What resistors should I use for my led build

[mcghee33]

I am trying to finish up my led build and I thought people were using 5 watt 1ohm resistors but I cannot find them anywhere at RadioShack. I am using 2 meanwell 90 48ds to run 72 LEDs in parallel. Each driver with 3 strings of 12. Please help.

 

[mcghee33]

I had the hardest time finding 1 ohm 5 watt resistors but finally found them at a local electronics shop.

 

[Reef Breeders Verified Sponsor]

You do not need the resistors for anything other than testing. I would reccomend 700ma fuses

 

[Young Frankenstein]

You need to know voltage input, LED ma, and LED voltage

here is a link LED calculator for single LEDs

 

[Young Frankenstein]

I think you need help with this, post detail what you are building so we can help you.

 

[Reef Breeders Verified Sponsor]

I think you need help with this, post detail what you are building so we can help you.

The resistors are used to test the meanwells in parellel strings, the voltage drop accross the resistor tells you the ma output while you dial the driver in via an internal potentiometer.

 

[Young Frankenstein]

The resistors are used to test the meanwells in parellel strings, the voltage drop accross the resistor tells you the ma output while you dial the driver in via an internal potentiometer.

? no comprende lol, LED bias voltage is anywhere between 1.5 to 3.5v dc, if feeding from a 12v power supply a resistor typically of 560 ohms to 1k is installed to drop the voltage to 3 v. LED for a reef fixture are wired in series, so you wont need resistors. A potentiometer is used wired to the driver so it can vary the voltage regulator input.

 

[Onedesign1]

? no comprende lol, LED bias voltage is anywhere between 1.5 to 3.5v dc, if feeding from a 12v power supply a resistor typically of 560 ohms to 1k is installed to drop the voltage to 3 v. LED for a reef fixture are wired in series, so you wont need resistors. A potentiometer is used wired to the driver so it can vary the voltage regulator input.

Well fortunately the OP found what he was needing....although my Rat Shack carries the big white ones in 5W 1ohm.

Young Frank....I take it you haven't ventured into DIY LEDs yet? The LED wizard you linked is a terrible example for this application. The drivers we use are constant current and auto adjust the voltage to maintain that current. They operate within a large range such that they can be used with a series string of 8 to 13 LEDs. If outside that range the drivers will kick in auto protect features. We then run multiple series strings in parallel of a single driver. This cuts the current by the number of strings in parallel and allows us to run more LEDs off a single driver since max LED count on a 48v driver is about 12-14 depending in which type of LEDs one uses. He's running 36 off each driver in 3 parallel strings and that driver puts out 1.8A so each string in his application is seeing 600ma.

We're not trying to get a voltage drop via a resistor for the LEDs....the drivers handle that. We use one at the head of each parallel string to measure the current through that string. Simple ohms law....I=V/R...so if the resistance is 1 then the current equals whatever the voltage reads...and so you just measure the voltage across that 1 ohm resistor and that equals your current.

Why are we interested in this? Because if there are multiple parallel strings you want them relatively balanced in terms of current each string sees from that driver and usually try to get them within 50ma of each other.

Fuses are always a good idea and should be rated above the running current and at or below the LEDs max rating. This way if you lose an LED in a string the resulting rush of current to the remaining strings will be contained by popping fuses rather than popping LEDs.

Another way to measure current and forego the resistors is to use that fuse holder and put your meter in place of the fuse on an appropriate current setting. This way you are measuring actual current as opposed to calculated current. I've done both and the resistors are always dead on.

OP...great to hear your build is coming along. Keep us posted!

 

[Young Frankenstein]

I am working with LED's for 25 years LED's are LED's and do not require a resistor with the corect voltage, drivers i have builded over the years using the LM317 or other voltage regulators, it sounds like the resistor is needed for the driver ? I have never worked with prefab market drivers or any programable drivers that field is blank, and sooner than later will do some project using them.

 

[Onedesign1]

I am working with LED's for 25 years LED's are LED's and do not require a resistor with the corect voltage, drivers i have builded over the years using the LM317 or other voltage regulators, it sounds like the resistor is needed for the driver ? I have never worked with prefab market drivers or any programable drivers that field is blank, and sooner than later will do some project using them.

I don't want to hijack this thread anymore plus you didn't read my explanation of why we use resistors in this application. OP found what he needed is all that matters. Before you venture into DIY LEDs study up on it though. What you're thinking of is completely irrelevant here.

 

[Reef Breeders Verified Sponsor]

I am working with LED's for 25 years LED's are LED's and do not require a resistor with the corect voltage, drivers i have builded over the years using the LM317 or other voltage regulators, it sounds like the resistor is needed for the driver ? I have never worked with prefab market drivers or any programable drivers that field is blank, and sooner than later will do some project using them.

Frankenstein, meanwells are the standard in DIY lighting. The resistor at 1ohm goes by ohms law, the voltage drop is equal to the ma. Say 2.5v=250ma, or 7v=700ma. That is all they are used for, the dirver has a pre-programmed voltage range, the voltage is determined by the ma that the LEDs are driven at. An XT-E Cree at about 700ma uses 3 watts, at 1500ma, 5 watts.

 

[Mason Dixon]

You do not need the resistors for anything other than testing. I would reccomend 700ma fuses

Sorry for the newb question, but if I am using 3 strings of 12 leds in parallel off a Meanwell 90d 48, do I put a fuse on each string or can I just use one on the single positive wire and branch the 3 strings off of it? Just need to know how many fuses/holders to buy.

 

[Onedesign1]

Sorry for the newb question, but if I am using 3 strings of 12 leds in parallel off a Meanwell 90d 48, do I put a fuse on each string or can I just use one on the single positive wire and branch the 3 strings off of it? Just need to know how many fuses/holders to buy.

No worries about the question! It's an important one. You need to use 1 on each string.

 

[Reef Breeders Verified Sponsor]

No worries about the question! It's an important one. You need to use 1 on each string.

Yup, ons on each, if you branch off, you will get the total ma, and then you can divide by three. 1 on each is better, as you can test for differences in current also