Can this circuit works What changes I have to make to Drive 1w Leds on same circuit

Discussion in 'LEDs and Optoelectronics' started by sanbadgujar, Aug 28, 2014.

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2. Harald KappModeratorModerator

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Welcome to our forum.

This circuit needs to be modified: the 470Ω resistors have to be in series with the LEDs, not in parallel.
What is the nominal voltage of the 1W LEDs you are going to use? From the voltage of the LEDs we can determine the current (for 1W operation) and the required resistors and possibly transistors.
Note that you may need a higher supply voltage than 3.6V - depending on the LED's voltage.

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Looks like the LEDs don't have any current limit in the diagram. Is that right?

4. davennModerator

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Yep, that's why Harald said it needs modifying to have the 470 Ω resistors in series with the LEDs not in parallel

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Where did I put that blonde hair dye now? That's scan reading for you. Sorry Harald

6. KrisBlueNZSadly passed away in 2015

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That circuit may work as drawn with some combinations of LEDs and battery voltages, but it's bad practice to drive LEDs without current limiting resistors. When your battery voltage starts to drop, the LEDs will become a lot dimmer quite quickly.

The best way to drive high power LEDs is using MOSFETs but they don't tend to work well at 3.6V unless you get specialised modern ones that are only available in SMT (surface mount). For your case I suggest using two PNP transistors driven from the collectors of the NPNs and acting as buffers, with current limiting resistors, like this.

I've increased the base resistors from 4k7 to 47k and reduced the capacitors accordingly, just to save power. You don't need to make that change if you don't want to.

Q3 and Q4 are driven from the outputs of the astable multivibrator (oscillator) circuit and they provide buffered drive to the LEDs. You can calculate the LED current limiting resistors using the guidelines and formulas in Steve's tutorial at https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

Q3 and Q4 will drop about 0.2~0.3V so you need to take that into account.

You may need a higher battery voltage to get a reasonable voltage drop across the current limiting resistors for stable LED brightness.

7. bigone5500

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Kris, this is off topic but what do you use to produce those schematics like that one?

8. KrisBlueNZSadly passed away in 2015

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I use an old MS-DOS package called OrCAD/SDT III from 1988. What do you think of them?

9. Harald KappModeratorModerator

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Used to be a fine tool at the time.
Still seems to be not outdated when it comes to drawing schematics (of course readability depends on the person drawing the schematics - I like yours ). I wonder: does it run on Windows 7 and higher?
Lacking such a tool I (ab-)use LTspice for drawing schematics. It gives me the advantage of being able to simulate the circuit should I feel like it.

10. KrisBlueNZSadly passed away in 2015

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Yeah, I like it. I still haven't found a single bug.

Thanks Harald

It doesn't run natively under Windows 7 - at least, not Win7 x64. It runs fine under DOSbox though.

Yes, I use LTSpice for drawing schematics too, especially when I want to simulate them. But LTSpice is really short on components and it's fairly easy to add new components to OrCAD/SDT III. Also OrCAD/SDT III gives you a lot of control over how your components and schematics come out. I like it. The schematics are uncluttered and I can ensure they have just the information I want - no less, no more.

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Ok cool.

14. bigone5500

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I like the way they are done. I found an old copy of it online but can't figure out how to run it. Does it need administrator priv to run? I'd like to think I'm a little tech savvy but this one got me.

15. KrisBlueNZSadly passed away in 2015

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What OS are you running? It runs on XP but with Win7 and later you need DOSBox (http://www.dosbox.com/). It doesn't know about privileges; it's an MS-DOS program!

There's no need to "install" it, but you need to set up the directory paths to the libraries, schematics, etc, and they need to be "8.3" filename compatible. I just point them to the same folder, and use subfolders for schematics - you type the subfolder as part of the filename you want to load or save.

The main program is called DRAFT and you run it with DRAFT /C to configure the settings.