Llow part count buck that come in dip package?

Discussion in 'General Electronics Chat' started by spinnaker, Nov 12, 2012.

  1. spinnaker

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    I need to regulate around 600ma at 12VDC for an LED light. I have some pt4115s but they are SMD. I'll give it a try as I have nothing to lose but my hands really are not all that steady and a dip would be much easier to work with.

    The other requirement that it needs to be fairly compact so a circuit of discrete components might be out of the question.
     
  2. takao21203

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    You can put the coil on top of a MC34063 IC. And it is possible to solder a SMD diode directly to the IC pins as well.

    You need one of these square, high current SMD coils. They cost $1 to $2 approximately.

    For the connections you can use small pieces of bell wire, or cut off LED leads (these are more hardy than other components wires, probably not made from copper). So the coil kinda sits on small stalks.

    I always solder this stuff freehand. If you use tools to hold the assembly in place, to solder wires to SMD components, it may take quite a long while to assemble it.
     
  3. spinnaker

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    That was going to be my question if I could use a MC34063. The datasheet does not seem to be real clear on this. But I guess it does say current limiting.

    What is confusing is it is a step up / step down regulator. I don't want to step up or down. I have 12v and need it to stay 12V.

    I ordered some MC34063s for a 5V step down supply. If I need 5V plus my current limited 12V, I assume I would need 2 of everything? One circuit for the 5V and one for the current limited 12V?
     
  4. takao21203

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    Normally this chip is used in step down configuration. It is also possible to step up voltage, but requires different circuit.

    If you want two voltages, using only coils, you need two chips. There are of course tricks which allow you to use only one chip, but only for lower currents. Isn't so much worth the efforts.

    What is the forward voltage of that LED? If you have 12V DC input, you never get 12V DC output. Depending on the forward voltage, you might not be able to light it at full brightness.

    If you want a fixed 12V output, use a LM2576-12 maybe. They are also sold as DPAK, with very short leads. They can be bent easily as required. Normally intend for SMD PCBs.

    There are many such regulator ICs but most interesting one's are actually SMD.

    The LM2576, MC34063 and TL494 are all nearly two decades old, nothing new in terms of technology. Newer chips often use MOSFET, but as said, are SMD, more expensive, or also can be more difficult to buy.
     
  5. spinnaker

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    Don't worry about the LED. I already have an off the shelf buck regulator that I am trying to replicate. The current configuration works fine.

    So since I am going 12V to 12v, I cannot use a MC34063. Correct?
     
  6. spinnaker

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    It looks to me like the MC34063 is a voltage regulator. I am not wanting to regulate voltage, I want to regulate current.
     
  7. takao21203

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    It can be used with a current sensing resistor.

    What is the specified LED voltage?

    AFAIK most simple buck reglators drop voltage. With a MOSFET you can get more close.

    34063 has a NPN switch.

    It is also possible to use external transistor.

    AFAIK for a saturated PNP you can easily substitute a p-ch MOSFET. Indeed I have done it for a TL494 circuit.
    The schematic I used was quite old so I had to modify it.
    Later I reversed back to a large power PNP because the MOSFET wasn't so much capable to withstand the inrush currents. With a 2 kW capable input, the MOSFET just popped up immediately.

    I have not tried MOSFET with MC34063, though.
     
  8. spinnaker

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    Looks like if I want to do this, I will need to use those pt4115s. That is what was on the original board that came with the light. It also used a MOSFET to switch the voltage on and off to the light.
     
  9. takao21203

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    OK I saw the IC.

    where do you buy it?

    You'll also need a SMD adapter for that or a PCB.

    You wrote above that you would prefer not to use SMD.

    Farnell and RS have 100s different types of such ICs.
     
  10. spinnaker

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    Got it from ebay but mouser has them.

    RS as in radio shack? I don't think so.
     
  11. takao21203

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    RS Components (Radionics).

    I have run red 3W LEDs directly from a 5V electronic transformer.
    They are a bit adjustable but that only changed the brightness a little.

    Other LED forward voltages don't match so well, a driving circuit is required then.

    I have ordered some 10W LEDs a few hours ago. Maybe I should get some of these ICs too. But I need to use a large 500VA toroid with more than 50V for each output winding (2x).

    The problem is, red and blue LEDs have different forward voltage. Maybe I put them in series combinations so I can use a single voltage for driving. And yes I would also use a current limit.

    I have one large drawer here only with switching power supply PCBs.
     
  12. takao21203

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    OK bought some of these ICs.
    Only have 680 uH toroids here but they are large, wire can be taken off.

    So I use my TL494 board to step down to 20V or something, and then I use 1 IC for 2 LEDs in series. For the shunt I will maybe use a piece of wire.
     
  13. spinnaker

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    Where do you get those ICs so fast on a Tuesday morning?
     
  14. takao21203

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    Guess- from eBay. They are indeed valueable, so it is not worth to try to use something else.
     
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