120VAC rectified power +thermistor, drive LED's

Discussion in 'The Projects Forum' started by timstolt, Aug 8, 2014.

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  1. timstolt

    Thread Starter New Member

    Apr 7, 2010
    3
    0
    Hi, everyone! Can you help me finish designing a LED circuit which is protected by a thermistor? Schematic attached.

    LED Light
    1. 120VAC supply
    2. protected/fused at 3A
    3. Rectified into DC
    4. Capacitor smoothes
    5. Resistor slow-bleeds capacitor to prevent shock at plug (right ohms?)
    6. 14 LED’s are the load, and used to drop all the supply voltage: prefer to keep at <120 Celsius
    7. PTC thermistor nested in LED’s to aid in current control, especially runaway current due to heat.


    Questions:
    1. Is this circuit a good idea?
    2. Is my R1 resistor placement and specifications a good idea?
    3. Can someone help me choose a thermistor for this setup?
    4. It seems that the circuit would ideally have around 126ohms resistance to limit current to the LED’s.
    a. Is it possible to use the thermistor for this resistance?



    BOM

    ITEM DESCRIPTION SPECIFICATIONS PN
    C1 Capacitor Aluminum, 400V, 560uf 380LX561M400A052
    B1 Rectifier Bridge, 800V, 4A GBU4K-BPMS-ND

    R1 Resistor 1Mohm, 1watt FMP100JR-52-1M

    D1 Blue LED 9-11V, 900-1050ma ebay
    F1 Fuse Fast blow. 3a, 125V
    T1 PTC Thermistor specs??


    LED Light
    1. 120VAC supply
    2. protected/fused at 3A
    3. Rectified into DC
    4. Capacitor smoothes
    5. Resistor slow-bleeds capacitor to prevent shock at plug (right ohms?)
    6. LED’s are the load: prefer to keep at <120 Celsius
    7. PTC thermistor nested in LED’s to aid in current control, especially runaway current due to heat.

    :confused: Questions:
    1. Is this circuit a good idea?
    2. Is my R1 resistor placement and specifications a good idea?
    3. Can someone help me choose a thermistor for this setup?
    4. It seems that the circuit would ideally have around 126ohms resistance to limit current to the LED’s.
    a. Is it possible to use the thermistor for this resistance?

    Schematic attached.

    Thanks very much for any help!!
     
    Last edited: Aug 8, 2014
  2. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello timstolt

    According to the scheme:
    You will get ((VAC * Sqrt 2) = 169.70 VDC). at the output of the bridge rectifier.

    Now: That DC voltage applied to 14 LED’s would have a voltage drop of 12.12 V each.
    But. . . How much current requires one of your LED’s, to fully illuminate?
    Say, what is important in the management of LED’s is the bias voltage and current required to fully illuminate.

    Moreover, what kind of thermistor intend to use: NTC or PTC ??.

    First, the thermistor must support the current flow through the LED’s.
    Also: if you use a NTC thermistor, its resistance is inversely proportional to its temperature.
    If the temperature rises, its resistance will decrease.
    With the consequence that the current in the LED’s rise.

    Otherwise the PTC thermistor, its resistance is directly proportional to its temperature.
    If the temperature rises, its resistance will rise.
    With the consequence that the current through the LEDs will drop.

    There is another important thing
    How many ohms per °C the thermistor varies?
    Is necessary to know the electrical characteristics of the thermistor.

    I think the "Basic" idea is good but you need to ripen to achieve the objective: To protect the LED’s against temperature rise.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Guys, this is a fancy version of "LEDs to mains", or "transformer less power supply". Read the terms of service (linked at the bottom of this page). Such topics are not allowed.
     
    timstolt likes this.
  4. timstolt

    Thread Starter New Member

    Apr 7, 2010
    3
    0
    sorry, I should have read the terms more carefully. I would edit the question if I could...system won't let me. I'll clarify the question in my reply to Carlos below. Thanks!
     
    Last edited: Aug 8, 2014
  5. timstolt

    Thread Starter New Member

    Apr 7, 2010
    3
    0
    Thanks, MrCarlos! I would be using a PTC thermistor, but am confused about how many ohms it should be at 25c.

    ---as a side note, are not the commercial LED strings connected to mains without a transformer? Why the rule? I dug around a little, and it seems to amount to "galvanic isolation".

    Also, I could construct this same circuit seemingly more safely with a 1:1 120VAC isolation transformer such as N-48X. ...although it's still 120VDC then going through that LED series which I suppose is kind of high, though it would be insulated even better than your houselamp cord (which is directly to mains, so I don't know what gives).
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    It's about safety, and it doesn't always make sense, but rules don't have to. Many things discussed here naturally involve products that connect to the mains. But when you start talking about building a project, especially one regarding LEDs (they tend to attract the kids), the line gets crossed.
     
    timstolt likes this.
  7. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    And wayneh is right.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    This can be found in our Terms of Service (ToS)

    Please do not bother to reword the question. It will not fly under the radar and will most likely result in a ban.
     
    timstolt likes this.
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