Electronics Forum

by **Mavruss** » Mon Dec 05, 2011 7:52 pm

I ran the 12VDC circuit through the simulator. I took measurements on just one of the LED buses.

LED1: 9.96V
LED2: 8.3V
LED3: 6.64V
LED4: 4.98V
LED5: 3.32V
LED11: 1.66V

Current draw: -204ma

by **Mavruss** » Mon Dec 05, 2011 8:09 pm

I also tinkered with a new design. Instead of series, I went parallel.

The simulator shows a -198.84ma draw and 1.6V on the LEDS. I dumped the 10 ohm resistors for 52 ohm.

by **admin** » Tue Dec 06, 2011 3:17 am

It is important not to connect the LEDs in parallel as it would lead to their ultimate failure. Keep the two branches of series connections and measure voltage across each LED and the voltage across resistor and post it here. I think the LEDs are dropping much more voltage than specified.

by **Mavruss** » Tue Dec 06, 2011 2:59 pm

LED1: 9.96V
LED2: 8.3V
LED3: 6.64V
LED4: 4.98V
LED5: 3.32V
LED6: 1.66V

Power source 12V
Current draw -204ma

this is for only one branch.

If you dont mind me asking, why would running in parallel burn out the LEDs faster?

by **admin** » Tue Dec 06, 2011 4:45 pm

I meant measure the voltage across LEDs & resistor in the physical circuit which was not working as expected.

When you connect diodes/LEDs in parallel there is a chance of thermal runaway and diodes getting damaged.

by **Mavruss** » Tue Dec 06, 2011 5:18 pm

Ah, got it.

i will do that tonight once I get home from work.

by **Thomas W** » Thu Dec 08, 2011 8:19 am

Mavruss, did you measure the voltage on the output of the battery, when the circuit was running?:

[quote="Thomas W"]Are you using the battery or the AC/DC adapter? What is the voltage output?
It should be close to 9 V or 12 V.[/quote]

If it isn't close to 9 V the current will drop.

by **Mavruss** » Thu Dec 08, 2011 5:17 pm

I measured the voltages last night but I forgot the paperwork at home. :evil:

Anyway, I will post them tonight when I get home.

I might connect that 12V transformer tonight to see what happens.

by **Mavruss** » Fri Dec 09, 2011 1:54 am

LED1: 6.89V
LED2: 5.46V
LED3: 4.07V
LED4: 2.72V
LED5: 1.38V

by **admin** » Fri Dec 09, 2011 3:03 am

So the LEDs seem to be OK. Please measure the actual resistor value and post it here. Also measure the actual battery/adapter voltage when connected to the circuit & post it here. It looks to me as if the resistor value may be too high or the battery/adapter voltage is too low.

by **Drew1** » Sun Dec 18, 2011 8:25 am

I am new here and was reading your thread about the IR light source and wanted to offer some input.
First thing you need to consider is that the 9V Battery is only 9V fresh from the box. I looked at a typical 9V battery, the Duracell MN1604 (http://datasheet.octopart.com/MN1604-Du ... 306548.pdf) and am basing my calculations on it.
Your LED’s have a max voltage drop, known as Forward Voltage or “Vf” of 1.6V and an absolute maximum rating of 140mA. I do not recommend running your LED’s at this current as it will limit their life. I would maybe do 100mA or less if you can get away with it.
The MN1604 battery has an internal impedance of 1.7 ohms so you will need to consider this when you design your circuit.
I suspect that the reason the circuit that Thomas W recommended was dim was because your battery source voltage was low or not rated at 240mA. The design should have worked IF the source was delivering 9V under a 240mA load. The NMH Battery Thomas W has would not be a good choice to power your circuit as the voltage will likely drop significantly as it discharges and given the relatively small Amp hour rating, it likely has an internal resistance that is fairly high. This means your LED’s will probably turn off long before the 38 minutes he calculated if they even turned on at all, remember you will need a minimum of 8V to forward bias the diodes and this does not count the voltage drop across the current limiting resistors.
I agree with what Thomas said about the parallel diode design, it will result in diodes burning out.
Your last post on 12/9 indicates that your battery is not beefy enough. 6.89V/5 is only 1.379V per diode and this means the diodes are not fully on.
What I recommend is the 12V AC/DC option as non-rechargeable batteries will give you brighter light for longer than a rechargeable but @ $3+ per battery it can get expensive. The rechargeable would be cost effective but would give you limited light for 1 hour or as much light as the former but only for minutes not an hour like you specified. The AC/DC will make things so much easier. 1 leg with 7 diodes and a 6.4 ohm .25W resistor will give you fully illuminated LED’s. The drawback is you would not be able to adjust the brightness. I do not recommend a 2 leg design as 200mA is your max capacity for the transformer and for the sake of not overworking the transformer, I would only run 150mA which only leaves 75mA per leg. This may make the LED’s to dim.
One option is to use an adjustable constant current regulator for the leg(s). This will enable you to turn the brightness up or down with a pot. These are easy circuits using a transistor, 2 fixed resistors and a pot. A constant current regulator; should you go with the battery option, will also allow you to maintain a stable current regardless of what the battery voltage is doing. Steady current means stable light output for the life of the battery charge not steadily dim over the time you use it.

by **Drew1** » Sun Dec 18, 2011 5:02 pm

I thought of one other potential problem that you might run into. These 12V Converters (Wall Warts) are typically not well regulated so under different loads, the voltage could vary by quite a bit. Using the 7 diode 6.4 ohm combo would require a pretty well regulated 12V source as the "overhead" voltage is only 0.8V (1.6V x 7 LED's = 11.2V). If the voltage drops under load (which I bet it will) by more than .8V, it will prevent the LED's from turning on. The best way around this is to use 5 or 6 LED's and recalculate the resistor value. You may have to experiment with your transformer to see what it is doing under load.

If you are heart set on the 9V battery option; the key to a 1-hour life is the battery voltage. Research the different batteries and depending on which one you choose, determin what the output voltage will be at a given constant current at the end of 1-hour. Then divide that voltage by 1.6V to give you the maximum number of LED's you can have in series. Be mindful of the resistor power rating as well. If you use 4 LED's in series with a 9V Battery at 140mA, the resistor will have to initially handle .364 Watts so a 0.5W resistor will be needed. If you use a wattage less than this, you will smoke it.

by **Mavruss** » Mon Dec 26, 2011 5:50 pm

Thanks for the input!

Sorry it took me so long to respond. I did a last minute vacation and was gone a week. Then, I returned and got sick for a week. This is the first time I have had the energy to look at anything online that wasnt work related.

Anyway, I will try the 6 LED one leg option with the 12V transformed and see what happenes.

I will post results soon!

by **Mavruss** » Tue Jan 03, 2012 5:43 pm

It works. I have the basics up and running. Now I have to work on optics. Mirror adjustments and diffusers. Im going to test it tonight in a large open area to see how well it lights that up. Seems plently bright for a smaller room.

I will post some pics once I get a housing built for it.

Now, the next phase is to either build a new animal or just make a second one to really illuminate a large room. Ballroom size.

by **Mavruss** » Wed Jan 04, 2012 3:21 pm

I did my test last night. This light is slightly brighter than the light I currently have with the 9VDC battery.

I'm going to have to double the amount of LEDs to get the light coverage I'm looking for.

What would you guys suggest? Should I just make two of these lights with their own PSUs or re-work the circuit with a different transformer?

I came across this:

http://www.instructables.com/id/Super-N ... dset-Hack/

This guy is using the same LEDs that I have and he is running off of 9VDC batteries. I take it his setup would run about 15 minutes?