Hot Resistors on LEDs

[EmmienLightFan]

I have loads of store brought LEDs that run on 24vac which is rectified to DC inside a multi function controller box that I have cut off.

 

I measured the output of the controller box as 31vcd, and have brought a 30v 4 amp DC power supply.

 

The resistors on some of the lights get very warm. I never felt them before so perhaps this is normal, but I am worried now that it is something to do with too much current. I don't know much about resistors and LEDs and constant current drivers. Everything I know about it is read from people burning LEDs up by doing something wrong, and I don't want that to happen to me.

[EmmienLightFan]

Anybody?

[Nomad2275]

well i will take a guess, if i remember my schooling and such turnung 24ac to 30 dc seems a bit off, i seem it mostly the same voltage if not lot less, like half. so like i would use a 16vac trasformer to get 12vdc, and if i wanted to make 24vdc i would have to have like a 24vac or 30vac to get the 24vdc. but you wanted a quick answer so theres my guess with out pulling out all my books to find the references

 

it would not be the current thats the issue, as it will only draw what it wants its all in the voltage.

Edited September 19, 2015 by Nomad2275

[EmmienLightFan]

I measured the output as 32vdc with my multimeter, so I think that it is correct. I thought you got more voltage when you turned AC to DC, but I could be wrong.

[Nomad2275]

you may want to connect the original power supply back to the lights and retest, as i have also seen it different when no loads are attached compaired to no load attach.

 

best example for power covertion i can pull oyt of my head is these, a 12vdc/24vac camera, the camera really runs on 12vdc but there is a rectifer in them that changed the ac to dc and such powers it, another is for electric strikes, 12vdc/24vac, in these all there is a coil nothing else so tghe coil works on 12vdc or 24ac with out no difference except is chatters on ac cause of the alternating currents.

[EmmienLightFan]

you may want to connect the original power supply back to the lights and retest, as i have also seen it different when no loads are attached compaired to no load attach.

 

best example for power covertion i can pull oyt of my head is these, a 12vdc/24vac camera, the camera really runs on 12vdc but there is a rectifer in them that changed the ac to dc and such powers it, another is for electric strikes, 12vdc/24vac, in these all there is a coil nothing else so tghe coil works on 12vdc or 24ac with out no difference except is chatters on ac cause of the alternating currents.

But the lights are "connectables" that means that the lights have a socket on the end and you can connect another set.

 

Wouldn't the power supply have to be regulated, as somebody may just plug in one string, but somebody else may max it out and put 30 or 40 strings on. The voltage would have to be the same whatever the load.

[Max-Paul]

Actually AC and DC are two different worlds. When using a DVM and reading AC voltage. Your actually reading what is known as RMS (Root Mean Square). Because AC Voltage is a sine wave. The valleys dont produce as much power as the peaks do. While DC is a constant voltage, power.   Ok now some examples, A 100 volt peak AC voltage will read 70.7vRMS. You dont see the peak voltage with a DVM (would see it with an O'scope though).

 

I took some 18" spriral trees and removed the 35 ican bulbs and made some red, yellow, orange, green, blue and white trees. Now Red, Orange, & Yellow LED are about 2.4vDC each. Your Green, Blue, White, Pink and a few more LED are rated at about 3.6vDC each.

 

Another point to be made. Even though you have recitified the A.C. voltage/current to a D.C. voltage and current. It is a DIRTY voltage with those peaks and valleys. This will trick the DVM into thinking the voltage is lower than the peaks that actually are coming out from the rectifier. Now if you put a filter capassitor on it the voltage would be a true D.C. voltage with no peak voltage. But a filter cap. will cause problems with fading and in worse cases catch on fire as some of use have seen over the years with some brands of big box store lights in the past.

So, you have to take in account this peak voltage and then apply some ohms law. Frankly I have tried this and found that the resistance is still off some. instead I would put my meter on current and put it in series with the LED. 14mA RMS is actually 20mA peak. Now one more consideration, is the wattage of the resistor. You can get close enough by doing this. 120Volt RMS multiply by 1.414 is about 170Volt peak. Although I have found that a white LED times 46 is all the more I can put on a string with NO resistor.  So, I cant say I have the absolute answer. I understand the whys of it. But in the end, use a current meter and adjust the resistor so that you only have 15mA. Oh, ok for the wattage. Use the above info about voltage drop per LED. Add up all of the voltage drops. Take 160 volts minus total voltage drops, this equals to voltage that needs to be dropped across the resistor. Take this voltage and multiply by .02A. This is the minimum wattage rating of the resistor. Lets say you get something like .33 Watts. Go to the next higher rating of .5 watts. Do not drop down to .25 watts cause it will heat up and try to burn down your house. Well maybe just your wiring.

 

If that was clear as mud, ask questions. I'll try to reword it so it becomes clear.

 

In a nutshell A.C. and D.C. voltages are not the same. Also, Dirty D.C. and a filtered D.C. voltages are not the same either. Filtered D.C. is the only voltage that Ohm's law can be directly applied to. And with A.C. and Dirty D.C. you have to take in account the Peak voltages and Peak current, and meters only read RMS or 70.7% of actual value.

[EmmienLightFan]

I tried the lights on 24 volts and they were very dim. Several people on the Australian Christmas Lighting Forum said that this is what they do, and that 31v is a very common voltage.

I didn't put in the resistors. The resistors are inside the LED and shrink wrapped/Heat shrinked. Other ones are inside rope light.

[EmmienLightFan]

I tried the lights on 24 volts and they were very dim. Several people on the Australian Christmas Lighting Forum said that this is what they do, and that 31v is a very common voltage.

I didn't put in the resistors. The resistors are inside the LED and shrink wrapped/Heat shrinked. Other ones are inside rope light.

[Max-Paul]

Did you check the current flowing through the circuit? And do you have any idea as to how many series circuits are wired in parallel? Oh and lets try to establish what is hot. If you burn your fingers with just a moment contact that is too hot. If you can hold onto the resistor for more than 5 seconds then its ok. About 130 degrees F (54.4C) is the limit of most people to heat.

 

Or your string was poorly engineered. without more specific info like the current flowing through your sting I cant address what might be going on. Might also help to know how many series circuits are in a parallel configuration. example might be that you have a 100 LED string consisting of 2 33 LEDs and one 34 LEDs. Current will most likely be around 60mA.

Edited September 28, 2015 by Max-Paul

[EmmienLightFan]

I can touch them fine at first, but after they have been turned on for a while they get hotter. I can touch them for about 15 seconds

[Max-Paul]

Then this might be a non-issue.

[EmmienLightFan]

I tested the original transformer and multi-function controller again.

 

I got different readings. This time I got around 24v (Measuring DC) and 40v (Measuring AC). I tested both with & without lights and actually got slightly higher voltage(1.2volts higher) with no lights.

 

I will test them on a 24vdc SMPSU, but I think that I will get the same results that I got on other lights, and the first ones will be dimmer the further along the string I go.

 

Just now some lights on the second set stopped working. They were on the original controller and transformer.

 

It is annoying. I think it is a bad connection.

I am now sure that the lights are in two parallel series, as it is every other light that has stopped working, and only until half way into the string.

Edited September 30, 2015 by EmmienLightFan

[EmmienLightFan]

I'm not sure if I am correct, but if there are 12 lights that have stopped working, the lights are in series of 12.

 

Each LED needs about 2.4v (I think they are red, but could possibly just be white ones with caps on, and they are special LEDs) so if there are 12 of them I need to supply about 28.8v. Should supplying them with 2.5 each make any difference?

 

 

The resistors are there to limit the current to the LEDs, aren't they? How come they get hotter depending on the voltage?

 

Also, there is a little twisty thing labelled "ADJ" on the 30v power supply. Is this to adjust the output voltage or is it something I shouldn't touch?

[Max-Paul]

Instead of asking so many questions. And wasting time here. Go and get a book or search the web and read all you can get your hands on about Ohm's Law and Watt's law. I am sure you will start thinking like an electrician.

 

What happens if you change the voltage to a resistor. What does the current do? Now what happens to the wattage if the the voltage stays the same but the current changes? If you knew Ohm's law and Watt's law you could answer these basic questions on your own. All you appear to know now is a few sound bites like current limiting resistor. Which by the way is a stupid statement. How do you limit current??? not by just a resistor.  And BTW I am not saying you made a stupid statement. It has been a saying that has been repeated over the years. Heck I have said it many of times myself. The fact is the current flowing through a resistor will change if the voltage applied the resistor changes.

 

Hint, think see saw and you will be able to visualize what I am talking about.

Edited September 30, 2015 by Max-Paul

[Max-Paul]

Actually the see saw does not work as well as i was thinking at the moment. Here is the formulas

Ohm's law:  E=I*R, R=E/I, I=E/R: Volts, Amps, Ohms

Watt's law: P=E*I, E=P/I, I=P/E: Power/Watts, Volts, Amps

 

Cheers

Max

[EmmienLightFan]

I will do some reading.

 

One simple solution that has been suggested to me would be a voltage regulator. I could then lower it to around 28 volts.

[EmmienLightFan]

I was looking on the company's website and I saw this:

 

http://www.festive-lights.com/accessories/small-white-power-pack-with-built-in-controller-connectable-uk-plug.html

 

When I zoomed in, on the multi function controller power supply it says it has an output of 26.3v. I think this must be the correct voltage for the lights.

[EmmienLightFan]

Anyone?