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Author Topic: LED help in ciircuit power side  (Read 6062 times)

Posts: 15

« on: February 10, 2013, 11:54:52 AM »

I am taking a new direction for my electronics besides ham radio of course.

What I want to learn is to build a simple power supply for led lights to use as simple back lighting of stained glass signs.

These signs will be callsigns or anything radio related or for that matter anything at all.
MY 1st sign I want to do is 1 for myself to use as a learning curve.
I wish to use 5mm led's or even 3mm for simple low lights to just light up a 3" basic sized letter .

now comes the fun part, I believe they are 3.3v to 6 aprox. I do not want to use battery parts at all.
these letters KE5HTB will be mounted 2 different ways.

#1 will be a stand like most other sign shops use or desk stand. put led's into base and shine lights up thru the glass.
this will be fine except for the fact that if I use non-solid pieces of glass or cut letters and numbers in parts and solder back together this will not work.  of course I do try 100% to not cut anything up if I can make it in 1 piece for kletters and #'s it will be.
in the above case I use copper tape and solder letters back to 1 piece.
Sure wish I could use the copper tape as power lines also but they are all 100% connected together. I do plan on running the wires in these channels if needed.

#2 is a solid background of plexi or even 1/4" plywood painted up some how. But this is my goal at 1st to set the letters onto plywood and light via holes thru plywood behind letters. I am debating raising letters 1/4" off plywood to allow light to travel more also.

OK What I need is a decent small power source to run aprox 20-30 leds off the 1 supply. I have some 9v wall warts I am going to try but I hate wall warts and to keep my head and myself moving forward on building small things I like a suggested kit, place to look at them, or any ideas at all. I know the basics not a real up to date tech at all for ic chips and what they do or dont do etc. older tech times for me was in between tubes and ic chips. basic transistors and such.
thanks for any ideas at all.

Posts: 4534

« Reply #1 on: February 10, 2013, 12:58:54 PM »



Posts: 3312

« Reply #2 on: February 10, 2013, 01:02:07 PM »

Fred -

Here is your desktop reference (and training manual):
United States Navy Electricity & Electronics Training Series - NEETS*

SparkFun has a number of simple kits for your project.

Spark Fun tutorials

Power Supplies

What I need is a decent small power source to run aprox 20-30 leds off the 1 supply.
I have some 9v wall warts I am going to try but I hate wall warts
You know the voltage you require (VDC), now need to determine the current draw (measured in Amperes) to properly size a power supply.

« Last Edit: February 10, 2013, 01:06:26 PM by W9GB » Logged

Posts: 15

« Reply #3 on: February 10, 2013, 02:17:24 PM »

actually the navy manual was cool to look thru and such. but more reading then I need to do right now at least. thanks for the link I will use it for future references.

total amps will be around max 1 amp I am kind of guessing on that right now as we all know depending if I go series or parallel circuit will determine some of this fun also.

As for the 1st comment, this is why I dont post to forums hardly ever. cause people like you that make folks like me who are handicapped needing some small help and wonder why I never post any more questions here at least. If you know how great for you, I asked simple ? got 1 reasonable reply to read on and it is appreciated even though not what I was hoping for.
I would like more links to kits mostly for now as I am in no way a pcb man at all. besides my soldering skills are not great since I got hurt in a 20' fall.

I looked at allot of links from here already, but I get tired easy and keep gooing back to sites I have been to and get lost. so I asked for some help.
 I think 1st guy need to get a life or maybe a GF to get rid of some of that rage in him.
leave us handicapped guys alone we too enjoy the ham radio hobby and I will never be a tech on them, but I want to learn basics and try hard to build my own products for my own home and my stained glass shop.

if the moderator thinks I am out of line it is ok to remove this totally as I think I am going to get more and more dumb remarks and not 1 bit of help cause people are mainly rude here and love good forums wars only.

thanks for the 1 link I will bookmark and use it for sure.


Posts: 4534

« Reply #4 on: February 10, 2013, 03:16:24 PM »

Apparently you didn't realize the top line of my reply....... The text in blue......... Is a link to the spec sheet for an LM-350K three pin regulator.  Possibly one of the easiest ways to build a regulated power supply.  My apologies for not making that painfully obvious.



Posts: 1371


« Reply #5 on: February 10, 2013, 04:16:04 PM »

Question: why do you need voltage regulated supply for LEDs that are lighting a sign?

now comes the fun part, I believe they are 3.3v to 6 aprox. I do not want to use battery parts at all.

The first step is to know what kind of diodes you have. The rule of thumb is that the higher the intensity, the higher the "knee voltage".
That being said, LEDs are CURRENT devices, they do not work like light bulbs! The brightness is controlled by the current flowing through them. Current limiting by using a series resistor, or some other device is required. They do have a maximum, so you will have to find out what that is, or ramp up the current experimentally.

With higher input voltage than the 3.3v knee, you may have to put them in series. The less power dissipation  in the control device, the better. A 5V suppy of sufficient current with an adjustable current source output would be one way to do it

The other way is to use a 5v input to that LM350, regulate to just above the knee with small limiting resistors in series with each LED. That way if one dies, the overall brighness would stay the same. you may outstrip the 3a max on that regulator with several high intensity LEDs, though.
« Last Edit: February 10, 2013, 04:40:57 PM by AC2EU » Logged


Posts: 607

« Reply #6 on: February 10, 2013, 04:37:01 PM »

A Google search for "power supply kit" gets quite a few hits.  These look OK;

but there are several others.

These are just regulator boards; they require a transformer.  A plug-pack is actually quite useful; it removes heat, bulk and weight from the device it powers.

I built a "plug-pack" for one of my projects for just those reasons;

Don't dismiss the humble plug-pack.

An AC plug-pack would drive a regulator PCB like the ones in the link above but note my later comments about voltage.

You should read about/Google for information on LEDs, particularly driving multiple LEDs.

LEDs are not usually driven in parallel; small differences between them mean that different currents flow in each one.

They can be paralleled (I have done it) but it's not good practice and can blow the LEDs that "hog" the current.

Series connection is usual; that means that a supply voltage greater than the sum of the LED drops is required.  Say six in series, each with a 2v drop; that's 12v @ 20mA.

(LED voltage drop depends on the colour and varies over a range; I used 2v for simplicity.  20mA is the usual current for ordinary LEDs).

To drive those six series LEDs through a current-limiting resistor, about 15v would be required; that gives a resistor value of 150 ohms.

To use 20 to 30 LEDs as you propose you could use a series/parallel circuit; series connection of, say 5 LEDs minimises the current-hogging problem so four parallel strings of five LEDs each string would work for 20 LEDs.

Each parallel string should have its own resistor; in the above example, a 15v supply with a 250 ohm resistor in each series string would work.

(Hope my maths is correct; I just did it in my head).

Another thing to consider; you may not need a regulated supply, just a single diode or a bridge with a transformer/plug-pack and a capacitor.

Millions of LED garden/path lights and similar lights work this way; it's much cheaper than a regulated supply and just as effective for non-precision applications.


Posts: 6994

« Reply #7 on: February 12, 2013, 07:44:38 AM »

Like 5UP I am a believer in the regulated supply; even going just one step further and using an LM-317 variable regulator chip.  The reason is the 3-legged regulator will give you a fixed voltage no matter what you hang on it up to it's current limit.

With the variable voltage regulator, which requires only one or two additional components, the voltage can be set to whatever is necessary for your project.  These 3-legged regulator power supplies are almost as simple as the wall wart.

Speaking of which, I would power the regulated supply with a wall wart.  They're readily available cheap (or free) and are very reliable.

A Pessimist is Never Disappointed!

Posts: 58

« Reply #8 on: February 19, 2013, 02:05:07 PM »

Don't dismiss the utility of the wall warts.  Do you want to learn about LED's, or learn about building a regulated power supply, or both?  As others have noted, you sure don't need a fancy regulated supply for LED's.

If you haven't already done so, take your 9-volt (DC) output wall wart and experiment with one LED, and with two and three series LED's, and a series voltage dropping resistor.  You can see how the LED brightness varies with the current through voltage dropping resistors of various values, and the percentage of system power wasted by the dropping resistor.  A cheap resistor substitution box will make it easy give you a good understanding of LED operation.  Then try the same thing with a 12-volt wall wart.

Be aware that the life of your LED's can be shortened a LOT by operating them quite close to their current limit.  That is another choice to make.  Stringing an appropriate number of LED's in series with a 24VDC supply can simplify your wiring of many LED's.

Then, perhaps it will be easier to choose your power supply option, and your desired output voltage.

Posts: 6994

« Reply #9 on: February 20, 2013, 08:41:56 AM »

As others have noted, you sure don't need a fancy regulated supply for LED's.

If you haven't already done so, take your 9-volt (DC) output wall wart and experiment with one LED, and with two and three series LED's, and a series voltage dropping resistor

First, you want to measure the output of the 9-volt wall wart, or any wall wart for that matter.  You will find the voltage much higher than the rating.  The wall wart rating is the voltage stamped on the case at the rated current output!

Since an LED draws only about 20ma you can see that the unregulated output of the wall wart is going to affect your dropping resistor calculation.  Which is OK, until you start adding LEDs or other loads.

With a regulated fixed voltage, you can calculate your dropping resistor value and as you add more LEDs and or load, this dropping resistor value remains the same for the same LED brightness.

For those who have never dissected a wall wart, it normally consists of a small transformer, a single diode and am electrolytic capacitor.  The no-load output voltage quite often is 50% higher than the rated voltage.


A Pessimist is Never Disappointed!

Posts: 58

« Reply #10 on: February 20, 2013, 09:45:19 AM »

Probably none of the cheap LED devices from China incorporate a regulated supply.  Since the load on the supply is fixed, as sounds to be with the OP's decorative lights, regulation is not germane. Of course, learning about regulated supplies is a different thing.

If you do this experimentation, another thing that you will realize is that most LED devices from manufacturers (such as handheld flashlights) that claim something like "Super Bright" are often doing little more than operating the LED at, or even higher than, maximum rated current.  This means short LED life - because the LED then generates heat, and it is the excess heat that degrades the diode.  But for a compact flashlight that is used infrequently, that might be a reasonable tradeoff.

Note that the dropping resistor also functions as a simple regulating device.  It is a tradeoff.  The more power the dropping resistor dissipates, the more the regulation effect.  This is an interesting decision to contemplate as you choose your circuit values.

Check on eBay.  There you will find great prices for bulk LED's of any color, any size, and any candle power when shipped direct from China.  I have ordered many of these, and never been disappointed.  You can buy for 1/10th the price at Radio Shack, or even less, including the cost of shipping.  Shipping is reliable. You only need the patience to wait three weeks, or a month.
« Last Edit: February 20, 2013, 09:55:18 AM by KF7Z » Logged
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