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is .35 volts at 35 amps possible? — Parallax Forums

is .35 volts at 35 amps possible?

GeeksGoneBadGeeksGoneBad Posts: 100
edited 2012-12-13 09:17 in General Discussion
I am working on a project with a friend - and he's done the math and says we need .35 volts at 35 amps and I told him I didn't think it was possible and that nothing exists that would do that

so I'm looking to be set straight or looking for back up :)

Jamie

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2012-12-12 11:06
    Sure it's possible. Ohm's Law gives R = E / I = 0.35V / 35A = 0.01 Ohms = 10 milliOhms.

    The fact that it's possible doesn't make it practical. That's not much resistance. You need pretty large wires made of very conductive material to keep the circuit resistance below 10 milliOhms total over any significant distance. You need fairly large area connectors, again using very conductive material, to make connections ... and not too many such connectors. Another issue ... how are you going to get that much power produced at that voltage with that sort of low source resistance. Remember that any sort of power generation device has its own internal resistance.

    If you've got a load of 10 milliOhms that you need to run 35 Amps through, you can generate the 35 Amps at a higher voltage and compensate for the voltage drop through your wires and across any connectors as well as compensate for the generator's internal resistance. This is commonly done by using a voltage regulator and measuring the voltage to be regulated at the load itself. The regulator takes into account any voltage drops across wiring and connectors, etc. Remember that 35A through any conductor is going to generate a significant amount of heat. The bigger the conductor, the less the resistance and the less heat would be produced. Think about automotive starter cables or plugs/wiring for electric dryers or ovens.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2012-12-12 11:14
    Interesting. What would you need such a low voltage for at such a high current?
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-12-12 11:37
    I did forget to mention that there is already resistance in play - a short piece of nichrome wire like 1.5 inches which gives a resistance of .09 ohms or so

    we need that wire to get to more than 260 degrees Celsius so that's (apparently) 35 amps? (I really know nothing of nichrome wire)

    the end goal is to build a better heating element for 3d printers
  • TubularTubular Posts: 4,706
    edited 2012-12-12 11:50
    Definitely possible. How tight does the voltage regulation need to be ( + / - ? ) , and how constant is the load current?

    The single cell solar cells parallax sells put out ~.6v at ~ 5 amps each. You could use an array and store the collected energy in a supercap (array) and chop it out from there to get 0.35v average.

    Or you could modify a classic linear power supply stage (mains->transformer->bridge rectifier->capacitor->dc out) to use two transformers, ie two 10:1 gives 110:11v followed by 11v to 1.1v, then rectify and chop from there. You'd waste heat in your rectifiers but you could manage that.

    Ok, so its for 3D printing. The 3v3 rail of an ATX power supply should put out ~30 A, you could just store that in a big supercap and chop the output and let the nichrome wire itself smooth it out thermally.
  • Mike GreenMike Green Posts: 23,101
    edited 2012-12-12 11:56
    Are you sure the resistance isn't 0.009 Ohms? Remember that resistance in a wire increases as the wire heats up, sometimes substantially. If the Nichrome wire does have a 0.09 Ohm resistance, you'll need 3.5V to get 35A through it. It sounds like you want an adjustable power supply with current and voltage limiting and rated a little in excess of 35A like 40A or 50A with a voltage output of at least 5V, maybe more. You'd set the current limit to 35A and the voltage limit to something low, maybe a couple of volts. You could probably find some schematics for an appropriate current limited power supply on the internet, maybe in the datasheet or application notes for a voltage regulator.
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-12-12 12:02
    Nope I'm not sure of anything ;) hehe

    the Nichrome wire said it had a resistance of 1.67 per foot so I *assumed* I would divide that to get the ohms for an inch - could be completely wrong there

    but the consensus I'm hearing is that it's doable - glad I'm wrong about that then :)

    the big thing for me was wasting time on something that was impossible - but if it's possible then I'll work it out

    Thanks for the info guys!
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-12-12 12:07
    Hi Geek;
    GeeksGoneBad wrote: »
    I did forget to mention that there is already resistance in play - a short piece of nichrome wire like 1.5 inches which gives a resistance of .09 ohms or so

    we need that wire to get to more than 260 degrees Celsius so that's (apparently) 35 amps? (I really know nothing of nichrome wire)

    the end goal is to build a better heating element for 3d printers
    Don't forget that the resistance of the nichrome will increase as the temperature increases.

    Duane J
  • Mike GreenMike Green Posts: 23,101
    edited 2012-12-12 12:11
    You might look at constant current regulators designed for use with high power LEDs. TI has some and they have evaluation boards. There was one I saw that can handle 40A and normally uses a 12V input. There are also chargers for large model battery packs. There was one with a 14V output at 30A max. I'm sure you can find others.

    You're playing around with quite a bit of power ... as others have mentioned, there's not a lot of difference between what you're handling and an arc welder.
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-12-12 12:19
    well right now we are just playing with the math of such things - but when I get to playing around with actual juice I will be careful ;)
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-12-12 12:20
    Also you might be able to find a large low voltage filament transformer, say 6.3V or lower, then control the output with a110VAC Variac, variable voltage transformer. Look for a transformer with a 220VAC primary.

    Duane J
  • GadgetmanGadgetman Posts: 2,436
    edited 2012-12-12 13:20
    I'm wondering more why you're using Nichrome wire as a heating element...

    The more popular choice these days is a 5.6Ohm, 5W Resistor embedded in the printhead itself.
    http://reprap.org/wiki/J_Head_Nozzle

    As the resistor is inside the head, heat transfer can be made much more efficient.

    You'll be dumping around 12W into that little piece of wire, and you'd need THICK (4mm square or more) cables from the PSU to the head...
    The 5.6Ohm resistor draws around .9A at 5V (4.46W) to heat up.
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-12-12 13:56
    If you run 35 amps through a 0.09 ohm resistor you will get a voltage drop of I*R = 3.15 volts. I'm not sure where you got 0.35 volts. However, that's not even right if you are using 1.5 inches of nichrome wire with a resistance of 1.6 ohms per foot. 1.5 inches of that wire would give you 1.5 * 1.6 / 12 = 0.2 ohms. To get a current of 35 amps you would need a voltage of 35 * 0.2 = 7 volts.
  • User NameUser Name Posts: 1,451
    edited 2012-12-12 16:05
    JamieJamieJamie...if you're okay with doing things Old School, there are all sorts of easy ways to get 35A at a fraction of a volt...AC. My Dad's old Wen soldering gun had several turns of very heavy copper wire wound over a rather conventional 110 VAC primary. It'd be easy to find where to place taps on such a secondary that would deliver 0.35VAC RMS into a suitable load.

    In fact, almost any AC transformer has space to add a turn or two of heavy gauge wire (or wire braid) around the core.
  • Mark_TMark_T Posts: 1,981
    edited 2012-12-12 16:54
    Duane C. Johnson wrote: »
    Hi Geek;

    Don't forget that the resistance of the nichrome will increase as the temperature increases.

    Duane J
    Pretty negligible though - nichrome alloy is specifically for heating elements because its resistance varies so little with temperature upto 1000C (6% or so) and because
    it doesn't burn in air when at orange heat.

    Anyway I think this is all academic because a bare wire heater will cause localised overheating and charring of the plastic, a
    heated nozzle is probably most reliable as a copper tube with an insulated heater winding around it - spreading the heat evenly
    over a large surface area and in a way that is compatible with temperature sensing - temperature control is all important surely?
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-12-12 21:15
    I wonder if you would actually want to use a heater material with a high thermal resistance coefficient? That way, you can measure the resistance of the heater in order to know the temperature of the tip.

    A while ago, I designed an analog circuit that would regulate the temperature of a heater by maintaining its resistance, then had an output voltage proportional to the power going into the heater; the intent, at the time was as a hot-wire mass-airflow meter.
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-12-13 03:43
    Thanks for all the posts guys :)

    I'm not really here to discuss exactly what or how I'm trying to build the hot end - the hot end I have now has the resistor and that's fine - but I'm trying to build something different... may not work out that it's any better (or works at all) but that doesn't mean I'm not going to try some different things :)

    please note that this one idea is not the whole part of my idea - just one little part :)

    I do appreciate all the suggestions and advise - but I'm not going to discuss my master plan just yet - I just wanted to know about the voltage at that amperage :)

    Thanks for understanding :)
    Jamie
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-12-13 05:20
    I could only find 50,000 amps. But I believe this guy say is half a volt

    http://hackaday.com/2012/11/03/hijinks-at-50000-amps/

    It wouldn't do what you require, but it looks fun.
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-12-13 06:24
    Wow glad to see there are people MUCH crazier than me! LOL
  • LawsonLawson Posts: 870
    edited 2012-12-13 09:17
    Yea, It's pretty easy to push amps like that if you use AC and a transformer. Especially easy if you're willing to make your own 1-10KHz power supply. In that case a ~2 inch outside diameter ferrite ring makes a great transformer core. I've made a Smile-tastic 100:1 transformer to make a 0.010 ohm load look like 50ohms to a 50ohm function generator. (literally 100 primary turns of random wire, with the secondary passing through the center of the core) Once over 1KHz I estimate it pushed about 35 amps at 0.3-0.6 volts. (below 1KHz you could see some nice saturation effects if you fed the drive voltage/current into an oscilloscope)

    Lawson
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