Click to expand...

Hello, Yes it's a good idea that if you run parallel strings that you check that the current through each is roughly the same. If you have 19ma in one string and 21ma in another string that's going to be just fine, but if you have 10ma in one string and 30ma in another that's not so good.

polashd · Jan 22, 2011

MrAl said:
polashd said:

Hello, Yes it's a good idea that if you run parallel strings that you check that the current through each is roughly the same. If you have 19ma in one string and 21ma in another string that's going to be just fine, but if you have 10ma in one string and 30ma in another that's not so good.

Click to expand...

Dear MrAL
current through each string should be same (20mA). But forward voltage of each string are little different (one may need 18.8V, another 19V or 19.3v). tell me whether I made it clear!
thanks for reply

SABorn · Jan 22, 2011

Why not just balance each string with a resistor and forget the current limiter.

If needed just add a voltage regulator to keep the voltage constant.

The only reason the current requirement will change is if the voltage changes.

So if we said a 24 volt supply and a 18.8v string and a 19.3v string all you would do is.........

24 - 18.8 = 5.2v
5.2 / 0.02 = 260..........so you would add a 260 ohm resistor to this string

24 - 19.3 = 4.7v
4.7 / 0.02 = 235...........so you would add a 235 ohm resistor to this string ( in reality a 220 ohm would do)

Pete.

polashd · Jan 22, 2011

SABorn said:
Why not just balance each string with a resistor and forget the current limiter.

If needed just add a voltage regulator to keep the voltage constant.

The only reason the current requirement will change is if the voltage changes.

So if we said a 24 volt supply and a 18.8v string and a 19.3v string all you would do is.........

24 - 18.8 = 5.2v
5.2 / 0.02 = 260..........so you would add a 260 ohm resistor to this string

24 - 19.3 = 4.7v
4.7 / 0.02 = 235...........so you would add a 235 ohm resistor to this string ( in reality a 220 ohm would do)

Pete.

dear SABorn
This is the best solution I can think of. As I'm not expert in electronics I want to make some confusion to be cleared:-
-as the resistors have some Torrence and 100% matching resistor can not be found- will it cause any problem.
for example:
5.2 / 0.02=260 ohm
5.3 / 0.02=265 ohm
- any slandered resistor has some small Torrence (small %), how to handle this.
-I can use small / flat variable resistor for precise value (does these too have Torrence?), these are of low Watts (I assume 1/4Watt). what if the string requires, say 217 ohm 0.5watt or something like this.
please suggest me
thanks again.

audioguru · Jan 22, 2011

You cannot buy a 260 ohm or 265 ohm resistor so use a 270 ohm resistor. One LED string will have 5.2V/270 ohms= 19.26mA and the other LED string will have 5.3V/270 ohms= 19.63mA and they will both look exactly the same. Double and half the current will be noticeable.

Use ordinary 5% (tolerance not torrence) resistors then calculate the very small current difference if one is 5% high at 283.5 ohms and the other is 5% low at 256.5 ohms. You will not see the small difference.

SABorn · Jan 22, 2011

A small variance in resistor values will make no real difference to the operation.

You will never really get the exact value resistor to the calculated value so we just use the nearest value to it.

As i quoted a calculated value of 235 ohm i would use a 220 ohm resistor instead, or even a 240 ohm, the fractional difference wont matter much.

You can do the math to find the difference if you want......

0.02 x 220 = 4.4v
0.02 x 235 = 4.7v

4.7 - 4.4 = 0.3 volt increase

At 20mA you would not be running at maximum so a 0.3 volt increase across the string of leds would be nothing to worry about.

Pete.

nickelflippr · Jan 22, 2011

I agree that the resistor method would be sufficient, and one wouldn't be able to discern the difference. But browsing for 20ma CC led drivers, I came across a most interesting part NSI45020T1G. At that price, you could let the led driver do the talking.

The OP would have to take into consideration the package thermal and power limitations, and plan accordingly.

LED driver circuit with LM317T for parallel strings

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