Smps.....switched mode power supply.....if it is Mains Isolated so if for example you put yourself between the -output and mains ground and dont get a little tickle.....I think smps power supplies are a needing a section all of their own as regards construction, building and repair/testing. ....stick with a properly designed transformer (lo voltage)....while your at it add a bridge rectifier and capacitor and sort your series resistor to suit and away you go.....

 

Not the right thread for this, but yes, they do. If you study SMPS they have a isolation transformer after a high frequency DC switching section. Circuitry that run on HV is allowed, but the isolation is still required. I believe if you really study the design of them you will find this is still the case.

 

Yep. Good catch. I've corrected it.



Short answer: Yes.

There are actually several things place limit on the allowed current, heat being the main one most of the time. But there is also a limit on the maximum instantaneous current that is allowed, however in most cases using pulses of several times the average current will not cause a problem.

Take note of the warning from Bill Marsden, the moderator. If you are talking about actually running LEDs from the mains voltage, then this would be a violation of the forum rules. But if you are using a transformer to step the mains voltage down to something reasonable, say 9V to 24V or something like that, then that's a different matter.

I'll add my $0.02. As you will see people will give you somewhat different answer with the devil being in the details. Looking at the differences can really help you learn.

1) Roughly. Two red LED's that are the same wavelength but are very different in size will likely have forward voltage drops that are much closer to each other than a red and a blue LED that are the same size. A given LED will typically see the voltage increase by about 100mV when the current increases by a factor of ten (there is quite a bit of variability between LEDs in this rule of thumb, but they are all in this ballpark).

2) Yes, within reason. And LED is a diode (that's what the D stands for) that conducts current in one direction and blocks it in the other. So if presented with an AC signal the LED will conduct during half of the cycle and light up during that portion only. As long as the frequency is fast enough (more than 30 Hz or so) you will probably perceive it as being on continuously but at about half the brightness. The big caveat is that LEDs, unlike their rectifier diode cousins, generally cannot tolerate significant reverse voltages and will typically break down with a reverse voltage in the 5V to 7V range while rectifier diodes typically have reverse breakdown voltages that range from dozens to thousands of volts.

3) Not much at all. The current through an LED will increase by a factor of ten for increases in voltage across it of just a few dozen millivolts. This is why most LED circuits are designed to establish a current through the diode and not a voltage across the diode -- in reality, of course, you are doing both, but the circuit design assumes an approximate voltage across the LED and then establishes a desired current that changes very little as long the LED voltage is anywhere close to that assumed value.

4) Not sure what you mean by a "clear" LED. The term "clear" usually just means the lens/encapsulation color and has no bearing on the LED's electrical characteristics. This depends primarily on the color of the light emitted by the LED. I'm not aware of any LEDs that operate at a forward voltage of only 1.2V. The lowest that I know of are the infrared LEDs that are typically in the .15V range. As a general (but not rigid) rule, the shorter the wavelength (i.e., going from the red end of the rainbow to the blue end) the higher the voltage. The current depends entirely on how much power the LED is designed to handle, which is directly related to how much light it is intended to give off. The more light, the more current. Currents can range from a few milliamps to tens of amps (maybe more, I'm not sure what the highest current LEDs are at -- they are primarily limited by the ability to get the waste heat off the die).

5) You need to learn about basic DC circuit theory. The LEDs only know about the voltage difference between their terminals and the current flowing through them. So all 3 LEDs in your example see the same thing and none of them behave differently because they are "before" or "after" the others. In this series circuit, if there is any current flowing, then the three LEDs will collectively drop a total of ~7.5V (3x2.5V) leaving the remaining voltage, 1.5V, to be dropped across the resistor. It doesn't matter where the resistor is located as long as it is in series with the LEDs -- it could be "before", "after", or "in between". The current will be set by the 1.5V across the 75Ω resistor which will make it right about 20mA. None of the LEDs are at any higher a voltage than the others because each of them only sees the 2.5V that appears across their terminals. If you add a fourth LED, then the four LEDs will each be dropping about 2.25V, which is 250mV less than their "on" voltage. Thus we can expect the current to be less than about 0.1mA (making the voltage across the resistor less than 10mV). So there is still current flowing, but it is not giving off any perceivable light.

6) The resistor imposes a relationship between the voltage across it and the current through it (Ohm's Law). Similarly, the LED imposes a relationship between the voltage across it and the current through it (it happens to be exponential). By putting them in series (order doesn't matter), they have to have the same voltage but the voltage across the combination is the sum of the voltages across either. When we apply a voltage to the combination we force the sum of the voltages to be a particular value and the current takes on whatever value it has to in order to satisfy both relationships simultaneously.

7) There are different kinds of dimmers, but most allow current to flow only during an adjustable portion of the voltage waveform. In theory you could use one to control the voltage on a circuit board, but the circuitry on the board would have to be carefully designed specifically to do this (as would an intermediate power supply). Do NOT try to do this naively as you will likely destroy your circuit board and may possibly expose yourself to dangerous voltages in the process.

Yep. Good catch. I've corrected it.



Short answer: Yes.

There are actually several things place limit on the allowed current, heat being the main one most of the time. But there is also a limit on the maximum instantaneous current that is allowed, however in most cases using pulses of several times the average current will not cause a problem.

Take note of the warning from Bill Marsden, the moderator. If you are talking about actually running LEDs from the mains voltage, then this would be a violation of the forum rules. But if you are using a transformer to step the mains voltage down to something reasonable, say 9V to 24V or something like that, then that's a different matter.

A few more questions:


1) Is a clear LED same thing as a white LED? (Lens cover is 'white' on my clear LED)

2) Once a 2.5 volt LED is lit can the voltage decrease and the LED still remains lit?

3) I think I read that even when an LED is not lit it still draws current. How much current for a single 2.5v LED?

4) Will a 2.5 volt AC incandescent (xmas bulb) work on DC if it is put in series with a bunch of LEDs?

Jan

 

1) No.
2) It will fade to zero.
3) Depends on the LED characteristics and the applied voltage.
4) Yes, if it and the LEDs are suitably rated for the current they're passing. For example, if the LED maximum current allowed is 30mA then the bulb must be rated to glow adequately, but not blow, when passing 30mA RMS.

 

A few more questions:


1) Is a clear LED same thing as a white LED? (Lens cover is 'white' on my clear LED)

2) Once a 2.5 volt LED is lit can the voltage decrease and the LED still remains lit?

3) I think I read that even when an LED is not lit it still draws current. How much current for a single 2.5v LED?

4) Will a 2.5 volt AC incandescent (xmas bulb) work on DC if it is put in series with a bunch of

1) No.
2) It will fade to zero.
3) Depends on the LED characteristics and the applied voltage.
4) Yes, if it and the LEDs are suitably rated for the current they're passing. For example, if the LED maximum current allowed is 30mA then the bulb must be rated to glow adequately, but not blow, when passing 30mA RMS.

Thanks Alec t:

1) My 'clear' LED is on a solar light and draws 55ma when the rechargeable battery is at 1.25v.(fully charged). The solar light is fairly new(2 years old). Any guess what the voltage might be?
2) Any guess on what the draw on the LED is when out.? The LED goes out when drawing below 4ma and the battery is 0.8v however I get 0 ma draw on my digital multimeter when the LED is out. Is the draw in fractions of a ma?

Jan

 

1) My 'clear' LED is on a solar light and draws 55ma when the rechargeable battery is at 1.25v.(fully charged). The solar light is fairly new(2 years old). Any guess what the voltage might be?

Clear means the surrounding plastic is uncolored. For instance my white, red, green and blue LEDs are indistinguishable until they are lit. Some LEDs have colored plastic shells.
How do yo know the LED is drawing 55mA? That's unusually high so I am skeptical. The voltage across the LED will depend on that particular LED but it's likely ~3V. You could measure it.

2) Any guess on what the draw on the LED is when out.? The LED goes out when drawing below 4ma and the battery is 0.8v however I get 0 ma draw on my digital multimeter when the LED is out. Is the
draw in fractions of a ma?

Yes, I think the draw will be very small when you cannot see it. Usually an LED will be plenty bright all the way down to 1mA or even less, so I'm again surprised by your 4mA number. Some solar lights will actually cut out to prevent deep discharge of the battery, but most will just run until they die.

One thing you should realize is that the power to the LED is pulsing. The voltage of the battery is not enough to light the LED, so there is a tiny boost circuit (sometimes known as a joule thief) that creates pulses at a higher voltage to light the LED. If you measure the voltage at the LED, however, you'll see the average of the pulses and this will be very close to the battery voltage.

 

Clear means the surrounding plastic is uncolored. For instance my white, red, green and blue LEDs are indistinguishable until they are lit. Some LEDs have colored plastic shells.
How do yo know the LED is drawing 55mA? That's unusually high so I am skeptical. The voltage across the LED will depend on that particular LED but it's likely ~3V. You could measure it.
Yes, I think the draw will be very small when you cannot see it. Usually an LED will be plenty bright all the way down to 1mA or even less, so I'm again surprised by your 4mA number. Some solar lights will actually cut out to prevent deep discharge of the battery, but most will just run until they die.

One thing you should realize is that the power to the LED is pulsing. The voltage of the battery is not enough to light the LED, so there is a tiny boost circuit (sometimes known as a joule thief) that creates pulses at a higher voltage to light the LED. If you measure the voltage at the LED, however, you'll see the average of the pulses and this will be very close to the battery voltage.

Thanks Wayne H:

I measured the 55ma with a digital multimeter drawing from the battery. The voltage across the LED is equal to the battery voltage ranging from .8v-1.3 volts depending on how 'full' the battery is. As the volts of the battery is going down the ma drawn from the battery also goes down and the LED goes dimmer until at 4ma the LED goes out.

I will be starting a new thread (today or tomorrow) with circuit board info on my solar light so may be that will give more of a clue on what is happening.

Jan

 

Search this forum for solar LED lights. There are a couple thread that collect a number of circuits commonly found. Posts in those threads by me and ErnieM.

 

A few more questions:


1) Is a clear LED same thing as a white LED? (Lens cover is 'white' on my clear LED)

2) Once a 2.5 volt LED is lit can the voltage decrease and the LED still remains lit?

3) I think I read that even when an LED is not lit it still draws current. How much current for a single 2.5v LED?

4) Will a 2.5 volt AC incandescent (xmas bulb) work on DC if it is put in series with a bunch of LEDs?

Jan

1) No. There are two "colors" involved (as well a some other things). The first is the color of the light that is emitted by the LED. There is no such thing as "clear" light (though some might use that term for an IR LED or perhaps a UV LED, but I think it would be a poor choice). The second is the color of the encapsulating material. This can either be colored or clear, but "clear" can also refer to whether the lens is diffuse or not. If the lens is "clear red" then it will look like clean red glass, but if it is "diffuse red" then it will look cloudy. I'm not aware of any standardized verbiage, so often you just need to look at how the manufacturer describes their different LEDs an you can get a pretty quick feel for what THEY mean when they use these various terms, particularly if you can find a website with pictures of their parts next to the descriptions.

2) Recall what I described in an earlier post. As the voltage falls below the forward voltage of the LED the light emitted will fall of exponentially. For rough numbers, if it is 20mA at 2.5V then it might be about 2mA at 2.4V and 0.2mA at 2.3V.

3) Again recall what I said earlier. The current is exponential and there is a current below which the LED will be perceived as being off even though there is some current flowing. But it is exponential. For the LED in the prior answer, by the time you get to 2V the current might be a few hundred nanoamps and by the time you got to 1.5V it might be in the picoamp range.

4) Yes, with caveats. The incandescent bulb will be seen as a resistor (albeit one that has a pretty low value when first turned on and that increased significantly (a factor of ten or more) when fully lit). The current in the LEDs and the current in the bulb will be the same, so it is important that both the LEDs and the bulbs be happy with that current. If the LEDs need 30 mA and the bulb needs 100 mA, then things won't work well. The same is true if thy are the other way around. Also, note that being 2.5V bulbs has no special meaning just because the 2.5V agrees with what you have been using as your example LED forward voltage. If they are in series then the voltages add.

 

1) No. There are two "colors" involved (as well a some other things). The first is the color of the light that is emitted by the LED. There is no such thing as "clear" light (though some might use that term for an IR LED or perhaps a UV LED, but I think it would be a poor choice). The second is the color of the encapsulating material. This can either be colored or clear, but "clear" can also refer to whether the lens is diffuse or not. If the lens is "clear red" then it will look like clean red glass, but if it is "diffuse red" then it will look cloudy. I'm not aware of any standardized verbiage, so often you just need to look at how the manufacturer describes their different LEDs an you can get a pretty quick feel for what THEY mean when they use these various terms, particularly if you can find a website with pictures of their parts next to the descriptions.

2) Recall what I described in an earlier post. As the voltage falls below the forward voltage of the LED the light emitted will fall of exponentially. For rough numbers, if it is 20mA at 2.5V then it might be about 2mA at 2.4V and 0.2mA at 2.3V.

3) Again recall what I said earlier. The current is exponential and there is a current below which the LED will be perceived as being off even though there is some current flowing. But it is exponential. For the LED in the prior answer, by the time you get to 2V the current might be a few hundred nanoamps and by the time you got to 1.5V it might be in the picoamp range.

4) Yes, with caveats. The incandescent bulb will be seen as a resistor (albeit one that has a pretty low value when first turned on and that increased significantly (a factor of ten or more) when fully lit). The current in the LEDs and the current in the bulb will be the same, so it is important that both the LEDs and the bulbs be happy with that current. If the LEDs need 30 mA and the bulb needs 100 mA, then things won't work well. The same is true if thy are the other way around. Also, note that being 2.5V bulbs has no special meaning just because the 2.5V agrees with what you have been using as your example LED forward voltage. If they are in series then the voltages add.

Thanks WBahn:

I like the way you explain things and I think I am slowly absorbing and understanding stuff a bit. Would you say that a standard household incandescent bulb puts out 'white' light?

 

Thanks WBahn:

I like the way you explain things and I think I am slowly absorbing and understanding stuff a bit. Would you say that a standard household incandescent bulb puts out 'white' light?

In rough terms, yes. To be truly "white" requires that the light contain all wavelengths of light in equal power density across the entire spectrum and no source (not even the sun) accomplishes that. But incandescent bulbs operated at their normal level produce very broad spectrum and reasonable flat output and can therefore reasonably be lumped into a group called "white". But even so, they are not all the same and the differences can be quite noticeable, so they are classified according to "color temperature" and other things for people that care about those differences.

 

I think this thread is getting a bit long so it is continued on the following new thread http://forum.allaboutcircuits.com/threads/solar-light-circuit-board.106073/"Solar Light Circuit Board"