LED Light Probe - Easy Soldering Project
08-23-2015, 06:04 PM
#12
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Since I am no way handy enough to make one my son picked me up this little beauty at the store. It has a flexible end and can get inside my bobbin area when I need it.[ATTACH=CONFIG]528578[/ATTACH]
08-24-2015, 02:48 AM
#13
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Originally Posted by Tartan
Since I am no way handy enough to make one my son picked me up this little beauty at the store. It has a flexible end and can get inside my bobbin area when I need it.[ATTACH=CONFIG]528578[/ATTACH]
08-28-2015, 05:01 AM
#14
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I'll do this in more than one installment, starting with the parts needed. I've put a lot of information here about the parts and their electrical operation, just for completeness if anyone needs it. Don't let it scare you into thinking this is complicated. It's not. Just skip over what you don't need or aren't interested in.
It is my hope in posting this that it can provide some experience with soldering. There will be an example of soldering to a piece of metal (the brass tube), which will absorb and radiate heat a little differently than wires do. It will involve tinning of wires and tack soldering techniques. We will start by tacking the parts together to make sure they work. This gives an opportunity to do a few connections that are only temporary, allowing you to get the feel of soldering before the final assembly.
Brass Tube
The brass tube used was obtained from Ace Hardware, although it might be obtained from a hobby shop or a home improvement center as well. It is 7/16 inch in diameter and 12 inches long. It was priced at $1.99. I have included a scan of the label that came on it.
(Brass Tubing Label)
[ATTACH=CONFIG]529056[/ATTACH]
LED
LEDs are fascinating devices. At their heart is a tiny semiconductor chip, maybe the size of a large grain of salt, and only a thin slice of that used. The chip is mounted into a reflective cavity, not unlike the inner surface of a headlight assembly. The mounting absorbs and conducts heat away from the chip. A hair fine wire connects the top surface of the semiconductor chip to the other terminal. Heat from the glowing chip is then conducted down the negative or short lead, and out of the LED. A diagram of a typical LED is shown below.
(LED Diagram)
[ATTACH=CONFIG]529057[/ATTACH]
LEDs now come in several colors. There have been red and green for decades, allowing both of these colors as well as yellow, by combining red and green. Years later in the nineties, the process for manufacturing blue LEDs was perfected. Finally, with the three primary colors available, white light became possible by combining all three colors.
You want a WHITE LED for this application. I have posted a scan of the LED package here, since it lists several useful pieces of information. It was obtained at Radio Shack and was priced at $2.49. You get 2 LEDs in the package. Hopefully you can find a Radio Shack near you that carries this exact item. If not, you may need to substitute something else, noting the information you find on the package.
(LED Package - Front and Back)
[ATTACH=CONFIG]529058[/ATTACH]
LEDs of this particular style usually have a flat spot on one side of them and have one lead that is shorter. Both should be on the same side. The short lead is the negative (-) terminal. The other lead (longer) is positive (+). The 7000 mcd specification is the relative amount of light the LED is designed to emit. You want this to be as high as possible for a good amount of brightness. The 25 mA specification is critical to the correct operation of the LED. Too little current won't light the LED very brightly. Too much current will burn it out. We want to get as close as we can to the recommended amount of current, which will be determined by a resistor (coming up). The FW voltage is what would be measured across the LED during proper operation at the rated 25 mA.
Transformer
We obtained the transformer from a local thrift store. You can see it still has the label on it. Look for one that is 12 volts DC. You may want one that is as small as possible. Any 12 volt DC transformer above 25 mA will do. We paid 99 cents for this one.
(Transformer)
[ATTACH=CONFIG]529059[/ATTACH]
Resistor
A resistor is used to limit the current that the LED draws. If it were a larger value (more ohms), less current would flow and the LED would not be as bright. If it were a lower value (fewer ohms), more current would flow and the LED would shine more brightly, perhaps overheating and burning out. The resistor is calculated to allow the specified 25 mA of current to flow when 12 volts DC is applied.
The value of the resistor used here is 560 ohms. This can be determined from the colored bands around it. With the colored bands toward the left, the bands should be green, blue, brown. If there is a fourth band, ignore it. In the color code scheme, the first two bands are digits, green for 5 and blue for 6. The third band is a multiplier, or an easier way to think of it is the number of zeros you add. Brown is 1, so add one zero. A 5 and 6 with one zero is 560 ohms.
Resistors come in different wattages, too. The higher the wattage, the larger a resistor is physically, the more heat it can dissipate and the more current it is able to carry without overheating. This resistor is an 1/8 watt size, small enough for it to fit inside the brass tube.
(Resistor)
[ATTACH=CONFIG]529060[/ATTACH]
Almost any departures from the LED and transformer used here can be made up for by changing the value of the resistor. You might want to be sure you can get this particular LED and a 12 volt DC transformer before getting the resistor. Any variations you need or want to make will require a resistor having a different number of ohms, that's all.
More soon...
John
It is my hope in posting this that it can provide some experience with soldering. There will be an example of soldering to a piece of metal (the brass tube), which will absorb and radiate heat a little differently than wires do. It will involve tinning of wires and tack soldering techniques. We will start by tacking the parts together to make sure they work. This gives an opportunity to do a few connections that are only temporary, allowing you to get the feel of soldering before the final assembly.
Brass Tube
The brass tube used was obtained from Ace Hardware, although it might be obtained from a hobby shop or a home improvement center as well. It is 7/16 inch in diameter and 12 inches long. It was priced at $1.99. I have included a scan of the label that came on it.
(Brass Tubing Label)
[ATTACH=CONFIG]529056[/ATTACH]
LED
LEDs are fascinating devices. At their heart is a tiny semiconductor chip, maybe the size of a large grain of salt, and only a thin slice of that used. The chip is mounted into a reflective cavity, not unlike the inner surface of a headlight assembly. The mounting absorbs and conducts heat away from the chip. A hair fine wire connects the top surface of the semiconductor chip to the other terminal. Heat from the glowing chip is then conducted down the negative or short lead, and out of the LED. A diagram of a typical LED is shown below.
(LED Diagram)
[ATTACH=CONFIG]529057[/ATTACH]
LEDs now come in several colors. There have been red and green for decades, allowing both of these colors as well as yellow, by combining red and green. Years later in the nineties, the process for manufacturing blue LEDs was perfected. Finally, with the three primary colors available, white light became possible by combining all three colors.
You want a WHITE LED for this application. I have posted a scan of the LED package here, since it lists several useful pieces of information. It was obtained at Radio Shack and was priced at $2.49. You get 2 LEDs in the package. Hopefully you can find a Radio Shack near you that carries this exact item. If not, you may need to substitute something else, noting the information you find on the package.
(LED Package - Front and Back)
[ATTACH=CONFIG]529058[/ATTACH]
LEDs of this particular style usually have a flat spot on one side of them and have one lead that is shorter. Both should be on the same side. The short lead is the negative (-) terminal. The other lead (longer) is positive (+). The 7000 mcd specification is the relative amount of light the LED is designed to emit. You want this to be as high as possible for a good amount of brightness. The 25 mA specification is critical to the correct operation of the LED. Too little current won't light the LED very brightly. Too much current will burn it out. We want to get as close as we can to the recommended amount of current, which will be determined by a resistor (coming up). The FW voltage is what would be measured across the LED during proper operation at the rated 25 mA.
Transformer
We obtained the transformer from a local thrift store. You can see it still has the label on it. Look for one that is 12 volts DC. You may want one that is as small as possible. Any 12 volt DC transformer above 25 mA will do. We paid 99 cents for this one.
(Transformer)
[ATTACH=CONFIG]529059[/ATTACH]
Resistor
A resistor is used to limit the current that the LED draws. If it were a larger value (more ohms), less current would flow and the LED would not be as bright. If it were a lower value (fewer ohms), more current would flow and the LED would shine more brightly, perhaps overheating and burning out. The resistor is calculated to allow the specified 25 mA of current to flow when 12 volts DC is applied.
The value of the resistor used here is 560 ohms. This can be determined from the colored bands around it. With the colored bands toward the left, the bands should be green, blue, brown. If there is a fourth band, ignore it. In the color code scheme, the first two bands are digits, green for 5 and blue for 6. The third band is a multiplier, or an easier way to think of it is the number of zeros you add. Brown is 1, so add one zero. A 5 and 6 with one zero is 560 ohms.
Resistors come in different wattages, too. The higher the wattage, the larger a resistor is physically, the more heat it can dissipate and the more current it is able to carry without overheating. This resistor is an 1/8 watt size, small enough for it to fit inside the brass tube.
(Resistor)
[ATTACH=CONFIG]529060[/ATTACH]
Almost any departures from the LED and transformer used here can be made up for by changing the value of the resistor. You might want to be sure you can get this particular LED and a 12 volt DC transformer before getting the resistor. Any variations you need or want to make will require a resistor having a different number of ohms, that's all.
More soon...
John
08-28-2015, 08:12 AM
#15
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Join Date: Mar 2015
Location: Denver, CO
Posts: 4,254
I need to make a slight correction. In the first paragraph of the brass tube above, I incorrectly stated that the diameter was 7/16 inch. It is in fact 7/32 inch in diameter. It shows that clearly on the label. Apologies to all and thanks to my proofreader.
John
John
09-01-2015, 04:44 AM
#16
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Join Date: Mar 2015
Location: Denver, CO
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Soldering Can Be Simple and Easy
If you're willing to undertake this small project, by the time you are done you will feel confident enough to pick up a soldering iron whenever the need arises. Soldering is not difficult if you follow a few common guidelines and I'd like to offer a couple of important tips. I've been involved with electronics and soldering for a long time and believe I can make you feel comfortable with it very quickly.
Use a soldering tip that is a good size for the work you are doing. A temperature of 700 degrees Fahrenheit is about right if you have any choice.
Use only 60%tin/40%lead solder, and a small enough diameter to let you regulate its application and flow easily.
If you get too much solder on a wire, simply swipe it away with the tip, letting the tip collect it. For larger items, such as the brass tube, heat the solder until it s liquid and then quickly move it to the edge of a wastebasket (or other safe place) and tap it gently. The solder will fly off. You have to be fast before it cools. Be careful doing this. Protect or avert your eyes. Another thing you can try is wiping away excessive liquid solder with a wet paper towel. The solder is hot, so if you try this, watch where it goes. Keep it in the paper towel or let it drop somewhere safe. For difficult solder removal, there are suction devices, called solder pullers, which will remove hot liquid solder from a connection.
There can't be ANY movement in a soldered connection until it cools and hardens. Usually three to five seconds. If there is, it will be a bad solder connection. It may be dull and may come apart. Reheat it and let it cool without movement. Look for a more smooth and shiny appearance.
Tip#1 - Use a damp sponge to wipe the tip before each connection that you solder.
A clean tip transfers heat much better. Make it a habit to pick up the iron, wipe both sides of the tip, then continue on to your soldering. Dedicate a small sponge as part of your soldering kit. Keep it just damp, gently squeezed out. It will get dirty and collect old solder. It will sizzle when you clean the tip. That's normal.
Tip#2 - Use the iron to heat your work. Let the work melt the solder.
Metal must be up to a certain temperature for the solder to "wet" the metal and bond to it. Flowing hot solder onto colder metal creates what are called "cold solder" connections. Solder, on cold metal, behaves like water drops on a waxed surface. It won't adhere.
Do more than "touch" your work with the soldering tip. Slight pressure will help the heat to flow. If two items are being soldered, try to find a corner or "V" where you can place the tip to heat both items at the same time.
Items with little mass, such as wires, heat and cool more quickly. And the heat will travel through them more quickly. Items with more mass, such as a brass tube, will take longer to heat up and longer to cool down.
With these things in mind, you're ready to start soldering.
Tinning the Resistor
Get the soldering iron warmed up and have solder ready. And a sponge. I suggest a small piece of wood to solder on. It doesn't pull heat out of what you're soldering. And, little burn marks let you know you're soldering at a good temperature.
(Tinning the resistor)
[ATTACH=CONFIG]529453[/ATTACH]
Place the resistor on the wood. Place the soldering iron tip at the end of the resistor's wire, pressing gently. Apply the solder to the wire slightly above the tip. Let the solder flow onto the wire. As soon as it does, take the iron away. Let it cool. You should have a blob of solder firmly adhering to the wire. It should look fairly shiny. You can re-heat it a second time if you need.
(Resistor after tinning)
[ATTACH=CONFIG]529454[/ATTACH]
When satisfied, do the other end too.
As you're doing this, try to think how the heat flows. You heated the end of the wire. Heat traveled up the wire, past the place you soldered, but also beyond, into whatever component is further up the wire. That's why you don't want to apply heat for any longer than necessary.
A resistor is a good component to learn with because they're designed to get warm and dissipate heat. If you think you need more practice, cut a 1/2 inch or so off each end of the resistor and try it again.
Tinning the LED
Tinning the LED will be much like the resistor, but you want to get it done without taking too long. LEDs are more heat sensitive. You'll be OK if you don't have the iron on it for a long time. Probably no more than a couple of seconds, less time if you can.
(Tinning the LED)
[ATTACH=CONFIG]529455[/ATTACH]
Do both wire leads. You can separate them a bit to make it easier if you want.
Below is a picture of what the results of your tinning the resistor and the LED.
(Both LED and resistor tinned)
[ATTACH=CONFIG]529456[/ATTACH]
Transformer
Remove the connector by clipping it from the end of the transformer's cord and discard it. Split the two wires apart for a few inches. Strip and twist the ends of the wires.
(Transformer with stripped wires)
[ATTACH=CONFIG]529457[/ATTACH]
With the wires stripped, tin them too, like you did with the resistor and LED. The wire is very long so you don't have any heat concerns.
Tack Soldering the Parts Together
To be sure everything works, and to give some tack soldering practice, we will temporarily tack solder the parts together.
Place the resistor and the LONGER wire of the LED together, parallel and overlapping a quarter inch or so. If you have trouble and want to tape down or lay something on the components, that's OK. Anything's fair.
Only a brief moment with the iron should cause the solder on the two wires to melt together. Do not move them at all until the solder is cool. As before, it should look fairly shiny. If there was movement during cooling, the solder may look dull and the connection may even crumble. Re-do anything you're not certain of, allowing cooling between tries.
(Tacking LED and resistor together)
[ATTACH=CONFIG]529458[/ATTACH]
When the resistor and LED are together, it will make a convenient test device for what we need to do next.
Schematic Diagram
(Light probe schematic)
[ATTACH=CONFIG]529459[/ATTACH]
Above is a schematic diagram of the light probe circuit. We are going to tack together the essential parts, without the brass tube, just to be sure they work.
First off, we need to determine the polarity of the transformer wires. We can do this by trial and error, using what was just put together. We know that the short lead and flat side of the LED is negative(-). A unique feature of LEDs is that they don't care if you connect them backward. They simply don't work.
Connection points:
Opposite end of the resistor from the LED (+)
Short LED lead (-)
DON'T CONNECT ANYTHING to the LED/resistor soldered connection.
You may want to tack solder the wires. Crosswise tacking is sometimes easier, and works for temporary purposes just as well.
You can also touch the wires to the LED and resistor by hand, to get them right before you tack solder. The isolated 12 volts is completely safe to do this with, even with the transformer plugged in. Just don't let the transformer wires touch each other.
With the wires connected one way or the other, it should work. When you find it, mark the wire connected to the resistor as (+). No need to mark both wires, but the wire connected to the short lead of the LED is (-).
(Light probe component test)
[ATTACH=CONFIG]529460[/ATTACH]
Soldering Brass Is Fun
If you've made it this far, you have proven that you have the right parts and that they work together. And you've proven that you can solder. Next post we'll get the parts fitted and soldered into the brass tube and the light probe will be finished.
When you see how easy brass material is to solder, you may think of other craftsy kinds of things to do with it.
More soon...
John
If you're willing to undertake this small project, by the time you are done you will feel confident enough to pick up a soldering iron whenever the need arises. Soldering is not difficult if you follow a few common guidelines and I'd like to offer a couple of important tips. I've been involved with electronics and soldering for a long time and believe I can make you feel comfortable with it very quickly.
Use a soldering tip that is a good size for the work you are doing. A temperature of 700 degrees Fahrenheit is about right if you have any choice.
Use only 60%tin/40%lead solder, and a small enough diameter to let you regulate its application and flow easily.
If you get too much solder on a wire, simply swipe it away with the tip, letting the tip collect it. For larger items, such as the brass tube, heat the solder until it s liquid and then quickly move it to the edge of a wastebasket (or other safe place) and tap it gently. The solder will fly off. You have to be fast before it cools. Be careful doing this. Protect or avert your eyes. Another thing you can try is wiping away excessive liquid solder with a wet paper towel. The solder is hot, so if you try this, watch where it goes. Keep it in the paper towel or let it drop somewhere safe. For difficult solder removal, there are suction devices, called solder pullers, which will remove hot liquid solder from a connection.
There can't be ANY movement in a soldered connection until it cools and hardens. Usually three to five seconds. If there is, it will be a bad solder connection. It may be dull and may come apart. Reheat it and let it cool without movement. Look for a more smooth and shiny appearance.
Tip#1 - Use a damp sponge to wipe the tip before each connection that you solder.
A clean tip transfers heat much better. Make it a habit to pick up the iron, wipe both sides of the tip, then continue on to your soldering. Dedicate a small sponge as part of your soldering kit. Keep it just damp, gently squeezed out. It will get dirty and collect old solder. It will sizzle when you clean the tip. That's normal.
Tip#2 - Use the iron to heat your work. Let the work melt the solder.
Metal must be up to a certain temperature for the solder to "wet" the metal and bond to it. Flowing hot solder onto colder metal creates what are called "cold solder" connections. Solder, on cold metal, behaves like water drops on a waxed surface. It won't adhere.
Do more than "touch" your work with the soldering tip. Slight pressure will help the heat to flow. If two items are being soldered, try to find a corner or "V" where you can place the tip to heat both items at the same time.
Items with little mass, such as wires, heat and cool more quickly. And the heat will travel through them more quickly. Items with more mass, such as a brass tube, will take longer to heat up and longer to cool down.
With these things in mind, you're ready to start soldering.
Tinning the Resistor
Get the soldering iron warmed up and have solder ready. And a sponge. I suggest a small piece of wood to solder on. It doesn't pull heat out of what you're soldering. And, little burn marks let you know you're soldering at a good temperature.
(Tinning the resistor)
[ATTACH=CONFIG]529453[/ATTACH]
Place the resistor on the wood. Place the soldering iron tip at the end of the resistor's wire, pressing gently. Apply the solder to the wire slightly above the tip. Let the solder flow onto the wire. As soon as it does, take the iron away. Let it cool. You should have a blob of solder firmly adhering to the wire. It should look fairly shiny. You can re-heat it a second time if you need.
(Resistor after tinning)
[ATTACH=CONFIG]529454[/ATTACH]
When satisfied, do the other end too.
As you're doing this, try to think how the heat flows. You heated the end of the wire. Heat traveled up the wire, past the place you soldered, but also beyond, into whatever component is further up the wire. That's why you don't want to apply heat for any longer than necessary.
A resistor is a good component to learn with because they're designed to get warm and dissipate heat. If you think you need more practice, cut a 1/2 inch or so off each end of the resistor and try it again.
Tinning the LED
Tinning the LED will be much like the resistor, but you want to get it done without taking too long. LEDs are more heat sensitive. You'll be OK if you don't have the iron on it for a long time. Probably no more than a couple of seconds, less time if you can.
(Tinning the LED)
[ATTACH=CONFIG]529455[/ATTACH]
Do both wire leads. You can separate them a bit to make it easier if you want.
Below is a picture of what the results of your tinning the resistor and the LED.
(Both LED and resistor tinned)
[ATTACH=CONFIG]529456[/ATTACH]
Transformer
Remove the connector by clipping it from the end of the transformer's cord and discard it. Split the two wires apart for a few inches. Strip and twist the ends of the wires.
(Transformer with stripped wires)
[ATTACH=CONFIG]529457[/ATTACH]
With the wires stripped, tin them too, like you did with the resistor and LED. The wire is very long so you don't have any heat concerns.
Tack Soldering the Parts Together
To be sure everything works, and to give some tack soldering practice, we will temporarily tack solder the parts together.
Place the resistor and the LONGER wire of the LED together, parallel and overlapping a quarter inch or so. If you have trouble and want to tape down or lay something on the components, that's OK. Anything's fair.
Only a brief moment with the iron should cause the solder on the two wires to melt together. Do not move them at all until the solder is cool. As before, it should look fairly shiny. If there was movement during cooling, the solder may look dull and the connection may even crumble. Re-do anything you're not certain of, allowing cooling between tries.
(Tacking LED and resistor together)
[ATTACH=CONFIG]529458[/ATTACH]
When the resistor and LED are together, it will make a convenient test device for what we need to do next.
Schematic Diagram
(Light probe schematic)
[ATTACH=CONFIG]529459[/ATTACH]
Above is a schematic diagram of the light probe circuit. We are going to tack together the essential parts, without the brass tube, just to be sure they work.
First off, we need to determine the polarity of the transformer wires. We can do this by trial and error, using what was just put together. We know that the short lead and flat side of the LED is negative(-). A unique feature of LEDs is that they don't care if you connect them backward. They simply don't work.
Connection points:
Opposite end of the resistor from the LED (+)
Short LED lead (-)
DON'T CONNECT ANYTHING to the LED/resistor soldered connection.
You may want to tack solder the wires. Crosswise tacking is sometimes easier, and works for temporary purposes just as well.
You can also touch the wires to the LED and resistor by hand, to get them right before you tack solder. The isolated 12 volts is completely safe to do this with, even with the transformer plugged in. Just don't let the transformer wires touch each other.
With the wires connected one way or the other, it should work. When you find it, mark the wire connected to the resistor as (+). No need to mark both wires, but the wire connected to the short lead of the LED is (-).
(Light probe component test)
[ATTACH=CONFIG]529460[/ATTACH]
Soldering Brass Is Fun
If you've made it this far, you have proven that you have the right parts and that they work together. And you've proven that you can solder. Next post we'll get the parts fitted and soldered into the brass tube and the light probe will be finished.
When you see how easy brass material is to solder, you may think of other craftsy kinds of things to do with it.
More soon...
John
09-01-2015, 05:55 AM
#17
Senior Member
Join Date: Mar 2013
Location: NW MO
Posts: 591
John, thanks so much for this tutorial. I have picked up a few pointers needed to try this project. Your instructions are easy to follow for us neophytes. Any other neat projects will be appreciated.
Edit-please post relationship of ohms and voltage. E.g., if one used a 6 volt/50 mA transformer would one use a 280 resister (if there is such a thing)?
Edit-please post relationship of ohms and voltage. E.g., if one used a 6 volt/50 mA transformer would one use a 280 resister (if there is such a thing)?
Last edited by Vridar; 09-01-2015 at 06:05 AM. Reason: Another question.
09-01-2015, 04:38 PM
#18
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Thread Starter
Join Date: Mar 2015
Location: Denver, CO
Posts: 4,254
Ron (Vridar),
This is getting more technical than I ever intended, but I'll try to keep it is as simple as I can.
Your question about a transformer that is 6 volts and a resistor of 280 ohms is certainly the right idea, except for the fact that the LED behaves peculiarly. LEDs tend to maintain a fixed voltage across them regardless of current flow. This one remains about 3.3 volts (shown on the package) across it all the time. You need to make a calculation based on the difference, something on the order of 8.7 volts across the resistor. I calculated it this way and when I measured the current, it wasn't quite what I expected. With electronics, reality rarely exactly matches the math on paper. It's just a starting point for design and testing.
Ultimately I measured the current with a meter while trying descending values, but starting above the range I had calculated. So as to not feed an LED too much current, it is always good to start with a higher resistance than you calculate and work down. Give me a day or so to post how to make this measurement with a meter.
More generally, for the purpose of understanding volts, ohms and amps, I would draw an analogy (for this instance only) to water flowing in a pipe:
Voltage is like water pressure. The pressure is separate and distinct from the water itself. The higher it is, the more flow there is.
Resistance (in ohms) is like the size restriction of the water pipe. The greater the restriction, the lower the flow.
Amperes (amps) are like the water itself flowing. One amp is a huge number (I won't give it) of electrons passing a given point in one second. Amps are always a result. You can't alter current flow directly. All you can do is alter voltage and resistance to produce the amount of current flow that you want.
On the transformer you propose, 50 mA would be fine. Only 25 mA will be drawn or used, no matter how high the mA capability of the transformer is.
Because I work with electronics, I'm advantaged by having most standard resistor values on hand to test with. But most people don't. Radio Shack has them in 5 packs. Maybe you could get a few well selected values to try. You can experiment with smaller resistors connected in series (end-to-end). You simply add all the values together. 280 ohms isn't a standard value, but I believe that 270 is. You could get some 47 ohm or 100 ohm resistors and make test combinations. You could work down from around 560 ohms to whatever produces the desired current. More tack soldering practice! When you know how many ohms you need, use one resistor that replaces all of them.
I'll get pictures of the meter setup ASAP. John
This is getting more technical than I ever intended, but I'll try to keep it is as simple as I can.
Your question about a transformer that is 6 volts and a resistor of 280 ohms is certainly the right idea, except for the fact that the LED behaves peculiarly. LEDs tend to maintain a fixed voltage across them regardless of current flow. This one remains about 3.3 volts (shown on the package) across it all the time. You need to make a calculation based on the difference, something on the order of 8.7 volts across the resistor. I calculated it this way and when I measured the current, it wasn't quite what I expected. With electronics, reality rarely exactly matches the math on paper. It's just a starting point for design and testing.
Ultimately I measured the current with a meter while trying descending values, but starting above the range I had calculated. So as to not feed an LED too much current, it is always good to start with a higher resistance than you calculate and work down. Give me a day or so to post how to make this measurement with a meter.
More generally, for the purpose of understanding volts, ohms and amps, I would draw an analogy (for this instance only) to water flowing in a pipe:
Voltage is like water pressure. The pressure is separate and distinct from the water itself. The higher it is, the more flow there is.
Resistance (in ohms) is like the size restriction of the water pipe. The greater the restriction, the lower the flow.
Amperes (amps) are like the water itself flowing. One amp is a huge number (I won't give it) of electrons passing a given point in one second. Amps are always a result. You can't alter current flow directly. All you can do is alter voltage and resistance to produce the amount of current flow that you want.
On the transformer you propose, 50 mA would be fine. Only 25 mA will be drawn or used, no matter how high the mA capability of the transformer is.
Because I work with electronics, I'm advantaged by having most standard resistor values on hand to test with. But most people don't. Radio Shack has them in 5 packs. Maybe you could get a few well selected values to try. You can experiment with smaller resistors connected in series (end-to-end). You simply add all the values together. 280 ohms isn't a standard value, but I believe that 270 is. You could get some 47 ohm or 100 ohm resistors and make test combinations. You could work down from around 560 ohms to whatever produces the desired current. More tack soldering practice! When you know how many ohms you need, use one resistor that replaces all of them.
I'll get pictures of the meter setup ASAP. John
09-01-2015, 06:51 PM
#19
Senior Member
Join Date: Mar 2013
Location: NW MO
Posts: 591
John, thanks. I will be following this thread with reverence. Please continue to enlighten. You seem to be a patient person we can all learn from. An asset to our forum, for sure.
Last edited by Vridar; 09-01-2015 at 06:53 PM. Reason: Grammar
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10-27-2014 07:23 AM
