Electronics - advice on circuits and LEDs
- Thread starter dbov22
- Start date
Ah yes, I hadn't looked closely enough at those transformers. Yes it does matter ... sort of. At 50Hz, the LEDs will appear on constantly, but dim. It wouldn't really be great practice either. You could use the rectifier circuit mentioned above, but for your purposes you might want to try and find a simple DC output transformer.
I really don't think you should be giving information on electronics with all due respects.
Hmm - so you rectified the AC source using a diode bridge?
Yes I just used some 1N4148 which seemed to do the trick. A DC supply is generally the same thing as an AC supply jut with the addition of a diode bridge, unless it's a SMPS which adds more complexity.
So to the OP if you have no suitable DC supply then you could easily use the AC ones with only a small investment for 4 diodes and a cap if you dont have some to hand.
So to the OP if you have no suitable DC supply then you could easily use the AC ones with only a small investment for 4 diodes and a cap if you dont have some to hand.
Care to elaborate?
The LED's i'm using are white -3.4-3.7 Vf, 0.020 A. I am not interested in using the AC adaptor - seems like too much fuss. There's a DC adaptor on ebay which is 12V, 5A and 60w.
That will certainly power my 70 LED's, just got to work out which resistors to put where
That will certainly power my 70 LED's, just got to work out which resistors to put where
With a 12v DC power source you'd be looking at 3x LEDs in series with a 62 Ohm resistor.
For your 10 cluster it would be something like this.
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------ 430 Ohm-----0v
for a total of 80mA@12v for the 10 LEDs.
If you want them in a line just run the +12v and ground wires to where they need to be for each run of LEDs.
Or you could wire them all like this...
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
for a total of 100mA@12v for the 10 LEDs. It's easier as you could pick up 100x 240 Ohm resistors for £1 rather than having to buy a few different types.
A 5A power supply will provide enough current for many hundreds of LEDs in series configurations like those above.
Looks a bit like this with the 240 Ohm and White LEDs obviously.
For your 10 cluster it would be something like this.
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------>|------>|-----62 Ohm-----0v
+12v------>|------ 430 Ohm-----0v
for a total of 80mA@12v for the 10 LEDs.
If you want them in a line just run the +12v and ground wires to where they need to be for each run of LEDs.
Or you could wire them all like this...
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
+12v------>|----->|------240 Ohm -----0v
for a total of 100mA@12v for the 10 LEDs. It's easier as you could pick up 100x 240 Ohm resistors for £1 rather than having to buy a few different types.
A 5A power supply will provide enough current for many hundreds of LEDs in series configurations like those above.
Looks a bit like this with the 240 Ohm and White LEDs obviously.
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Thanks, I really appreciate the effort, I'll get to it. Can you clarify one thing though. In my original method of wiring the LEDs this is wrong and I should be wiring them like yours where literally the LEDs are joined to each other rather than me joining all the annodes together then all the cathodes together...
For discrete low power LEDs such as these the generally recommended way of wiring LEDs is with 1 or more LEDs in series with a single resistor of sufficient power rating, repeated several times for however many LEDs you need. It's more efficient in terms of light output/vs heat output to have 2 or more LEDs in series as less power has to be dissipated by the resistor as heat and more can be converted into light.
What you initially wired up was 40 LEDs directly off a current limited, but not 'controlled' power source. I'm not sure what advice you've taken but it's not good advice.
This is what I understand....
LEDs are not completely identical and one will have a slightly lower forward voltage than the others and will happily take additional current, so it gets hotter, and as it gets hotter it's forward voltage drop decreases further, so it gets more current still, so it gets hotter and you get a thermal runaway situation and eventually it'll blow. Now you have 39 LEDs left and that extra 20mA of current is shared amongst the LEDs. The one with the next lowest forward voltage will start to heat up further and will blow faster than the first one did, and then the next and the next. After the first few failures they'll basically go quickly. bang bang bang.
A few years ago, before I got into electronics, I bought 12x 10mm White LEDs to light my keyboard as I was using a computer desk with keyboard tray and it was tricky to see in the dark. I wired them up to a 4.5v plug in charger with 300mA or whatever current capability. I wired them just as you did and over the coming weeks I first noticed flickering and then eventual failure of first one, then a short time after two and then three before I stopped using it.
If I'd wired them in series with a resistor properly they'd still be working today, and probably 10 years from now.
Here's an illustration of the issue with not using a resistor.
The Red LED (right) has a lower Vf (1.8v) than the Orange (9 of them on the left at Vf 2.13v). I have a current controlled source of 250mA. Because the Red LED has the lower Vf it is happy to steal all the current that would normally be shared amongst all the LEDs. In fact in this example it takes all but 15mA of the 250mA. Ok it's maybe not a perfect example but I haven't been able to work out how to change a LED Vf in this software.
What you initially wired up was 40 LEDs directly off a current limited, but not 'controlled' power source. I'm not sure what advice you've taken but it's not good advice.
This is what I understand....
LEDs are not completely identical and one will have a slightly lower forward voltage than the others and will happily take additional current, so it gets hotter, and as it gets hotter it's forward voltage drop decreases further, so it gets more current still, so it gets hotter and you get a thermal runaway situation and eventually it'll blow. Now you have 39 LEDs left and that extra 20mA of current is shared amongst the LEDs. The one with the next lowest forward voltage will start to heat up further and will blow faster than the first one did, and then the next and the next. After the first few failures they'll basically go quickly. bang bang bang.
A few years ago, before I got into electronics, I bought 12x 10mm White LEDs to light my keyboard as I was using a computer desk with keyboard tray and it was tricky to see in the dark. I wired them up to a 4.5v plug in charger with 300mA or whatever current capability. I wired them just as you did and over the coming weeks I first noticed flickering and then eventual failure of first one, then a short time after two and then three before I stopped using it.
If I'd wired them in series with a resistor properly they'd still be working today, and probably 10 years from now.
Here's an illustration of the issue with not using a resistor.
The Red LED (right) has a lower Vf (1.8v) than the Orange (9 of them on the left at Vf 2.13v). I have a current controlled source of 250mA. Because the Red LED has the lower Vf it is happy to steal all the current that would normally be shared amongst all the LEDs. In fact in this example it takes all but 15mA of the 250mA. Ok it's maybe not a perfect example but I haven't been able to work out how to change a LED Vf in this software.
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But say I'm just using 3 LEDs for example and a resistor (forget ANY calculations or volts etc). Would it be wire from +ve to LED, join other leg of LED to another LED then repeat the to resistor then from other end of resistor to -ve of power source?
I already illustrated it above.
The above was to signify a LED and I called the -ve power source as 0v, it can also be called ground, or return.
This page might help.
http://led.linear1.org/led.wiz/
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->|-The above was to signify a LED and I called the -ve power source as 0v, it can also be called ground, or return.
This page might help.
http://led.linear1.org/led.wiz/
It would have worked, with the resistors added, but Tealc's method is MUCH more efficient. Not sure why I didn't notice the alternative in the first place! Oh well ... live and learn
If you're going to use the AC power supply it may be worth designing your string to incorporate some of the led's as a full wave rectifier, you can capacitor smooth it if you want but the frequency would be twice supply so you probably wouldn't notice the flicker.
I can't see using LEDs as a rectifying bridge as being a good idea. LEDs don't have very high reverse voltage and besides wouldn't be able to handle the current involved to drive 70 LEDs, no matter what the configuration.
To prevent the LEDs from flickering you'd need a lot more than 120 Hz so I disagree and you would need a smoothing cap.
All academic really as the OP says that a DC unit will be used.
One thing to watch for with a DC power supply such as the cheap ones is that they are not regulated at 12v so will give a fair bit more than 12v at low load. As you increase the load it will get closer to the 12v. For this reason it's often better to use a power supply rated nearer to the desired load. At 750mA a 5A power supply should be within a volt or two of 12v so should be just fine, but if you had only a couple of LEDs on there it might be nearer 18v and could be an issue.
To prevent the LEDs from flickering you'd need a lot more than 120 Hz so I disagree and you would need a smoothing cap.
All academic really as the OP says that a DC unit will be used.
One thing to watch for with a DC power supply such as the cheap ones is that they are not regulated at 12v so will give a fair bit more than 12v at low load. As you increase the load it will get closer to the 12v. For this reason it's often better to use a power supply rated nearer to the desired load. At 750mA a 5A power supply should be within a volt or two of 12v so should be just fine, but if you had only a couple of LEDs on there it might be nearer 18v and could be an issue.
Swap out those for led's with some diodes then, just as cheap. I mention that method as I seem to recall Samsung or some other tech firm showed off a chain of Christmas lights with the rectifier surrounding the chain so only the two ends flickered. Though it did give a couple single points of failure.