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Post by David Murray on Mar 4, 2007 15:40:35 GMT -5
okay.. I'm finally back to my LED project. For a refresher, I am hooking up 25 LEDs in series directly to 120V A/C. Now Madmodder (or TLR, can't remember) told me it might help to have some capacitors, probably electrolytic. Trouble is, I couldn't find anything at Radioshack that was rated for more than 50 volts. So I bought 3 of them and decided to use them in series.
Well, I realized that I did not know how to tell the polarity on these because they are axial lead. It has arrows, which I am assuming the arrow points towards the negative side, right?
Next question. I noticed a diagram on the internet describing the use of capacitors in series. They said it is required to have a resistor on each capacitor (seems like that would defeat the purpose, but oh well) can somebody explain this to me and tell me what oms I should be using on the resistor?
Edit - Wow.. maybe more capacitence than I needed. I have three 1000uF caps in series (at 50V each) which should equal 1000uF at 150V.
Well I plugged it for just a second. I am not sure if it made it any brighter or not, but all 25 LEDs continue to shine for at least 30 seconds after unplugging them!!! I only wanted enough to help with the sine-wave problem.. Anyway, I haven't hooked it up for more than a second due to not knowing if these caps are going to blow up being that they are connected straight to 120V DC in series like this.. I'm still waiting for somebody to tell me if it is a good idea or not.
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Post by kaos116 on Mar 4, 2007 17:46:55 GMT -5
Are they connected to 120V DC or A/C? Your post says both. But either way it's not the safest of voltage/ cap rating. Usually when designing circuits the rule of thumb for cap voltage rating is twice the circuit voltage in this case 120V. Your caps should be rated for atleast 240V. This not being a common voltage rating 250v would be better that 200v. The next question I have is what is your resistive load on the LED's? Also What is the PIV (peak inverse voltage) rating on the LEDS? Is there a link to the begining of this thread? I'd be more than happy to add my 2 cents, but I need more info before I can try to help.
Todd
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Post by David Murray on Mar 4, 2007 19:27:38 GMT -5
This is a rough diagram of what the circuit currrently looks like. Yes, the whole setup is connected right to 120V AC. My idea is to eventually replace one of the lights in my room with LEDs so that the room is lit up by LEDs. However, it appears so far that 25 LEDs aren't enough to match the fluorescents I have right now. It is currently looking as if I might need closer to 100 LEDs. Still, if I can get this to work, I can modify the design to accomodate more. On this setup I have not plugged it in longer than about 10 seconds.
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Post by Jim Brain on Mar 5, 2007 2:26:15 GMT -5
DON'T plug that circuit in as shown for very long. The caps will pop and hurt you. You can;t gang up 50v caps and get 150V support. They are opposite resistors. For example: 3 100ohm resistors in series is 300ohm, in parallel, it is 33ohms. IN series, resistors need only handle the current through them, so as the overall resistance goes up, the wattage dissapated by eahc resistor goes down. But, caps are opposite. 3 1000uF caps in series is 333uF. They must be placed in parallel to get combined capacitance. And, at all times, they must be rated for maximum in-rush current, because they are reactance loads, not resistance loads. In fact, I believe you should remove them and throw them away, as they are probably already developing shorts across the plates. If you want to do 25 LEDs, here's someone who's already done it: ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#lineled.gifHere's a blurb on figuring capacitors: tpub.com/neets/book2/3e.htmAnd, you'd be best to ditch the resistor, as that is wasted energy. So, assuming your LEDs are 3v 20mA, 170/3 = ~ 57 LEDs. The cap won't waste much energy, it's mainly there to deal with inrush current and the value is not very relevant. The bridge rectifier is highly recommended, as it will eliminate wasting half the duty cycle and (with the capacitor smoothing the waveform) eliminate the 60Hz flicker. Do as the first post suggests and scavenge some old caps off a PC PS. Jim
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Post by David Murray on Mar 5, 2007 9:41:49 GMT -5
Okay.. this idea looks interesting.. Here is the diagram for reference: However, there is one problem with this diagram. The LEDs I'm using are 3.4V (3.6 max) So can you tell me what I'd need to change to make it the right numbers? I'm glad I have you brains around here (pun intended) to help me with this stuff. One other interesting tidbit. I was walking into the room yesterday and touched one of the leads as I was grabbing the LED strip. To my surprise I shocked the led (static from walking on carpet) and also the LEDs turned on. Not only that, because they had the capacitors there, they actually stayed on for about 20 seconds. The whole thing was unplugged at the time I did this, so my static discharge was actually stored in the caps and ran the LEDs.. I was amazed that there was enough power to do that. However, I was dumb. I thought that was so cool I decided to do it a few more times, on purpose. Well, next time I turned on the strip, the one LED next to where I had been touching it was no longer functioning. I guess it never occurred to me that an LED was ESD sensative. Guess I learned me lesson. Now I have to desolder that one and drill it out (as it is epoxied in place) Anyway.. I'll try that design shown above, if somebody can tell me what values to use to make it work for 25 LEDs that are 20ma at 3.4V. EDIT - I read the description on that page again and realized it actually shows the formula right there. So I went down to Fry's and got a nice capacitor, it was 250V / 47uF. I also bought some of those cement power resistors. I bought four 1K-Ohms and a 270 ohm. I put them all in series to try for around 4,200 ohms as suggested by the formula. Okay.. I put it all together. It works. I haven't let it run more than 10 seconds yet. However, I do not think it is as bright as it can be. I put my volt meter on a single LED and it is reading 3.2 Volts. So I am thinking of removing the 270 ohm resistor and seeing where I am at then. I need to be closer to 3.4 Volts to get the brightness I'm looking for. With my earlier setup I was actually reading 3.5V, which is still in tolerance since these support 3.6V max.
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Post by David Murray on Mar 7, 2007 13:46:44 GMT -5
Just an update here - I've redesigned it with the design in the previous post. It seems to work. I've left it running for up to 30 minutes and the LEDs and the capacitor are cool to the touch. The 5-watt cement resistors I'm using are slightly warm, but not close to being hot. I'd guess they are around 110 degrees. All seems good, except one thing I'm curious about. When pulling the power, the LEDs still run for about 20 seconds from the capacitor. They start dimming down immediatly, of course. What is weird is that 3 or 4 of the LEDs will turn black (well, not black, but having NO light) within 2 or 3 seconds of pulling the plug, the rest seem to stay evenly lit as they continu to dim down. Is this a sign that I might have damaged some of these LEDs, or is this normal irregularities in manufacturing (yes - I know "normal irregularities" is an oxymoron, but you know what I mean) How long should I run this thing before deciding the design is solid.. because I'm going to fill parts of it with epoxy before mounting it in my house somewhere.
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Post by Jim Brain on Mar 7, 2007 20:24:08 GMT -5
It's no doubt normal, due to bandgap tolerances on the LEDs. Can you sub them out just to be safe, though?
Jim
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Post by MadModder on Mar 8, 2007 12:26:16 GMT -5
Good thing you choosed a 250V capacitor. The effective voltage is 120V, but the top voltage is 120*sqr(2) = 170V. That is what you get when rectifying and connecting a smoothing capacitor. You can have several electrolytic capacitors in series, but you are recommended to use resistors in parallel with each one to even out the voltage difference, because the capacitors inner resistance is different, and you will most likely fry one of the capacitors otherwise. The size of the resistors depends on how high the voltage is over each capacitor, and how much current you are going to draw. If you draw 30mA, you can have maybe like 5mA through the resistors. In this case 10k is quite good. If you have four 50V capacitors in series to handle 170V, you want 42.5V for each one. It's a bit on the edge but... Anyway, four 10k resistors in series over 170V is 4.25mA. 4.25mA*170V ~0,7W. 0,7W heat just because you place four capacitors in series instead of one that can handle the voltage.
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Post by gmoon on Mar 17, 2007 17:44:48 GMT -5
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