Several questions about driving LEDs - Page 1
Forum section: electronics / projects

author: treespicer
author_rank: Newbie
timestamp: February 13, 2014, 05:29:16 pm
content: I recently had my kitchen remodeled and my contractor installed some ofthese LED stripsto use as under-cabinet lighting. They are garbage. Within 9 months, 5 out of 6 had failed. The reviews online suggest that this is common.So I decided to take one apart and see what's inside. These are powered directly from 120V mains and consist of aDB107Sfull-wave rectifier, 25 SMD3528 warm white LEDs and 24 91-ohm resistors in a single string. That's it. I probed around and one of the LEDs had failed open, so the string was dead.Q1: Is the rectifier necessary? LEDs are rectifiers, so you could just connect the string to mains to get half-wave, correct? I suppose it'd be slightly dimmer.Q2: Why are these failing so consistently? My best guess is EFT or surge on mains is causing over-voltage on the LEDs. I don't think there's been much lightning nearby recently. Is my washing machine killing my LEDs? And why wouldn't they just throw a zener or MOV on there and be done with it?So I looked for a replacement and it seems 12V LED strips are more popular. I assume there's a reason, so I went withthis strip of LEDsandthis LED driverbecause they were cheap.I didn't notice the bit about AC or the MR16/G4/MR11 bulbs when I bought the 12V driver, but I figure surely each of those bulbs is just a string of LEDs so I should be okay, right? It doesn't work. The drivers cannot drive the LEDs (they are extremely dim). I don't know if it is because they are broken or because they're not meant to drive a string like I have. So now I opened these up to see what's inside.This is the schematic as far as I can make it out (click for bigger):Q3: What is going on here? I see that it's rectifying mains, then there's some sort of oscillator/regulator circuit driving the output driver. Is there a name for this kind of circuit? The 10-megaohm resistor is a dummy load to get LTSpice to stop complaining. Also, the output transformer secondary is actually floating, LTSpice threw a fit about that, too. The simulation output is a fast-switching waveform with about 930mVrms voltage. The measured output waveform is shaped slightly differently, but about that voltage. The transformers on my schematic probably aren't very accurate.Q4: Why bother with any of this? Why not just have a single step-down transformer? Maybe there's some current regulation feature here I don't understand.Q5: How do I kludge this to work with my LEDs?

author: Corporate666
author_rank: Unknown
timestamp: February 13, 2014, 06:35:56 pm
content: 1) LED's also have a max reverse voltage and many LED's, especially the latest ones used in white lighting, specifically state they should not be operated in reverse... IMO it's always a bad idea to use the LED as a diode instead of a light source.2) No idea. LED's die from heat - i.e. too much wattage. Relying on a resistor and the forward voltage of the LED is, IMO, a bad idea. The LED biz is so competitive that it's also possible/likely they are overdriving the LED's for more brightness without more cost, meaning they die sooner.3, 4, 5) Why do you want to use this AC driver to power your DC LED's? A 12V AC signal is going to be, what, 17 volts peak-to-peak, so you're only getting a max of +8.5V to your LED's, and it's pulsing at 60Hz, so yeah, it's going to be very dim.120VAC -> 12VDC power supplies are really cheap... I don't think there is any reason to try to bodge this one to work.

author: treespicer
author_rank: Newbie
timestamp: February 14, 2014, 04:08:38 pm
content: Max reverse voltage should be about 5V per LED. I did find one datasheet that had a footnote that said they are not meant to be driven in reverse. But why? It still listed a max reverse voltage of 5V. Why wouldn't they just say 0V? I mean... it's still a diode.Forward voltage should be around 3.2V, which would put the current at about 40mA peak. Datasheets give peak DC forward current between 30~60mA, so it could be overdriving them. Peak power dissipation seems to vary quite a bit between manufacturers, also. One of the reviews online said the company switched to a different manufacturer, so my guess is the first guys were underbid by someone using cheaper parts. That answers that question.As for the AC driver, I want to understand the circuit more than anything else. And I don't want to make this AC driver work, I want to turn it into a DC driver because I already bought it. It's the principle of the thing.

author: Corporate666
author_rank: Unknown
timestamp: February 14, 2014, 10:03:06 pm
content: Quote from: treespicer on February 14, 2014, 04:08:38 pmIt is technically a diode, but you shouldn't confuse an LED with a diode. They have different construction and design. Most datasheets will have an "absolute maxmimum ratings" section where they state that because the device can achieve these levels, it does not mean the device will operate acceptably at those levels for any period of time. Same with LED's. It can take 5V of reverse voltage, but that doesn't mean it will last. Ergo, they are not mean to be driven in reverse.QuoteWell, not sure I'd assume it was a case of underbidding using crap parts... more likely an engineering failure, IMO... like using an LED with lower Vf and pairing it with a constant voltage supply or something. It is trivial to measure voltage, current and temperature, so if you have more than one strip, replace the defective LED with a working one and see what is going on.QuoteRectify the AC and put a regulator on the output - done.

author: treespicer
author_rank: Newbie
timestamp: February 17, 2014, 08:24:07 pm
content: Don't confuse a diode with a diode. Gotcha.As far as driving with a constant voltage, you have to. The LED string tape that I linked to is just a bunch of parallel strings of three LEDs and a 151-ohm resistor. Each string is about 2 inches long and it's made so that you can cut it at the dotted lines to whatever length you need. Since you don't know what your load is going to be, you can't control the current. Most under-cabinet lights are daisy-chainable, so you don't know the load in advance. They just specify a max number of lights you can chain together.Meanwhile, I think you missed the part where I said the driver is only giving me 1Vrms output, so I can't just rectify it. The reason I posted this thread is because I don't know if the driver is broken, or if it's working properly and I'm just using it wrong. Like I said, it specified some special bulbs, but I assumed they were just strings of LEDs.Theselinkssay otherwise and it seems like I need a specific type of capacitive load for the driver to operate correctly. That was my suspicion. Indeed, the output waveform looks very different when I substitute the load with a capacitor in LTSpice. Here it is cleaned up a little bit:So again, the question is, what is the easiest way to modify this to create a basic flyback converter to drive my LED tape?

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 18, 2014, 09:15:05 am
content: TreeSpicerThe circuit diagram you attached looks like that of an "electronic transformer". These are used in place of conventional iron cored mains transformers to power 12V halogen globes. The output is 12VACrms at +/-40khz. They are also "dimmer friendly" so can be used with a phase controlled triac based dimmer. They are a self oscillating topology (Jensen converter I think) that uses a saturable reactor control core. No control IC required. It is open loop and has rather good load regulation considering its simplicity. They use bipolar transistors rather than MOSFETS for cost reasons.Omron make a range.There is no way that this should be considered for use with an LED strip light.Supply the LED strip light with the correct DC supply it requires and, assuming its not Chinese crap, it should last.Dick

author: Corporate666
author_rank: Unknown
timestamp: February 18, 2014, 10:27:54 pm
content: Quote from: treespicer on February 17, 2014, 08:24:07 pmThat's not what I said. I said you should not confuse an LED with a diode - the LED is technically a diode, but using it for rectification is bad - as you discovered in the datasheet you mentioned. You asked if you can't just drive it off stepped down mains because it's a rectifier. The answer is no. Or, perhaps "you could but it is highly likely your LED lifespan will be shortened, perhaps dramatically".QuoteI was referring to your question about why your previous strips failed. LED's die generally from heat, meaning too much power flowing through them for their heat sinking capability. I don't think the theory of "crap supplier" is correct, because even crappy LED's should last much longer than you have experienced if they are driven within their limits.QuoteI don't see the reason to modify this driver into a flyback converter for driving your LED's? It's not as if it's close but just needs a couple of tweaks here and there. It's a different animal altogether. The right solution is to buy a 12V power supply, but if you are just embarking on a journey of what is technically possible, well, you have a transformer and some diodes... rectify the AC (i.e. part out this driver and re-use a few of the parts).

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2014, 12:36:10 am
content: QuoteI would go one step further in stating that this circuit, as an led strip driver, is commercially not viable - too complex, unnecessary and costly to implement. I don't believe for a second that it comes from a commercial product.

author: treespicer
author_rank: Newbie
timestamp: February 24, 2014, 08:38:56 pm
content: Quote from: dannyf on February 19, 2014, 12:36:10 amActually, as Dick mentioned, this appears to be acommon circuitfor electronic low voltage (ELV) transformers used in halogen lighting. It is also similar toelectronic ballast circuitsin CFLs.I realized not long after my last post that the self-oscillation circuit requires a minimum load to complete the positive-feedback loop. When I first tried out the driver, I was only using one string of LEDs to test it, which turns out to be way too small. From my simulations, the minimum load required is in the neighborhood of 12W. One of my sections of lights is 8 ft long, and at 1.5W/ft that's right at 12W. So I should be able to take Corporate666's advice and just rectify and regulate it.However, the only regulators I have lying around are 78T05s, which burn way too much energy. Also, I have another section of lights that's around a 1.5 ft or so, which is not a sufficient load. I could just throw a 10W load resistor on there, but that seems pretty ridiculous. And I don't want to have to heat sink anything. So I went back to the flyback route.I wanted to do this because the circuit is actually very similar to a simpleringing choke converter. I tweaked it for a while trying to find a configuration that would work and finally came up with this:I removed the L2 winding, shorted C1, beefed up C2, re-routed Q2 a bit and added D5/C4. It seems to regulate pretty well (in simulation). It's not very efficient, though. Q1 dissipates about 1W with a ~2.5W load. Also, the two drivers that I have are made by different manufacturers and one uses 13007 while the other 13005 BJTs. When I swap out the 13007s for 13005s in simulation, the circuit doesn't oscillate. So I guess it's fairly sensitive to component characteristics. I'd appreciate any input on why that might be.Meanwhile, I realize this isn't terribly practical, mainly from a safety perspective. It's more of a learning exercise. Does anyone have a recommendation for a cheap power supply? I can get a 12V 2A wall brick adapter for less than $5 online, but this will be hardwired.