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I have 20 x 3 Watt LEDs and have a 80W constant voltage (not constant current supply). I originally wired this up with 5 sets of series string of 4 LEDs from my 12V supply, however I have been told that I need to limit the current. I don't really want to change the supply and wondered if I could add a very low Ohm resistor to each string to regulate the current?

This is a schematic as it now is? If I added say a \$1\Omega\$ resistor before each of these LED strings would this do a good job in regulating the current?

Alternatively could I maybe use one of these on each string

http://www.ebay.co.uk/itm/DC-Buck-Step-Down-Converter-DC-5A-Regulator-Voltage-Current-Adjustable-DE-/321439418999?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item4ad7484a77

enter image description here

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  • \$\begingroup\$ Can you post the forward voltage and current spec's for the LEDs or, better yet, a link to their data sheet? \$\endgroup\$ – EM Fields Jul 20 '14 at 21:13
  • \$\begingroup\$ Sure they are Cool white they are attached to the listing cgi.ebay.co.uk/ws/… \$\endgroup\$ – user49734 Jul 20 '14 at 21:38
  • \$\begingroup\$ What I was asking for was the range of voltage drops across one of the LEDs with the manufacturer's recommended current through it, not its advertised color rendition. \$\endgroup\$ – EM Fields Jul 20 '14 at 21:46
  • \$\begingroup\$ Sorry the max recommended current is 750mA. The forward voltage stated as being between 3.5 - 4.5v. \$\endgroup\$ – user49734 Jul 20 '14 at 22:01
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The data sheet states that with the LED dissipating 2.63 watts, its forward voltage, Vf, will be 3.5 V, which corresponds to a forward current, If, of 751 mA.

With the LED dissipating 3.38 watts, its Vf will be 4.5V and its If also 751 mA.

That says that with 751 mA through the LED its Vf can lie anywhere between 3.5 and 4.5 volts, a caveat being that the LED's absolute maximum If is 750 mA.

Since the LEDs are going to be used as aquarium lights, there's probably no reason to blind the fish and run the LEDs on the hairy edge of their own demise.

The If VS Vf graph on Ebay, for the LED, shows that with an If of about 400 mA, Vf will be about 3.5 V, allowing the LEDs to dissipate a much more comfortable 1.4 watts each.

Going that way means that you can wire three LEDs in series for a total of 10.5 volts across the string, and drop the remaining 1.5 volts from the 12 volt supply with a 3.75 ohm resistor in series with the string. The resistor will dissipate 0.6 watts, so a 1 or two watt resistor would be a good choice.

A nice metal oxide 3.6 ohm 2 watt unit is readily available, and would up the current through the LEDs a little, but would be fine for the application.

So, since you can't get 4 LEDs in series to be properly driven by 12 volts, the array changes from 4 X 5 to 3 X 6 with two LEDs left over.

If the 3 X 6 array would work for you, then the supply would have to put out 2.4 amps at 12 volts, which is about 29 watts, well within its capabilities.

If you absolutely have to use all 20 LEDs, then you could connect the last two in series with a 12.5 ohm 3 watt resistor and wire that string in parallel with all the others, increasing the load on your supply to about 34 watts.

enter image description here

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  • \$\begingroup\$ thanks so much guys this is really incredible from all of you. This is such a great forum really do appreiciate all your help. I think I am going to try the resistor method suggested first and run a test to see what the temps are like on resistors. I might also add a fuse onto each string just to be double sure - probably overkill but best to be cautious and I can always resuse them. Thanks again for all your help, really is very much appreiciated. \$\endgroup\$ – user49734 Jul 21 '14 at 9:39
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Yes and no. Yes, you can add a suitable resistor, calculated the same as for a single LED, just sum the forward voltages.

NOTE: CALCULATE THE REQUIRED POWER FOR THE RESISTOR - you don't want to set fire to the resistor...!

BUT

A resistor is not recommended for high wattage LEDs as the resistance changes with heat, and at that wattage there will be a lot of heat about.

You really should consider either creating a constant current source, or using constant current sinks (note: for the latter you can keep your existing supply, just add the components to each chain to limit the sink current).

For example, a couple of circuits I found on another question on this site:

enter image description here

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  • \$\begingroup\$ Thansk for this. I like the idea of not changing my supply but I really don't really understand how the current sink work sorry I am obviously not a spark, just playing to build a small aquarium light. Could I maybe use a current regulator such as these ebay.co.uk/itm/… \$\endgroup\$ – user49734 Jul 20 '14 at 20:56
  • \$\begingroup\$ No, that's a voltage regulator. I don't know why they have stuck the word current in the title... There's many different ways of doing constant current sinks - ask google for some examples and schematics, there's lots. \$\endgroup\$ – Majenko Jul 20 '14 at 21:01
  • \$\begingroup\$ What about this one? It look like two pots on here one for current and the other for voltage. ebay.co.uk/itm/…. I am not trying to skip designing a circuit but this is for an aquarium and it could be a bit messy with adding lots of extra components \$\endgroup\$ – user49734 Jul 20 '14 at 21:45
  • \$\begingroup\$ @Majenko Both those eBay switching regulators have adjustable output voltage, as well as adjustable current limit settings - note the two blue presets on each module. Either could thus be used to limit current as the OP requires, by being set for current limit as required by the LED string, and output voltage adjusted to a bit above the string total forward voltage. \$\endgroup\$ – Anindo Ghosh Jul 21 '14 at 9:09
  • \$\begingroup\$ Hmm... a current limited constant voltage power supply - it might just work. Is running one of those regulators constantly at the set current limit advised though? \$\endgroup\$ – Majenko Jul 21 '14 at 9:26
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There is more than one correct answer to the stated requirements.

Solution 1: Current limiting resistors, as answered by EM Fields
Solution 2: BJT based constant current source or sink, as answered by Makenko
Solution 3: Current-limited switching regulators as proposed in the question ("how-to" follows)
Solution 4: Simple-to-use 2-terminal constant-current linear LED driver ICs
Solution 5: Switch-mode constant current LED drivers


Solution 3 how-to:

  • Choose a current-adjustable boost regulator, not a buck regulator. The module listed by OP will support just 2 LEDs per string (4.5 x 2 = 9 Volts), with the specified 12 Volt main supply, as its maximum output voltage is a couple of volts below supply provided to it. Use something like this one instead (35 Volt maximum output, CC adjustable)
  • Choose an LED string length for maximum total forward voltage a couple of volts lower than maximum output of module: for the suggested module, (35-2)/4.5 = 7.33, so 7 LEDs per string. 3 modules would be needed, one per string of 7, 7 and 6 LEDs each.
  • Set CC adjustment to the low side of the appropriate preset, say 10 mA, and hook up 6 or 7 LEDs in series with an ammeter, across the module output. CC indicator LED should light up.
  • Adjust CC preset till desired current is achieved - EM Fields makes a good case for using 400 mA per string. That's that.

Solution 4:

  • Use parallelled sets of constant current 2-terminal linear current regulators such as the SuperTex (now MicroChip) CL6 IC. For 400 mA per string, 4 CL6 ICs would be used in parallel for each string. Use heat sinks for the ICs.
    CL6
  • Being 2-terminal devices, these current regulators can be used on either the high or low side of the LED string, just like a resistor.
  • The key advantage of these devices over resistors is that the current regulation is pretty much independent of the supply voltage within the specified operating voltage limits. This is specially useful if a high voltage supply (up to 90 Volts) is available instead of the stated 12 Volt one.

Solution 5:

  • Design a circuit based on a switch mode step-up, preferably multi-channel, constant current LED driver IC, with one string of LEDs per channel. This is probably maximum overkill for the purpose specified.
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    \$\begingroup\$ Wow thanks Anindo this is great summary really helpful. The CL6 looks interesting. How ould you mount this on a heatsink - just thermal glue? \$\endgroup\$ – user49734 Jul 22 '14 at 18:21
  • \$\begingroup\$ @user49734 Regular heat sinks as used for 3 terminal linear voltage regulators. See the image I added to the answer, they have heat sink tabs. \$\endgroup\$ – Anindo Ghosh Jul 22 '14 at 19:00
  • \$\begingroup\$ arhh ok gotcha, like a Mosfet. Looks like could be a nice neat and tidy solution. great stuff. Many thanks \$\endgroup\$ – user49734 Jul 22 '14 at 19:11

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