Couple of Questions related to LEDs

subratabera

Newly Enlightened
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Jul 24, 2014
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Hello Friends,

I am new to this forum and very new to LEDs. I'll need a lot of your help with my journey to learn more about LED lighting.

Please find my initial queries listed below.

1) Can I drive a LED which requires 3.2V/150mA with a power supply which supply 12V/1A? With a little search I found that we have the formula "(Vout-Vf)/Vi". Now using this formula I am getting a registor value of 58 Ohms (12-3.2/0.15=58.67). Can this registance in series save my LED from high current supplied by the power supply?

+ "12V/1A" ~~~"58 Ohms"~~~"LED-3.2V/150mA"~~~ "-"

2) How to calculate the capacitor and registance value for a transformer less power supply where we need 48V/150mA and 48V/100mA output ?

3) How to keep the SMD LED 5630 (3.2V/150mA) cool. Can we drive it with lower voltage / current without loosing much brightness?

4) What do you think about this variable power supply to drive different LEDs ? (http://www.ebay.com/itm/5A-DC-to-DC...232?pt=LH_DefaultDomain_0&hash=item484c99fbf0)

5) How to make a simple constant current power supply with required voltage / current ?

Thanks,
SB.
 

DIWdiver

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Welcome to the forum, subratebera! I hope you find it as useful and welcoming a place as I did.

These are questions that reach far into several very different realms of knowledge. They can all be answered simply, or with great complexity. The questions show that you have an understanding of the problem, which is far more than many first posters have. I suspect that you will learn very quickly here.

However, each of these questions probably warrants a thread of its own, as each will probably elicit numerous responses that will take some effort to assimilate. Also, it kind of looks like you might be trying to take a 'short-cut', asking a lot of questions, some of which could be answered by reading, instead of asking.

I'd suggest you do some reading on the forum, to give you a better platform from which to launch more targeted questions, which would elicit replies more appropriate to your specific needs.
 

subratabera

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Jul 24, 2014
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Thank you very much DIWdiver for the reply and guidance. After searching and experimenting a bit I learned couple of things which I am listing below. I was indeed trying to avoid multiple threads and to keep it at one place for easy reference, however, I'll open a separate thread to keep it more focused.

1) For the first question I found that it can be done, and the registance will only pass the required current to the LED, provided, if the voltage is regulated.

2) I found that with capacitors you can only get required current to some extent. Getting the required voltage can be a bit tricky. I found the below link very useful.

http://homemadecircuitsandschematic.../how-to-calculate-and-deduce-current-and.html

3) This one is a bit difficult. I tried driving them with lower voltage/current but they are not as bright as when they are getting required voltage/current as per the datasheet.

4) I bought the power supply and experimenting with it. This video helped me a lot.

https://www.youtube.com/watch?v=piET0Biqo0I

5) Found this video useful. Searching for more articles.

https://www.youtube.com/watch?v=Ezf87w62FLg


Regards,
SB.
 

DIWdiver

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Messages
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Location
Connecticut, USA
Perhaps I was a bit hasty in saying these questions should be treated separately, as several of them are easily answered.

1. Yes, the resistor will limit the current. If the voltages are well known and constant, the current can be set precisely with the correct resistor. However, power supplies can be well regulated, or not, and LED voltages are always a bit squishy, meaning they change under different conditions. This will make the current change. If the supply voltage is substantially higher than the LED voltage (like 12V and 3.2V), a small change in voltage will make a small change in current. But if you try to drive a 3.5V LED from a 4V battery, a small change in either voltage results in a much larger change in the current. This is the fundamental limitation in using resistors to drive LEDs.

2. It depends on how big a capacitor you are willing to buy. The capacitor value required is proportional to the current you need, so theoretically you can get as much current as you need. The link you posted is basically accurate, but doesn't go far enough. He doesn't discuss Z1 in his schematic. This is a zener diode, and it will limit the voltage across C2. It will conduct all the 'excess' current not used by the load, keeping the voltage constant as the load current varies.

3. LED brightness is pretty nearly proportional to current. Heat generated also increases with increased current. So running them at lower current will reduce both the heat and the brightness. You can't have your cake and eat it too. The way to keep them cool is to use heatsinks. That's a concept for it's own thread.

4. I've seen several supplies like that, and thought they might be great for LED work. I've even been tempted to get one (or several - they are SO cheap) just to play with and report on. Please let us know how you make out. By the way, that video is excellent. Something every beginner should watch!

5. The supply you found is one way to create a constant current power supply (in the LED world we often call them 'drivers', but they are just special-purpose power supplies). There are literally hundreds, maybe thousands of others. There are only a handful of basic 'topologies', but each topology can be implemented with nearly infinite variations. The topology in the video is one more. There's enough to talk about in power supplies for dozens of threads. By the way, if you understood that video with no formal electronics education, do you need a job? I'd love to be able to hire someone with such natural ability!
 

subratabera

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Jul 24, 2014
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Thanks a lot for the detailed reply DIWdiver. That's very helpful.

I am experimenting with adjustable constant current driver and its really a great product. Waiting for couple of components for further experimentation.

I want to learn more about constant current drivers but before that need to improve my electronics skills which is just a hobby for me as of now. Thank you very much for the compliment and offer by the way. That made my day! :)
 
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