I'm trying to understand the best way to size a power supply for a longer LED strip installation.

I understand that the basic calculation is the strip's watts per metre multiplied by the total length, but I'm wondering how much headroom people normally allow for the power supply.

For example, if a 24V strip is rated at around 10W/m and the total length is 8m, would a 100W supply be a sensible choice, or would you normally go higher to avoid running the supply close to its maximum rating?

I'm also interested in whether voltage drop becomes a significant issue on longer runs and whether it's better to feed the strip from both ends or use multiple injection points.

What approach do you normally use when sizing the supply and wiring?

 

Hi Cedricuk! You're right I believe; 100 W supply gives you roughly 20 W of headroom, which can be reasonable if the strip’s actual load matches its rating. I’d still check the strip and supply datasheets, and avoid designing around the supply’s absolute limit especially if it’ll be enclosed or run for long periods. Voltage drop is a separate issue from supply wattage. On a long run, the far end can look dimmer even if the supply has enough total capacity. The strip’s copper thickness and layout matter, so check its recommended maximum run length.

 

Welcome to AAC.

20% headroom should allow even less-than-stellar supplies to work reasonably well. As far as voltage drop is concerned it can definitely be a real problem. Using two or more smaller supplies at points along the run rather than end-feeding with one large supply will prevent this.

For RGB(W) strips the voltage drop has a differential effect on brightness for the various RGB LEDs and so it shows as color shift and is especially noticeable. The most reliable method of sizing the suppl(ies) is empirically. Put one supply on the end and observe where the brightness/color shift is noticeable, then put a supply at that point, &c.

If you are buying inexpensive supplies, overrating them is a good way to increase their life since pushing them closer to their nominal rating will result in more heat and so shorter life—especially for the capacitors. For this purpose more that 20% might be called for. I would choose based on price. If you can get to 50% while paying a reasonable amount you will probably ultimately save money on replacements.

 

Hi Cedricuk! You're right I believe; 100 W supply gives you roughly 20 W of headroom, which can be reasonable if the strip’s actual load matches its rating. I’d still check the strip and supply datasheets, and avoid designing around the supply’s absolute limit especially if it’ll be enclosed or run for long periods. Voltage drop is a separate issue from supply wattage. On a long run, the far end can look dimmer even if the supply has enough total capacity. The strip’s copper thickness and layout matter, so check its recommended maximum run length.

Thanks, that's helpful. The distinction between the power supply capacity and voltage drop is particularly useful. I was initially thinking of them as part of the same problem, but they need to be considered separately.

The point about checking the strip's recommended maximum run length also makes sense. I suppose the actual strip construction can make quite a difference even when two strips have the same voltage and wattage rating.

 

Welcome to AAC.

20% headroom should allow even less-than-stellar supplies to work reasonably well. As far as voltage drop is concerned it can definitely be a real problem. Using two or more smaller supplies at points along the run rather than end-feeding with one large supply will prevent this.

For RGB(W) strips the voltage drop has a differential effect on brightness for the various RGB LEDs and so it shows as color shift and is especially noticeable. The most reliable method of sizing the suppl(ies) is empirically. Put one supply on the end and observe where the brightness/color shift is noticeable, then put a supply at that point, &c.

If you are buying inexpensive supplies, overrating them is a good way to increase their life since pushing them closer to their nominal rating will result in more heat and so shorter life—especially for the capacitors. For this purpose more that 20% might be called for. I would choose based on price. If you can get to 50% while paying a reasonable amount you will probably ultimately save money on replacements.

Thanks, that's a really useful point about RGB(W) strips. I hadn't considered that voltage drop could cause a colour shift as well as simply making the far end appear dimmer.

I also like the empirical approach of checking where the brightness or colour starts to change and adding another feed there. That seems more practical than relying entirely on the total length, especially when different strips can have different copper layouts and current requirements.

 

Several good answers so far! Certainly there is a voltage drop from the resistance inside an LED strip. At least that has been my observation with the two long strips that I have. I suspect that there are varied qualities of LED strips available. ALL wiring will exhibit some voltage drop along it's length, as will those flexible trie-color LED tapes. And LED brightness varies quite a bit with the voltage, as well. LEDs ARE diodes, and diodes are indeed non-linear devices.
( EDITED ADDITION ) "Remote sensing" is an option worth considering if there is much voltage drop anticipated in the connections between the supply and the load. Some REGULATED power supplies allow that option, others do not.

One simple trick is to power both ends of the LED strips, if they are longer ones. BUT beware of excesheat while soldering!!

 

I'm trying to understand the best way to size a power supply for a longer LED strip installation.

I understand that the basic calculation is the strip's watts per metre multiplied by the total length, but I'm wondering how much headroom people normally allow for the power supply.

I knew that for every 10 °C rise in temperature, it's half of the device's expectancy life that's erased, given that it operated its whole life at that temperature. Source.
So there you have it, you want it to survive for a long time? Run it cold/put it in cold weathers.

Now you might be thinking, how do I calculate the temperature of the components inside? Generally, a device will have its internal temperature at ~120 °C while at 100% capacity. The power loss on the device is proportional (generally) to the power consumed, so it preserves the efficiency. So I see that at 50% capacity it will dissipate half as much, and will be at the medium point between ambient temperature and ~120 °C. So if ambient is at 20 °C, it will be 70 °C.

 

In the design of systems that are not seriously cost-constrained, such as production equipment, sizing power sources to work at 50% to 75% of their rated capacity has been my choice. That usually is able to work with after-thought additions. For those seriously "cost-constrained" applications, where cost limiting matters more than reliability, running at 80% to even 90% was accepted. Mostly that choice worked, but there were exceptions! I have had to replace failed power supplies with higher rated ones when servicing equipment designed and built by others.

 

If you read the recommendation of the LED strip providers, they will all tell you that for longer strips, the voltage will drop over the length which then affects the colour.
The recommendation is always to feed the power at intermediate points on the strip, and have the power delivered by the thickest wire you can afford/fit in.
It also depends on the application. If its just lighting a room then colour may not be so important. If its lighting an art installation colour is more important.