Discussion in 'The Projects Forum' started by epic_newb, Apr 11, 2011.

1. ### epic_newb Thread Starter Member

Apr 10, 2011
44
0
Hello,

I am trying to set up simple led circuit nothing fancy just want to be able to light them.

A little back info: This is my second time trying to finish this particular project and I am very newby as my name implies so bear with me. I recently lit up an array of leds and it worked for about a minute until they started burning out one by one. I think my main problem was that I only used one voltage limiting resistor for the entire array. technically it should have worked but it is very unstable.

Also, I noticed that my battery which has a nominal reading of 9v was actually 9.9v when hooked up to the multimeter. I didn't notice this until after my leds start popping. Does anyone know if voltage on batteries is supposed to change much? If so how would I deal with this problem? I'm trying to get my measurements as exact as possible in order to have my leds as bright as safely aspossible.

This is a drawing of what I have my circuit set up to currently, any suggestions would be greatly appreciated.

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
Either change to 2 LED's per string (and change resistors to about a 130 ohm 1/2W resistor)
Or go to a 12V power source, keep the 3 LED's in series and change to about a 120 ohm 1/2W resistor.

3.2V x 3LED's=9.6V = too much for a 9V battery.. Your battery might be 9.9V with no load but as soon as you put a load on it your back down to 9V or so.. Your Vf of the LED's cannot be more than your source voltage.

3. ### epic_newb Thread Starter Member

Apr 10, 2011
44
0
Ohhhh ok thanks.

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
LEDs, 555s, Flashers, and Light Chasers

First two chapters are tutorials on LEDs, the second half of chapter 2 gets more into what you're talking about (but also more complex)

As mentioned, 9V batteries are not stable. They quickly drop to 7V, this is expected and should be designed for.

It is better to leave around 3V headroom when designing LED circuits. Online LED calculators have no common sense in this reguard, I discourage people from using them. LEDs are very easy to predict and design around, why use a flawed tool when your head works better?

5. ### epic_newb Thread Starter Member

Apr 10, 2011
44
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Wow very nice guide but yeah it gets complicated quick.I am definitely still learning and currently working on chapter 6 of the ebook as well. Which I believe you helped write, correct?

Also, with regards to my circuit. Would it be better to have a more stable power source such as that of an outlet? At least to keep it simple. Thanks for your input! I keep learning a ton from this forum.

6. ### Audioguru New Member

Dec 20, 2007
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896
Nobody makes a "3.2V" LED. The voltage for an LED is a range of voltages maybe from 2.8V to 3.5V (look on its datasheet). Then if all your LEDs are actually 2.8V the 15 ohm resistor has 1.5V across it and it limits the current to 100mA which burns out your 20mA LEDs.
If you use only two LEDs in series then the resistor will have more voltage across it which helps to reduce the increase in current when the actual LED voltage is low.

7. ### epic_newb Thread Starter Member

Apr 10, 2011
44
0
Well, the actual led voltage range is between 3.2-3.4, maybe I should be using the middle number (3.3) to acquire more correct calculations of resitance.

I ordered the leds from ebay:
http://cgi.ebay.com/100-pcs-X-5mm-U...033?pt=LH_DefaultDomain_0&hash=item5198b8ecb9

Unless the product description has the wrong information that should be the voltage. Also, I don't know where I would be able to find the data sheet. I think it may require exta voltage than normal leds because is a UV led? In any case the range is fairly small and I don't think it should be too large of a concern but sorry for not being clear and thank you for your time

Last edited: Apr 11, 2011
8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Most of the book was written by Tony Kuphaldt. He is a professor I'm not sure where. I suspect it was a bigger project than he had anticipated.

I've written a few chapters, along with some experiments. I'm pretty decent with the 555 timer chip. Mostly it is a group effort, the very definition of open source.

You can make or buy simple LED testers, and measure the Vf yourself. It is what I would do. A simple 9V with a 470Ω resistor will light most any LED, you just measure the voltage across the LED. Do 10 or so and average the number (possibly setting aside any that are too far out the limits). Semiconductors can have a wide range, this is true of transistors and many other devices.

9. ### Audioguru New Member

Dec 20, 2007
9,411
896
The LEDs were probably made by an no-name-brand unknown manufacturer "over there". Do you trust their information when they don't even have a datasheet?

Common sense and a little simple arithmatic shows that if they are 3.3V each then three use 9.9V and the 15 ohm current limiting resistor will have almost no voltage across it with a 9.9V battery then the current would be almost zero and the LEDs would barely light and will not burn out.

10. ### themindflayer Member

Oct 29, 2010
44
1
Set up your system turn it on, Measure the following for quick diagnosis:
- Battery voltage
- Voltage across each R
- Voltage across each LED

LEDs will burn out if they get hot .... check whether they do ...

11. ### epic_newb Thread Starter Member

Apr 10, 2011
44
0
Yeah, don't really trust the leds myself so I will just do what Bill said and see how much they use, I really just got them because they were cheap and thought it would be good for a start.

Also, I'm pretty sure the "9.9v" battery or what I thought it was, is incorrect anyways. Thanks to mcgyvr I learned that voltage drops when a load is placed on it. I am thinking of either abandoning use of a battery as a power source and using an outlet or using a voltage regulator and a higher voltage battery.

But thanks again guys, still learning

12. ### Audioguru New Member

Dec 20, 2007
9,411
896
A 9V alkaline battery is 9.6V when brand new and quickly drops to 9.0V then slowly drops to 7.2V.
If you connect two of your "3.3V" LEDs in series and in series with a 120 ohm resistor then the current will be about 25mA when the battery is brand new, 20mA when it is 9.0V, 12.5mA when the battery is 8.1V and 5mA when the battery is nearly dead at 7.2V.

13. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
I'd just get a 12V regulated wall wart and use that..

14. ### epic_newb Thread Starter Member

Apr 10, 2011
44
0
Would this do the trick?
http://cgi.ebay.com/12v-DC-Power-Su...096?pt=LH_DefaultDomain_0&hash=item35abdd6d70

Seems kinda spendy for my purposes but it will probably save me a couple headaches and disappointments and maybe I can use re-use it on some future projects

The only thing I would be concerned about is being able to determine positive wire and negative wire. It sure would be convenient if the wall wart came with the wires all ready but oh well

Last edited: Apr 12, 2011
15. ### Audioguru New Member

Dec 20, 2007
9,411
896
Do you want to buy the no-name-brand power supply that is not UL or CSA approved for use in North America?
If it catches on fire and burns your house down then your insurance company will say, "Too bad it is a cheap non-approved one so pay for the damage yourself".

I bought some UL and CSA approved Name-Brand similar power supplies (5.0VDC at 2A and small and lightweight) at a local surplus store. They were made for telephone line modems a few years ago. I paid \$1.25 each. Inside there is a trimpot that adjusts the output from 3.0VDC to 20.0VDC. The output is regulated and has a small level of a high frequency.

16. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It is pricey, you can do much better from your local Goodwill store (or any other used electronics place). Radio Shack tends to love their stuff a lot too much.

You haven't mentioned what part of the country you are. The eBay link suggests North America. It matters only in that we can recommend good vendors.

Digikey, Mouser, BG Micro (my favorite), All Electronics, are all good mail order and internet vendors.

Used laptop power supplies are also good sources.

17. ### epic_newb Thread Starter Member

Apr 10, 2011
44
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I'm from Minnesota, USA.

Yeah, I was lookin around on Amazon and saw
VideoSecu 12V DC 500MA Regulated CCTV Camera Power Supply AC to DC Power Adapter 1AR, it was even UL-listed. I put the link on here and it said a moderater had to approve it but I guess it never got approved. I was wondering if that would work?

But yeah, I plan on looking around at goodwill before I make any online orders. What are some of the online places that you would suggest?

18. ### epic_newb Thread Starter Member

Apr 10, 2011
44
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I meant to say: what are good vendors for the area I live in?