current limiting for very low resistance loads

Discussion in 'The Projects Forum' started by rft-hillview, Nov 19, 2008.

  1. rft-hillview

    Thread Starter New Member

    Nov 19, 2008
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    Hi
    Let me confess I am not an electrical wiz - on the contrary I build things I think will work based on what I have read.

    Having built lots of power supplies in the past I thought a new one to drive a pyrography pen would be simple - just a supply to give between 1 and 2 volts at a current of between 1 and 4 amps.

    Pyrography is when you decorate wood or leather by burning it (hopefully artistically) so the pyrography pen is basically a zero ohms filament.

    Actually the cold resistance is between 0.05 and 0.1 ohms and hot resistance is not much more.

    The problem is that all the devices I have looked at like the LM317K and LM338K have short circuit protection built in and I cannot find a datasheet which mentions loads with very low resistance. Pen 'nib' temperature needs to be controlled - I suspect by controlling the current.

    This cannot be a new problem - has anyone got a tested design that might meet my requirements - I do not want to admit defeat and spend £100 buying one !

    thanks
    Ralph
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I'm assuming your "iron" has a step-down tranformer. It's a much easier job to regulate the current on the primary side, which should be just about as effective as doing on the low voltage side.

    eric
     
  3. rft-hillview

    Thread Starter New Member

    Nov 19, 2008
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    Thanks Eric
    I had not considered current limiting as anything other than a downstream of the transformer activity in a classic regulator sort of way.

    Can you suggest some required reading please
    ralph
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hi Ralph:

    Are you looking for precise current regulation, or merely overcurrent protection? An incandescent lamp is a fairly decent current regulator if chosen carefully.

    Eric
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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  6. rft-hillview

    Thread Starter New Member

    Nov 19, 2008
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    The real problem here is that there is very little published about pyrography power supplies. What little there is suggests 1 volt at 4 amps is typical. However the "filament" of a typical nib is 25mm of 23 sgw (0.65mm dia) nichrome wire. This is 3.3 ohms per meter hence a filament resistance of between 0.05 and 0.1 ohms.

    Now even I can do the sums that says that at 1 volt we would draw 20 amps with the smaller filament resistance. I made the assumption that when sources described units at 1 volt and 4 amps that what the really meant "not a big voltage and about 4watts total" which led me to seek a current limiting solution that would not fall over with a .05 ohm load.

    Eric's suggestion about looking at the primary side has led to possibilities using dimmable "electronic transformers" which are nice and cheap etc but which still have potential problems with short circuit protection kicking in at such a small filament resistance - still early days on this line of research.

    As for the solution suggested by nomurphy it is precisely that type of pyrography that I trying to get away from - it is much too clumsy and uncontrolled for the more artistic and delicate work which I have in mind - I need something that behaves like a pencil rather than a chisel and that rules out the soldering iron approach but thanks for the thought.

    As an answer to Eric's question "Are you looking for precise current regulation, or merely overcurrent protection?" what I want is to be able to twiddle a pot and go from a black barely hot "nib" (filament) to one which is glowing very brightly (and back again) fairly quickly and smoothly. There is no special technical requirement other that nib temperature control.
    thanks Ralph
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hi again, Ralph.

    Ahh... Well, in that case, what you need is a simple Variac! Get one a little bigger than you need, and you'll have all the regulation you could ask for.

    eric
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Okay, so you didn't like that one. Are you familiar with modern-day soldering stations available with a variety of tips that are designed for fine-pitch SMD rework? You can get tips that are only 31 mil in diameter. I just measured my #2 pencil tip, and it's ~55 mil (slightly worn). Such stations are much cheaper than a variac, and have pretty fast temperature control. For a relatively inexpensive one, take a look at:

    http://www.action-electronics.com/wewes51.htm

    And, look at the ETP or ETO tips.
     
  9. rft-hillview

    Thread Starter New Member

    Nov 19, 2008
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    Hi
    I have a working solution to my problem:

    It consists of a regular 250 watt dimmer feeding an Osram Halotronic Mouse HTM 150W electronic transformer.

    This works even with the almost short circuit 0R05 filament because of the nature of the electronic short circuit and reset system which is part of its specification. Due to limits on both the dimmer and the electronic transformer the combination is operating near its shut down limit because the pyrography filament is not drawing enough power .

    After some experimental setups I added two 0.05 ohm 25 watt resistors in series with the filament and this brought stability and much smoother control from the dimmer.

    In operation the filament is operating at about 1.5 volts - assuming my meter is capable of giving accurate readings with the chopped up AC coming from the dimmers triac and passing throught the electronic transformer, and is drawing between 3.5 and 4.5 amps depending on the filament brightness.

    My thanks to those who took an interest in the problem
    Ralph
     
    Last edited: Dec 18, 2008
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