Will this work ? Transformers, Drivers and LEDs

Discussion in 'General Electronics Chat' started by ronan.l.n, Sep 10, 2010.

  1. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    Hi all,
    After reading loads of forums and pages online, I have a better idea of my project, but still needing a bit of help !
    I'd appreciate if you could "check" my set up and give me any advice, so here we go :

    I am trying to build a DIY video projector, from a LED, a LCD, a fan and a couple lenses.
    My LED is 30v,1A, my LCD is 12v,<3A and the fan is 12v,0.3A and I would like to power all this from a single power socket rather than having two different plugs.

    SO I thought about having 2 transformers connected to the mains, a 30v 1.07A one for the LED, and a 12v 5A one for the fan+LCD. Both circuit would be in parallel and the fan/LCD in parallel. Each "sub-circuit" would have a switch so the fan could be left on after the light has been switched off.
    The 30v and 12v transformers are here http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350390453297 and here http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=280521186928

    Would this work ? Can the 2 transformers be connected in parallel safely ? Does the LED need a proper LED driver or could a resistor in series be enough to have constant current/voltage ?

    Any advice would be very welcome !

    Thanks,
    Ronan
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    Two seperate power supplys with a comon ground will be fine but that isnt paralell!
    If you mean the input, primary windings, that no problem but you MUST have adiquate protection on the high voltage circuit.

    Your figurs dont look right though, a cluster of BIG leds might have a fowarwd voltage drop of 30V and require 1A but the LCD??

    I am only mentioning it because if the 12V power requirements you could just get it from the 30V rail with a standard voltage regulator.

    Post a schematic if you want a 'proper' qualified answer.
    Al
     
    Last edited: Sep 10, 2010
  3. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    0
    Thanks for your answer, I am attaching a schematic of what I was thinking.

    Meaning both positives on positives and both negatives on negatives, or what ?? and what would be an adequate protection ?

    Here is the LED I have :
    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160469308373
    and this is the LCD
    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=270626252242
    You are right, the LCD is 0.25A, or 3W

    Would the 30V 1.07A transformer be enough for that ? and how much work is required to have a voltage regulator ?

    Cheers

    Post a schematic if you want a 'proper' qualified answer.
    Al[/QUOTE]
     
  4. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    The schema should read 12v/0.25A for the LCD !
     
  5. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    You need to ask many more questions before building this or you will probably dammage the expensive componants.

    Sorry to be blunt but I am sure you are asking so you can learn, just like me.

    The transformers are configured correctly, in the drawing, but you are supplying AC voltage to all your componants, none of which will work like that.

    The LED would work as it happend because the 30V 'transformer' isnt!!
    It is actually a 30VDC power supply, probably switch mode.

    The 12V 'transformer' also isnt a transformer, its output is AC so the previous comment still stands but it is a supply designed for halogen lighting most of which need a fairly substantial minimum load.
    The upshot ... it probably won't power-up.

    Am I correct in assuming you plan to strip the monitor, tahe off the back light and put the high output behind it with the optics in front?

    Would you like some spaciffic help with what to buy?

    Al
     
  6. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    The absolute minimum forward voltage for that led is 30V It could be as much as 32V you should plan for this and use a higher voltage supply and some form of current limmit.
    I think the LED will overheat if its not fully on!
    I'l check that ... perhaps someone else will know better than me.

    Al
     
  7. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    0
    Thanks for your help Al, it is much appreciated !

    I hadn't noticed that the 12v 'transformer' was actually giving AC out, thanks for pointing it out !
    On the other hand, the 30v power supply is suitable right ? And should give a smoothed and regulated voltage out for the LED ?

    In that case, would it be possible to use a voltage regulator on the 30v branch, to get 12v out to power the LCD and the fan ? Would the power supply supply enough for the whole system ?
    I already have the 30v power supply so I would prefer to keep this one.

    You are correct indeed, that is the plan ! I have managed to gather all the components, but am lacking the knowledge for the connections !
    Could you point to a suitable 30v to 12v voltage regulator ?

    Since I already have the 30v power supply, I will try with this and could add a 32v LED driver if it really overheats. Yes ?

    Thanks again for the help !
    R.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    The LED must have an adequate heatsink (and possibly a fan) to work. This is not a typical LED, there is already some circuitry involved, probably in the form of many LEDs in series. However, LEDs are current controlled devices, while your power supply needs to be a minimum of 32VDC, it also needs a current regulator.

    You might check with the seller to see if they have a recommended regulator for this part, otherwise you will probably have to build your own, and it will require a bit more voltage than 32VDC to work.

    A common linear regulator for a job like this is the LM317. It is cheap and easy to use, and requires about 3V more than the maximum to work. In such a case the voltage needed would be 35VDC.

    [​IMG]

    The R for the above circuit could be qty 8 10Ω ¼W resistors (cheap and easy to get) or qty 1 12Ω 2W resistor (not so common and easy to get). The LM317 will also require a heat sink, as it will get hot.

    Again, I'd check with the seller first.

    Yours is not the first thread with this kind of problem. It resembles a project where another gentleman was building a projector light to save on replacement bulbs. It was a long thread, but he seemed happy with the end results.

    LED power questions - i know bog all about electronics...
     
    Last edited: Sep 11, 2010
  9. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    All that sits fine with me BUT ....

    The limmiting factor with led's is power disipation, yes for a constant current you can use the forward voltage drop and calculate as with any resistive device.
    If you want to be efficiant, same perceeved light less power or same power more perceeved light then you would use a pulsed supply.
    Still current limmited but, for example 50% duty cycle.
    If you look at the data sheet foe kust about any LED you will find a constant forward current and a peek forward current for some max time.
    Since the current and the chemestry are the things that affect light output for any given device you can run it longer, looking brighter, and and cooler by feeding it pulses that arv within its maximum operating range.
    Simple example ...
    An LED requiring 500 mA constant will probably be fine with 1A in 1mS pulses provided the duty cycle is 50 % or less. The average power disipated at 50% is the same.
    It will only be on 50% of the time but wll look almost twice as bright.

    If you want the LED to last longer, because you disipate less power with it, use this strategy to make it a bit brighter and use a bit less power.

    Look up white LED drivers on the net .... loads more than I can give you.
    I recomend targeting your spaciffic device.

    On the subject of the supply ....
    IF the LED drops 30V or less, AND you use the strategy above to reduce your average current requirement somewhat then a simple resistor 'might' work.

    Directly connected as a DC source with some current limmit added the supply is probably too small as it is, you would normally want sonmething at least 25% bigger than your max load.
    Worse still in order for a simple current limmit resistor, or better still the regulator circuit mentioned above, to work you would need 5V to 8V more than you have.

    By the way the simple circuit above can easily be modified to be turned off by some external signal which means the current limmit you need can also perform the PWM you want with the addition of 1 dual opamp.

    A small boost circuit to lift the voltage a bit is possible ... but well outside my comfort zone, I would have to do some research. better still find someone who knows about SMPS Switch Mode Power Supplies to advise, they are a specialist sublect in their own rihgt.

    You had better let me know how you feel about this before we get into it any further.

    Al
     
    Last edited: Sep 11, 2010
  10. Wendy

    Moderator

    Mar 24, 2008
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    I think you are off on a few things.

    Constant current supplies automatically assume the Vf of a LED. Generally they are the preferred power supply for an LED. A resistor is much simpler, but with currents this high a regulated current is better, since as the LED heats up the Vf will change a little.

    Pulsed supplies have no advantages with LEDs. Generally this is a side effect of a BuckPuck or other versions of a SMPS constant current source, but it has to do with efficiency of conversion, nothing else. You will see pulsed currents in terms of multiplexing, which doesn't apply here. It will not make the light brighter, as the average brightness is an average of the power.

    Over driving an LED, even with a 50% duty cycle, will reduce its life span, maybe a lot. Driving an LED at it's maximum rated current can reduce it's lifespan a bit. With LEDs of this class, heat dissipation is the thing. They need cooling. It you keep them cool, they will last. This device will generate about 30 watts, it will need a lot of cooling.

    A lot of this was covered in the link I gave on post #8.
     
  11. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    Hi,
    I am left a bit confused by this discussion ! SO if I understand correctly, both of you are saying that it will not be possible to use the 30v power supply I was thinking of ?
    Or could I use one of these (http://shop.ebay.co.uk/i.html?_nkw=step+up+30v) to get to 32v ?
    I have sent the seller a message to see if he has any regulator to recommend, I'll let you know.

    What do you guys think would be my best option for what I am trying to do ? It seems hard to find a driver or a transformer over 30v for an LED.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    I think a 30V 1A it will be 5V shy of working, and we've seen other people find out the so called specs weren't as accurate as they are supposed to be, so a couple of volts more may be good. The other side is it could swing both ways, so it may work just fine.

    Something else, you never want to use a component like a power supply right at the edge of its specs, it is a recipe for premature failure. Add at least ½ amps to what you get. Switching power supplies are fine, but the main reason you go with a LED is lifespan IMO.

    Me, I'd go for more to be on the safe side, something like 36VDC. Some of the PuckBucks (a specific, propitiatory device) use AC. This was the reason I was suggesting you call the manufacturer and ask what they might have. It is possible they have something off the shelf that will work well.

    So plan on 36V at 1.5A for the LED power supply.

    Then build or buy a current regulator (a LM317 is under $2). It will need a puny heat sink, as it dissipates around 2 to 6 watts of power at 36VDC.

    I can not emphasize this enough, plan on a really good heat sink for the LED. Something like a computer CPU heat sink or equivalent. You can drill and tap whatever you get to hold this LED.

    I had a relatively light weight 3.5VDC 700ma LED, and still had to use a heat sink. You will need to carefully solder wires (before you mount it on the heatsink!) before you assemble it. I noticed a couple of small solder pads on your LED, you need some solder skills or borrow someone who has them to attach your wires.
     
    Last edited: Sep 11, 2010
  13. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    Thank you for the advice, I had a look at the 36VDC power supply, and found these 2 :
    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=220641558825
    and
    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=320585161051
    Are they suitable ? Any reason to choose one or the other ?

    Then about the current regulator, does the LM317 get fitted as in your previous post, with the resistors in series ?
    Are the ones on eBay all the same ? (TO92/TO220?)
    And I understand I will have to plan for a couple of heatsinks too !

    I have a bit of soldering experience, so hoping I will be able to do it myself !

    Also, for the other part of my circuit, can I find a voltage dropper 36vdc to 12vdc to run the LCD and fan ? Would it be ok to just put it in series with the LM317 and the LED ?

    I'm learning a lot this week end !
    Thanks
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    If you get the expensive 2.1A unit you could use a LM317 voltage regulator for both applications, the cheaper lower cost unit couldn't do this. You can share the same heat sink for both regulators, but the LED needs to have it's own heat sink. Both heat sinks should have fans, though you may be able to get by without on the regulator (I'd plan on it though).

    While the LED and fans can share the same power supply, beyond that they are two different circuits. You will need some small caps for the voltage regulator (as opposed to the current regulator, both using the LM317). Figure 220µF at 63V or greater, and a couple of 0.1µF. You probably need a small breadboard to put the small components on (not the regulators). The exact type isn't critical, but here is an example of what I'm talking about.

    [​IMG]

    This is a 1.75" X 1.45" board.

    Can you show me the fans you're planning on using before you buy anything? You might be able to put the fans in series if they are similar enough and reduce the total amount of current used.

    Hope this has been of some help.
     
  15. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    "Pulsed supplies have no advantages with LEDs."
    Have to take issue with that sorry ....

    Even if you are looking at a simple white LED driver in a torch your eye will 'see' a bright flash as bright, collecting whatever image is available, in a relativly short time. It takes much longer for the lack of illumination to register, did you ever watch a film or look at a multyplexed display?

    I am not, saying you can get something for nothing. In fact I mentioned PWM which actually takes advantage of the fact that your perseption is of some 'perceeved' brightness and hence power. The point is your perception is likley to be greater than the electrical reality.

    I honestly do not know what the discrepancy is or what base freequency would best exploit it but its there!

    So far as damaging LED's by over driving them ....
    YES absolutly with you on that point. Total power disipation must be within limmits, and Junction temperature is of paramount importants, so gooling is a must.
    But you can achieve these goals with a driver that delivers a current greater than If Max (Cont') with less than 100% duty cycle an hence 'looks' brighter.

    LED bulb manufarturers do this all the time cool and bright is the holy grail of MR12 replacement lamp research. The cheep ones use resistors, better ones use the current limmit mentioned earler and the realy good ones are modulated.

    Al
     
  16. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    Not sure about the science 'here' but its well documented all over the place.
    http://www.ledsmagazine.com/news/5/5/11

    There are plenty of others
    Just thought I should qualify my comments a bit
    Al
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    5% compared to your recommended 50%, you're advice would burn the $28 LED out. Then there is the issue of strobing, which can cause some truly strange effects, most are undesirable unless they were planned for. This LEDs has no specs on over driving and strobing that I can find (if you read the specifications on a lot of LEDs it is there).

    So, it is still bad advice as well as questionable factually to my mind. Again, LEDs in numerical displays use this technique because they have to. It saves beaucoup parts. For solid state detectors I buy it since they detect instantaneous light levels, but you are talking subjective at best. I have a lot of experience with LEDs, the difference between a 10ma current and 20ma is barely perceivable, it is there, but hard to see. I also noticed the article didn't mention lifespan reductions.

    There are people on this and other sites that claim they can see the flicker up to 1Khz. I'm not sure I'm buying that either, but it enters into the planning.

    Are you even aware why you would not want to use a strobed display with a projection type display? The OP did state that video projection is his intended use.

    The main reasons most people use LEDs is longevity. Over driving defeats that, in any form. For many uses it doesn't matter, and isn't considered.

    It has nothing to do with the subject at hand, but I have an article on LEDs if you care to read it.

    LEDs, 555s, Flashers, and Light Chasers
     
    Last edited: Sep 12, 2010
  18. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    I do care, and will take your points on board thanks ....
    I dont want to hijack this thread so wont get into it any deeper here but it's obvious that it isnt as clear cut as I believed.

    Reading to do I think .... All good stuff that will probably help others too.
     
  19. ronan.l.n

    Thread Starter New Member

    Sep 10, 2010
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    Right, it all seems a bit clearer in my head, but still not sure about some things.
    If I understood correctly, the schematic I have attached nis what I should be building, with the LCD and the fans in parallel (series?) on the end of the schematic ?
    Now when trying to find the values of the different resistance, I used
    http://www.reuk.co.uk/LM317-Current-Calculator.htm
    for the first one, which returns a 1.3 Ohm 1.25W resistor, whereas you sugeested qty 8 10 Ohm 1/4W or qty 1 12 Ohm 2W resistors. Which one should I follow ? And would the 8 resistors be built in series where R1 sits ?

    The calculations for R2 and R3, for the voltage regulator side taken here :
    http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
    show that if taking the standard R3=240 Ohm, R2 should be 2064 Ohm. Is this correct ?

    And what is the method for choosing the correct capacitors ? Could you explain a bit more when you say :

    and how do they connect ?

    I was thinking about finding some old computer fans which run on 12V and have ~0.2A load to use with the regulators and the LED.
    Would they be suitable ? Do you have any to recommend ?

    Yes, extremely helpful ! Thanks
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Couple of things I'd add. Beware - there are LM317s out there with metal tabs which are internally connected to pin 2.

    That's an excellent eBay find on the supply, however I'd check its open circuit voltage before I'd hook it into a circuit. I realize you'll be loading this from the start but sometimes little time delays sneak into matters.
     
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