Help needed to understand wiring LEDs in parallel and power supply requirements.

Thread Starter

Spacey

Joined Mar 8, 2018
5
Hello, Thanks for taking the time to read my question. I'm a complete newbie and while I have read a lot of forum posts trying to research the answers, I clearly have basic gaps in my understanding and have just come away more confused than when I started.

I've been asked to help build a basic LED lighting circuits after somebody left a project, but I'm worried the original project assumptions are incorrect but I'm also significantly out of my comfort zone so can't be confidant in my assessment or understanding.

The project is to create a simple set of 160 LED lights in totally broken up across 4 parallel lines. Some of the hardware has already been purchased
LED's are RS (7133955) Nichia 500D White LED 5mm (T-13/4)
The datasheet states:
Max Forward Current is 30ma
Reverse Voltage is 5v
Forward Voltage is 3.2V at 20ma with range of Min 2.65V to 3.5V

RS Resistors 10 Ohms +-5% (Brown, Black, Black, Gold)

Power supply is assumed to be 5 volts but hasn't actually be purchased but no current calculations can be found. Is 5v enough given voltage drop?

We have made 4 lighting rigs consisting of 2 cables with 40 LEDs in parallel down the length (10 meters roughly) of the cable with each LED connected to a 10 Ohms resistor. I can't find any documentation from the original lead as to why a 10 Ohm resistor was chosen. I understand that having a resistor will help the LEDs be the same brightness. Which I believe is one of the most common issue when wired in parallel (assuming I've understood correctly). What I'm worried about is how the value of 10 Ohms was calculated. The original plan was to connect each one of the lighting rigs to a shared power supply but again no details of what that power supply was going to be can be found. So, I need to establish the power supply requirements in terms of 5v, 12v or 24v

Guess the first question is this the best way to achieve a total of 160 LEDs? or would it be better to do a parallel series of lights? The originally person has made many rigs already. So if its possible to do this in standard parallel configuration safely I'd like to avoid recreating all their work. Assuming the 10 Ohm resistors it actually correct.

I think this is a correct representation although, it would be times 4 rigs that would connect to the power supply.
Basic LEd.PNG


To establish what power supply needs I have. I've used information from https://www.petervis.com/electronics/led/led-resistor-calculator.html
"The total current is the sum of all the individual forward currents of each LED."
So assuming you use the Max Forward Current rather than the typical 20ma operating current.
Max Forward Current 30ma x 40 = 1.2a
Then as we've got 4 of these rigs 1.2. * 4 = 4.8a is that right?

Looking at the same page I can’t find any combination of input voltage that would get me to a 10 Ohms resistor. So, I'm even more lost now.
5V @ 3.2 VF = 90 ohms.
12v @ 3.2 VF = 440 Ohms.

I'm sorry this questions / post is probably all over the place and I'm sure I'm showing my huge naivety in general. So, I really do welcome all feedback and I thank you in advance for any help you can give me.
 

ericgibbs

Joined Jan 29, 2010
11,118
hi Spacey,
Welcome to AAC.
I would recommend that if your LED's are rated at a typical 20mA, you do not exceed that in continuous operation.
You may find that they are bright enough at say 15mA, which would give a longer life expectancy.
E
 

Thread Starter

Spacey

Joined Mar 8, 2018
5
hi Spacey,
Welcome to AAC.
I would recommend that if your LED's are rated at a typical 20mA, you do not exceed that in continuous operation.
You may find that they are bright enough at say 15mA, which would give a longer life expectancy.
E
Thanks Eric, said I'd showed my naivety. I don't wish to drive them at their maximum current. I thought you had to work from the Max current to ensure if it draws too much it wouldn't be an issue but even as I write that statement, I can see the error of my ways. The lower the amp's I can get away with the better from not just the component life span but the less heat I'll create. The plan is that these lights are on 24/7/365 so I'll take on board you recommendation thank you.

Which gives me
Forward Current 20ma x 40 = 0.8
Then as we've got 4 of these rigs 0.8 * 4 = 3.2a
Meaning a power supply capable of pushing 3.2a would be enough to power the rigs at around normal running conditions but something we can turn down to around 2.4a if bright enough would be better for long term running. That right?

In terms of the voltage that needs to be supplied, what is the calculation you use for when they are in parallel to allow for the voltage drop?

Assuming once I establish that, I'll know if the resisters are to low. Most calculators I'm looking at suggest it is currently. should be more 100 Ohms rather then 10. Leading me to wonder if the gent before me typo'd the order when buying the resistors originally.
 

ericgibbs

Joined Jan 29, 2010
11,118
hi S,
Do you have a 15Vdc 2Amp 'wall wart' power supply.
You could run 4 * 3.2V LED's in series, with single common current limit resistor.
E
 

Thread Starter

Spacey

Joined Mar 8, 2018
5
hi S,
Do you have a 15Vdc 2Amp 'wall wart' power supply.
You could run 4 * 3.2V LED's in series, with single common current limit resistor.
E
At the moment we've only got the LED's and the resistors. Oh and the hours of time that's already been spent making many parallel rigs.

We'll still have to source the power supplies. We have been looking at the typical wall wart power supply but its understanding what voltage we'd need to start from. Based on the 5V supply @ 3.2 VF = 89.99 ohms but I'm aware that doesn't allow for voltage drop of the cable (if applicable at 10 meters per rig) and each of the resistors.

Challenge I have is to convince my grouchy boss that we should bin everything that's already been done. Also if they are in series, wont I have a nightmare if a LED fails? As in they will all go off? The installation where this is destined for is very high up. So while the odd LED failing isn't a problem we wouldn't want to loose all off them if a single LED went.
If the 10 ohm resistor isn't the correct one then we'll obviously need to start again anyway. Just need to be sure in what I'm suggesting before we go to the expense of binning and starting again.

Sorry if I've missed the point along the way.
 

ericgibbs

Joined Jan 29, 2010
11,118
OK.
So 4 Rigs of 40 LED's at 20mA/LED, that's a total current/rig of 40 * 0.02A = 0.8Amps
If each rig has its own supply cable, regular 2.5A twin cable would be OK.

For each LED at say 3.2Vdrop, using a 5Vdc supply gives 5V-3.2V= 1.8v.... 1.8v/0.02 = 90R

Is that what you are planning.?
E
 

Thread Starter

Spacey

Joined Mar 8, 2018
5
Thanks for sticking with me Eric. I think your pain is almost over. As we do look to be heading in the same direction :).

So 4 Rigs of 40 LED's at 20mA/LED, that's a total current/rig of 40 * 0.02A = 0.8Amps
If each rig has its own supply cable, regular 2.5A twin cable would be OK.
Yes, that is what I was hopping for. Each rig will be connected to a set of shared terminals (4 wires to positive & 4 wires to negative). We will then connect the supply output of 1 power supply feed the positive and negative terminals.

You're breakdown below is also what I get.
For each LED at say 3.2Vdrop, using a 5Vdc supply gives 5V-3.2V= 1.8v.... 1.8v/0.02 = 90R
Which is why I can't fathom why the previous person used 10 Ohm resistors on each of the LEDs? As I believe, that should be 90 Ohms on each LED not 10 ohms. Would you agree? I can't find any input voltage that would operate the LED's and while still work with a 10 Ohm resistor.

Oh and one final clarification please. Currently each of the lighting rigs has a standard plug 3amp inline fuse on it. Would you recommend this or is it pointless given the power supply will have one within it and if its not pointless. I assume if we're pushing 3.2 amps based on the original calculation then the 3 amp inline fuse is just going to pop each time its turned on and we should change it out for a standard 20x5 mm glass 3.5a quick blow fuse?
 

ericgibbs

Joined Jan 29, 2010
11,118
hi S,
Based on a 10R resistor dropping 1.8V, that would mean a current of 180mA thru the LED.
That would suggest the LED's are power LED's for that other project, rated at approx 3.2V * 0.18 = 0.57Watts.

Why don't you fuse each 0.8A rig with say a 2A fuse, if one rig fails the others remain lit.
Look up inline fuse holders.

E
 
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