LED Lighting for a Fish Tank - Page 1
Forum section: electronics / beginners

author: tec5c
author_rank: Frequent Contributor
timestamp: August 02, 2015, 10:42:14 am
content: Posting on behalf of a friend of mine.I am trying to modify the LED lighting in my fish tank. I have two LED boards that run off 240v, rectified down to ~3.3v across each LED.Each board contains about 60 LEDS (blue and white), but in the middle of the board there is a 20 Ohm resistor. I can't for the life of me figure out why it's there.I am wanting to cut the board in half but am not sure if it will it will then draw too much current.I have attached a picture of the board and have (crudely) drawn in black how I think the PCB traces go (at the right angle you can see the traces under the solder mask) I haven't checked this with a multimeter though.Can anyone confirm the point of the resistor as well as whether I will have any problems with cutting the board in half (and using the half with the mains plug)?

author: LA7SJA
author_rank: Regular Contributor
timestamp: August 02, 2015, 12:37:45 pm
content: The 20 ohm is a more or less a fuse if my math is not completly off. If you give 30 LED 240V they will have 8V for a short while until the resistor release the smoke and/or one or more of the LED's selfdestructs with a bang. This construction would not be legal any place I like to live. The rectifier, if it is only one diode will make LED current 50% dutycycle and (240 * 1.4142)/60 = 5.65V max for one diode if you dobble this the current kn the diods could be 600mA and not the maximum 30mA the diode is rated for. If you chose to splitt it in two, remeber to but it in the trash.

author: picandmix
author_rank: Frequent Contributor
timestamp: August 02, 2015, 05:02:21 pm
content: Agree with LA7SJA, would avoid doing any modification to that light at all, far too dangerous.There are lots of Reef /Fishkeeping sites that have dedicated sections to diy led lights, run from much safer DC sources.Think one of the biggest near you is the MASA AU grouphttp://www.masa.asn.au/phpBB3/The biggest wordwide ishttp://www.reefcentral.com/forums/Here in the UKhttp://www.ultimatereef.net/forums/forumdisplay.php?717-LED-s-sub-forum

author: tec5c
author_rank: Frequent Contributor
timestamp: August 03, 2015, 12:54:11 am
content: It is not a one diode rectifier, rather it is a SMT bridge rectifier.

author: tec5c
author_rank: Frequent Contributor
timestamp: August 03, 2015, 11:40:59 am
content: Bump.

author: tec5c
author_rank: Frequent Contributor
timestamp: August 05, 2015, 09:04:55 am
content: Does anyone else have some advice on this?

author: picandmix
author_rank: Frequent Contributor
timestamp: August 05, 2015, 09:33:10 am
content: Quote from: tec5c on August 05, 2015, 09:04:55 amAs LA7SJA already said, the existing circuit relies onallthose leds being present to run properly.If you reduce the number of leds by half, the voltage must also be reduced by half otherwise they will just burn out/explode.Its not a practical modification unless you want to start adding lots of extra parts, which on a mains circuit is not something I would recommend.You say you have two banks of those led assemblies, why are you wanting to cut one in half ?

author: Kjelt
author_rank: Super Contributor
timestamp: August 05, 2015, 10:54:29 am
content: Quote from: tec5c on August 05, 2015, 09:04:55 amYeah scrap that POS. At least buy or build something (if it needs to be AC mains fed) with a tapped linear driver circuit (cheapest that I can advise).Better and best advice: buy some decent isolated led drivers with the correct current setting and chop the ledboard in to pieces that the driver can handle. For personal safety in wet environments I would keep the Voltage to the strings <= 36VDC (so 10 to 11 leds in series), so solder of 5 leds and you have 5 strings of 11 leds left.You can find drivers enough that can drive those strings.If that is too expensive for you, you can use a standard 36VDC supply with enough amps to feed all 5 strings and add in series with these strings a current limiter such as a LM317 circuit designed as constant current circuit. If the strings are from the same bin and have the exact same voltage across them you can even put some strings in parallel but for the few $ saved I would not do it.

author: BytesGuy
author_rank: Regular Contributor
timestamp: August 10, 2015, 01:05:58 pm
content: I upgraded my fish tank from tube lights to some retrofit led tubes and they work great.They are the 90cm type T8 tube, the LED ones use about half the power (2x 16W vs 2x 30W) which is a nice bonus plus a more even and brighter light - I grabbed one super bright (which is white) and one colour plus light (which is a pale pink) and both combined look great if you have colourful fish (I have fancy goldfish).You can probably get cheaper alternatives, but each one of these has it's own power supply (10V 2A rating) and they just mount in place of the old tube lights with no hassle.http://www.ebay.co.uk/itm/Superfish-T8-LED-Color-Plus-Retrofit-OR-Superbright-EVERY-SIZE-Fish-Tank-Bulb-/171509194204?pt=LH_DefaultDomain_3&var=&hash=item27eebe9ddcI was going to do something custom myself, but it wasn't worth the hassle when these are easy to get.

author: Unknown
author_rank: Unknown
timestamp: August 10, 2015, 03:46:49 pm
content: There's a big discussion on the web sitehttp://reefcentral.com/forums/showthread.php?t=2222702regarding DIY led lighting for marine tanks. Look for posts by user O2Surplus who designed a driver board. Another link from this forum:http://coralux.netwhich sells led driver boards, etc.I tried keeping a marine tank once but my fish mortality was too high (my failure). It's a lot of work.

author: ResR
author_rank: Regular Contributor
timestamp: August 10, 2015, 04:02:54 pm
content: You can use a 250VAC 220nF polymer capacitor with parallel 1MOhm resistor in series on AC side with the ½ board to limit the current to the LED's as shown in the attachment. Although be careful when testing, the circuit is not electrically isolated from mains so lethal electrical shock risk is still present.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 11, 2015, 12:53:53 am
content: 's suggestion sounds about right.But cutting the board in half implies that the enclosure will be altered too. And if it ever falls in the water, it's basically the same as putting a large amount of exposed AC wiring in your tank. Don't assume it will never happen if you're careful! I'd be embarrassed to admit how often it's happened to me in ~10 years of tank keeping. (Plus don't forget your spouse/children/houseguests/pets can do things you will *never* expect.)I might still use this over a freshwater tank, but only if those lights are plugged into a GFCI.I would not use this over a saltwater tank at all. Disaster more likely due to the much higher conductivity of the water. Even if never in direct contact with water, salt creep may still work its way in and provide a conductive path.EDIT: Something I forgot. Many cheap LED fixtures like this drive the LEDs at (or even beyond) their max ratings, to provide the greatest amount of light from the smallest number of LEDs. Combined with skimpy heatsinks, this leads to significant loss of light output in a year or two, rather than the 10-20 years you'd expect. If you undertake this modification and longevity is desired, you might check the current and temperatures, and possibly use a smaller capacitor than ResR recommended to further reduce the current to each half panel.

author: ResR
author_rank: Regular Contributor
timestamp: August 12, 2015, 06:49:30 am
content: The 220nF is the smallest I have seen used in LED bulbs, also bigclivedotcom from youtube said that the 220nF drives the LED bright enough to not overdrive it, I have taken apart EOL LED bulb that had 620nFdropper capacitor, a lot of times 470nF is used for current limiting that overdrives LEDs a bit too much. Of course the smaller the capacitor, the lower the current on LED's thus the longer the lifetime, but the light output is lower at the same time, it can be too low that the LED light doesn't light the tank at all.GFCI is also a very good idea and a must in places where device can and will get wet.

author: tec5c
author_rank: Frequent Contributor
timestamp: August 12, 2015, 07:18:44 am
content: As I recall, there is a poly cap and resistor on the bottom of the board. As this post wasn't for me, I don't have access to the board to take a look at values.When I get some free time I will catch up with my mate and give the board a better look over. Though my mate has said that he is considering going with an entirely different lighting setup so I'm not sure what his plans are.Thanks for the replies.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 12, 2015, 05:47:42 pm
content: , you're probably right about 220nF already being an ample reduction. I never really used this approach as I'm one of those who can see 120hz flicker. I should have said to reduce the capacitor value to less than what was in the *original* fixture.One more thought. It's possible the PCB that those LEDs is on has an aluminum substrate, rather than epoxy/phenolic, so that the entire PCB functions as part of the heatsink. In which case it would have a thin layer of electrical insulation between the traces and substrate. If you cut it and find this is the case, check for shorts between traces and substrate at the new edges; you may also need to carefully trim traces back from the edge to prevent arc-over, given the voltage this operates at.