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Transformerless power supply

Discussion in 'Electronic Design' started by mkr5000, Jan 25, 2013.

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  1. mkr5000

    mkr5000 Guest

    I have a simple circuit on a breadboard that has been doing fine 24/7 for months which is --

    120vac, hot into a 200k 1/2 watt that feeds a dropping cap (.47uf, 400v) that then feeds a bridge rectifier. The output of the bridge has a zener and a filter cap which supplies my DC.

    I breadboarded it because I really never used a transformerless design in a project and just wanted to see how reliable it was. (so far -- very reliable).

    My question is about the 100 ohm resistor (or around there) that is used in the neutral leg of the AC feeding the bridge that you often see in this circuit.

    I'm not using one, just tying the neutral directly to the bridge.

    So what is the purpose of this resistor? A safety precuation? Just to provide "some" isolation from ground (neutral 120vac?) Or maybe it acts as a cheap fuse?

    Of course, you'd be stupid to not have the ground isolated from the chassis or anything that you could touch (like they did in the 50's and 60's).

    I'll have to say that it's been very reliable on the breadboard anyway -- but I'm only drawing 25ma or so with my load.
  2. I don't know about the 100 ohms. But is this a one of? You're using
    the AC 'neutral' as your circuit ground? (That's a bit scary if so.)

    I'm also having a hard time adding up your numbers... the 200k ohm
    resistor is in series with the cap and then feeds the bridge? (25mA *
    200k ohm = ~5kV?)

    George H.
  3. mkr5000

    mkr5000 Guest

    that's what I thought Jim -- won't stop a surge but at least help.

    don't trust my numbers, actually can't remember the R value off hand -- even the 25ma is an exaggeration.

    really just wanted to know the purpose of the resistor in the neutral side.

    actually, it's been very reliable -- like I say, may end up using it in projects where i don't have room for a transformer.

    using (4) 1000v rectifiers, and plenty of headroom with the resistor power ratings.
  4. Phil Allison

    Phil Allison Guest

    "mkr5000" <

    ** What is the 0.47 cap for when the 200k does all the dropping ??

    I think you have stuffed up the schem.

    The 200k is meant to go across the 0.47 uF ( so it discharges when
    disconnected from the AC supply ) - the cap alone will pass about 20mA AC
    to the bridge and zener.

    The 100 ohm is simply a surge limiter and can be in series with the A or N

    .... Phil
  5. mkr5000

    mkr5000 Guest

    My Mother has a baby photo of me and she is giving me a bath in the kitchen sink.
    (late 50's)

    Right above me (and above the water), is a kitchen AM radio and guess what? It's plugged in !

    The power cord was hanging on the wall right behind me.
  6. mkr5000

    mkr5000 Guest

    Like I say, forget about my numbers, I was talking out of my &*& -- in a haste just referenced a schematic I saw that had a 200k in it.

    My circuit has (2) 20K 1/4w in parallel right before a .47uf cap and I'm just driving a couple of LED's with a 5.1v zener.

    Just a test circuit.

    You guys answered my question about the 100 ohm -- I'm not using it but sounds like good practice.

    Another thing about this circuit --

    I know it's used a lot for driving LED's. Sure seems a lot more practical sucking the 120vac for a few mils rather than a high frequency driver.

  7. Guest

    In which countries is this products sold ?

    In countries with polarized plugs, it is usually forbidden to put a
    fuse in the neutral wire.

    In an area with non-polarized plugs and thus possibly the full mains
    voltage to ground on each terminal, in case of a isolation fault, in
    the bad case, about 1 A will flow through the 100 ohm resistor,
    quickly evaporating the low power resistor.

    As long as you do not call that device a "fuse" it can be used in
    countries with both polarized as well as non-polarized plugs :).
  8. Guest
  9. mkr5000

    mkr5000 Guest

    Your 10K series is pretty high, lowers efficiency, but that's a lot of surge/spike limiting. The cap's reactance is 5K, so your current limiting is mostly resistive/lossy.

    so, that tradeoff may be a good one? protection vs efficiency?

    but of course, for higher current circuits, resistance plays a bigger role because of the heat generated in the resistor?

    what value of resistance would offer surge protection worth anything on an AC line? or -- forget about it? because a typical surge is going to wipe you out regardless? isn't that the nature of a transient on an AC line? (not talking over voltage conditions but say a lightning induced spike)

    John, that brings me to another question, I'm sure you know the answer to --

    can there really be smaller amplitude transients on an AC line depending onyour circumstances? (location etc from the problem) or is a spike generally "pretty nasty" close to the source of the problem and drop off quickly with distance?

    I hope I'm making sense. In other words, can the amplitude from a bad spikebe anywhere from a 1 to a 10 or is generally a 9 or 10 and then die off rapidly?
  10. mkr5000

    mkr5000 Guest

    now that I read my post, kind of a stupid question really --

    1 through 10 no doubt but I guess it's "when and where" along the length of the power line that I'm thinking about.
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