Hi all i have a very simple question which i'm sad to say is past my skills. I've spent all day trying to work this out but it's past my skill level.

can someone please advise what i need to buy from ebay for the following.

I'm using 6v to power a the Led. The only information i have on the led is below:-

10W High Power LED PCB Grow light Aquarium Heatsink White
Certification: CE,RoHS
Power Dissipation: 10W
Chip: Bridgelux
Life: 50,000h
Forward Voltage: 2.0-3.6V
Forward Current: 700mA.

Can any tell me exactly what resistor i need to buy on EBay please as the listings are hard to work out and don't make much sense to me. the link below is the best i've found for what i think i need (could be wrong)

Cement Resistors 5W - 2.2Ohms


  • 1
    \$\begingroup\$ With such a wide-ranging Forward-Voltage spec, what you need is a constant-current supply or driver - not a simple resistor. \$\endgroup\$
    – brhans
    Sep 6, 2016 at 21:13
  • 1
    \$\begingroup\$ And those specs don't make sense together. P=IV so the numbers don't work in the formula. You'd need over 14V to get 10W with 700mA. \$\endgroup\$
    – brhans
    Sep 6, 2016 at 21:17
  • 1
    \$\begingroup\$ I can't comment but I just want to add that you will need to dissipate the heat coming out of the 10W LED. Use a heatsink if your buying the LED without a metallic housing \$\endgroup\$
    – Latchup
    Sep 6, 2016 at 21:27
  • \$\begingroup\$ @brhans: I think the OP can't read the tables correctly. Ebay suppliers will sell these as 1W, 3W, and 10W devices. The OP picked up the 1W figures. Not the 10W figures. \$\endgroup\$
    – jonk
    Sep 6, 2016 at 21:28

3 Answers 3


Ebay specs are unreliable, and usually non-existent.

The Bridgelux 10W chip comes with no heatsink and requires a big one.

  • It is actually made from 9 chips in an array of 3x3 or 3S3P.
  • Thus you need more than 9V and >=1.2A
  • If using say a PC PSU at 12V with a 3V drop per 10W ~9V chip then 4V/1.11A= 3.6 Ohm 5W resistor
  • This needs a well regulated 12V supply and AWG 16 -18 wires for a short distance(3m)
    • Tolerance error on Vf of LEDs depends on quality at 10W. They are usually binned in 0.1V increments and these may be rejects at Vf>10V Then use Jonk's calculations for 2V drop with 1A or 2 Ohm
  • The exact answer may be around this. But you are better off focusing on a heatsink design like a CPU cooler. (est)enter image description here 10W High Power LED PCB Grow light Aquarium Heatsink White Certification

after doing some research on Bridgelux's website and their distributor , it seems the 3x3 chips here are obsolete and replaced with higher voltage arrays in their 3rd and 4th chips ,which favour lower cable loss. SO these chips are cheap for a reason. Good ruck.


You need to start at the beginning, and work step by step.

You have a 6 volt supply. Your LED apparently operates over a very wide voltage range, but let's assume that, since you're interested in running near maximum, that the LED voltage will be 3.5 volts.

Now, since you're not entirely sure what you're doing, let's not figure on getting maximum power from the LED. Let's aim for 500 mA. Then the voltage across the resistor will be (6 - 3.5) volts, or 2.5 volts. Voltage equal current times resistance, so $$R=\frac{V}{i}=\frac{2.5}{0.5} = 5\text{ ohms}$$ and the power in the resistor is $$P=i^2R=(0.5)^2\times{5} = \frac{5}{4} = 1.25\text{ watts}$$

So a 5 ohm, 2 watt resistor is about what you want.

  • \$\begingroup\$ thank you for the replys. i've added below a link to the item i'm refering to:- ebay.co.uk/itm/… \$\endgroup\$
    – Ben Hughes
    Sep 6, 2016 at 21:53
  • \$\begingroup\$ @BenHughes - The first thing you should be aware of is that the linked item is NOT a 10W LED. 0.7 A times 3.5 volts is 2.5 watts. \$\endgroup\$ Sep 6, 2016 at 22:39
  • \$\begingroup\$ I think, looking at the ebay pictures, and then going to the BridgeLux web site and looking at their discontinued models, that the device is probably a knock-off, anyway. The package is very similar to their "ES Star Array" but it looks just different enough to not be their own brand markings. If it is supposed to replace them, then these do operate at 350mA but with a forward voltage of 27.3V (for their 9.6W variety.) \$\endgroup\$
    – jonk
    Sep 7, 2016 at 0:03

You will want to find a different power supply, if possible.

I've taken a look at an ebay web page that supplies something called "1W 3W 10W High Power LED with PCB - Grow lights aquarium LED," which I take to be similar to what you are pursuing. On that link, they cite the use of (9) separate LEDs to make the module and that the current maximum is \$1.05A\$ and the voltage required is \$9-11V\$. Which works out correctly in terms of power rating.

This pretty much means that you can't use your \$6V\$ power source. If you wanted to keep your \$6V\$ power source, and assuming it can comply by supplying \$1A\$ into a load, then you will need a current limiting resistor of about:

$$R=\frac{6V - 10V}{1A}=-4\Omega$$

Negative resistors, unfortunately, are in rather short supply.

So I think you need to find an appropriately sized power supply for your LED. I'd recommend accepting a \$12V\$ power source rated to supply at least \$1A\$, though more current compliance is better. The resistor would then be:

$$R=\frac{12V - 10V}{1A}=2\Omega$$

And it would need to be able to support at least \$2W\$ dissipation. Here, a \$5W\$ resistor would now be a good choice. You will also need to have a way to dissipate the 10W that the LED itself generates.

This all assumes you really are considering a \$10W\$ LED module and that it is actually similar to the one I found on ebay.

NOTE: It is turning out, with the new link you added, that I'm wrong about the module. Your link is supposed to a BridgeLux of some kind. The closest I can find, by appearances, on the BridgeLex web site is their "ES Star Array," which operates with \$350mA\$ and \$27.3V\$. However, the pictures on the ebay site do not fully match up with the BridgeLux "ES Star Array," and I can't find an alternative that does match. Also, the ebay site says \$700mA\$, not \$350mA\$, so I frankly have failed to nail the specific device. (I'd have to buy one to work out the details and I'm not doing that.)

  • \$\begingroup\$ If he's going to buy a new power supply, he'd be better off buying a constant-current supply so as not to have to waste power in a resistor... \$\endgroup\$
    – brhans
    Sep 6, 2016 at 21:41
  • \$\begingroup\$ @brhans: That makes sense to me. I just haven't actually used any before. (I have constant current bench supplies, but that is entirely different than an active circuit using inductors to achieve this design goal.) Do you have a link to one I could examine? I'd appreciate seeing how they are designed to operate. (Also, 12V supplies are easy to find. But I'm interested in what you might point me to.) \$\endgroup\$
    – jonk
    Sep 6, 2016 at 21:43
  • \$\begingroup\$ @brhans: I still interested in an efficient constant current LED driver circuit design, if you know of one. The ones I've looked at so far appear to be boost/buck voltage sources providing a maximum voltage for compliance and a current limiter circuit. These are NOT efficient designs, as they must still dissipate based upon their maximum compliance voltage. I'd REALLY be interested to see a control circuit designed to regulate the actual current using magnetic energy storage and where the voltage developed is exactly as needed, not dropped away by a current regulator section. \$\endgroup\$
    – jonk
    Sep 6, 2016 at 22:54
  • \$\begingroup\$ The ones I'm thinking of are in the 85-90% range on efficiency and I'm almost certain they are direct current regulators, not voltage 1st then current as you describe. But sharing my company's IP would probably be somewhat frowned upon... I'll see what's been publicly released or disclosed in patents though. \$\endgroup\$
    – brhans
    Sep 7, 2016 at 1:52
  • \$\begingroup\$ @brhans: Most people looking for COTS systems aren't likely to encounter rare, relatively unpublished patented systems not known by universities and their teaching staff. I hope that if these things are COTS, then they are designs that could be found in the teaching literature. And if not, then... wow! I'd still like to know about them. To be honest, I've never laid hands on such a system. I could go through my older books (I have a switching supply teaching book I could examine again.) But any help would be very much appreciated. I'm interested. \$\endgroup\$
    – jonk
    Sep 7, 2016 at 2:48

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