Help With Variable Power Supply

Discussion in 'The Projects Forum' started by sspil, Feb 12, 2010.

  1. sspil

    Thread Starter New Member

    Feb 12, 2010
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    I am teaching a physics class at the local community college. They purchased a magnetics experiment (current balance) that requires a variable 5 amp supply to the equipment. The one the experiment maufacturer recommends is their own 25V, 5A supply, but they charge $280 each for them and funds in the department are tight. The college has several variable power supplies, but none that provide the amperage required. They do have a few 6V 5A supplies, but they are not variable and I need to be able to vary the current input into this experiment.

    I was thinking of building my own modifier for the 6V/5A supply that would make it variable, but I'm not sure how to go about it. I'm thinking I need a rheostat, but I want to make sure that will do the job and if so, that I get one with the proper rating.

    Thanks-Susan
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Give us more information about the experiment.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Susan,
    I appreciate your predicament - funding for schools is tight all over, and it's been that way for many years. I don't see the situation improving anytime in the near future, which is very sad.

    I have used MPJA frequently for things like this - unfortunately they are out of stock on the supply that would suit your needs:
    http://www.mpja.com/prodinfo.asp?number=14603+PS
    5 variable 0v-30v 0A-5A supplies for $95/each is about 1/3 the cost you were quoted.

    If the experiment requires 5A current with a variable voltage supply, then a laboratory-type supply is going to be your best bet. Trying to build your own high-current power supply can be done, but it involves much more than a simple rheostat.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    If the 6V, 5A fixed supply is unregulated then you could maybe run it from a variable auto-transformer (Variac), your school should have one of those somewhere. This is a crude method, and the voltage and current output are not regulated except by how you set the knob, and by the impedance of the experiment.

    If you use a rheostat the worst senario would be 6V x 5A = 30Watts. We need how the load of the experiment varies with voltage applied to be able to give a better recomendation as to Ohm and Watt rating.

    A linear regulator circuit attached to the fixed supply would give better control over the voltage. Goggle for lot of circuits.

    Good Luck
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    How much voltage do you need? Are you sure a 25V or a 6V supply will work?
    Are you trying to adjust the voltage or the current?

    For voltage adjustment a lot of supplies come with an output adjustment pin. Usually the adjustment is done by simple adding a potentiometer. Vicor makes an AC to DC
    converter that allows an output voltage adjustment from 110% to 10%.
    A 24V supply could be trimmed up to 26.4 or down to 2.4V. You will be
    current limited and power limited by the supplies ratings. They make
    devices from 50W to 600W.

    If a 5V supply would work and you don't mind taking a chance
    on surplus check out Electronc Goldmine (EG). There
    is a Vicor FlatPAC (5V/40A) on sale for $40. If these are new
    and in new condition (as the ad states) they are great deal.
    The EG link is http://tinyurl.com/yjg56sr
    You can get a datasheet and additional application
    information at http://www.vicr.com

    You can also buy these new from Vicor. The price would probably
    be around $150 or so. You could get the exact output voltage and power
    that you require.

    (* jcl *)
     
  6. sspil

    Thread Starter New Member

    Feb 12, 2010
    11
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    Thank you for all your suggestions. I had a feeling it wasn't going to be as easy as popping in a rheostat. Here is a link to the current balance experiment manual from Pasco.

    http://store.pasco.com/pascostore/showdetl.cfm?&DID=9&Product_ID=1681&groupID=317&page=Manuals

    The experiment demonstrates induced magnetic force with current. You can see that the manual doesn't really say much about voltage requirements/limits which I though was strange. The important feature is the current supply, but I would imagine there has to be some sort of operating voltage range. The only time they mention voltage is when they suggest their own power supply at 25V.

    The physics department at the college is non-existent. There is no full time faculty there, they staff it with part time adjunct faculty and they never seem to stay long. My boss is in the Chem. department and knows nothing about the equipment for the physics lab. I'm trying to kluge together labs with the hodge podge of equipment they have there. Luckily they have decent circuit trainers that will do most of what I need for our electronics units. Despite the lack of equipment and departmental support, I really like the students and the class. I want to do the best I can with what I have so your help is greatly appreciated.
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    That link shows the apparatus but not the manual :( Some sort of FTP error messages
    were displayed.

    A current source would make more sense in this application. Are you sure that the source is not a 5A current source with a 25V compliance?

    If a current source is required I would get a low voltage supply and build a current
    sink using an LM317 or a MOSFET and Op-amp. Not difficult to build.

    (* jcl *)
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A reostat is the easiest solution to this. Also, it makes more sense to the students rather using an electronic circuit. However, if you want to build an electronic circuits it is not very hard.

    What values of reostats do you have available?
     
  9. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Teaching equipment is some of the most overpriced electronics on the market. Look at the dyi market for much better deals. Or at equipment made for hobbiests. When I taught in the old days we had potiomenters that were cylinders about a foot long and 3 inches in diameter, a slider on top, if yours is an old school they may be kicking around. Can be wired as a rehostat and may have the right resistance for you ( or not ) Think the max current was in the 5 to 10 amp range.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Seems that their FTP server is down at the moment.

    However, after looking at just the image of the product, I don't see why you couldn't do with something a WHOLE lot less expensive than a bench power supply - after all, the boards in the experiment are simply traces of various lengths.

    I think that you would be fine using just a 12v-14v power supply, and use some automotive-type bulbs as current limiters. This could be done quite inexpensively. (I'm thinking 12v-14v due to the readily available source of auto light bulbs)

    You wouldn't even need that much voltage; the boards are just traces with terminals on them. All you really need is a low voltage source and a means of limiting the current so that you won't burn things up.
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    If you are comfortable with do it yourself you can get a decent SMPS similar to this, and use it to make what you need.

    Being in a university you should be able to get a student to make a large metal box for a case (I have a similar project on hold for just that) and go from there.

    With modified LM317 circuits you could do it pretty easily. The LM317 goes to 1.5A, but there are several add ons that will boost the current to your range easily. The down side is they don't go to 0V, generally 1.25V is the lower limit.

    If you decide to go this route the circuitry isn't very complicated. Add ons like current regulation will make it more so, but I don't know if you need that or not.
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    I have found an image of the experiment layed out for ad purposes , using PASCO power supply. They have it set to 1VDC.


    Have you the ability to borrow a variable PSU to see what minimums you need to purchase for the experiment?
     
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  13. sspil

    Thread Starter New Member

    Feb 12, 2010
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    Sorry about the link. It was working on Friday. Hopefully it's just a glitch and it will be back up soon.

    I don't have any rheostats available in the lab. At least I don't think I do. I need to do some more poking around. I will need to buy them if I go that route. There is a LOT of equipment there, some of it very old (think 1960's) and it's in disarray. I've been trying to organize as I go and toss out the items that are damaged.

    The automotive light bulb idea sounds fun! I may try that. I think the students would be interested in it. They are a curious bunch.

    I would rather keep this as simple as possible. I feel a bit out of my element with the electronics section. I'm good with the theory, but my background is in mechanical engineering and it's been a long time since I had a circuits class or any practical electronics beyond household basics. Thanks for your support!
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    Keep the broken stuff. Much useful stuff may still remain inside.
     
  15. John Luciani

    Active Member

    Apr 3, 2007
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    I didn't look at the specs for the power supply but a new 25V/5A adjustable supply with
    dual meters (as shown in the picture) for $250 does not seem out of line.
    This supply could be useful for other experiments as well.

    Since you need low voltage and high current I would go with a lower voltage
    supply. A rheostat that can dissipate 10V * 5A may not be cheap (unless you find it surplus).

    Using an LM317 with a small pot would be a few dollars. You could power
    it from an old PC power supply or the Vicor FlatPAC I mentioned previously.

    (* jcl *)
     
  16. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    That's a fairly nice design of an experiment. When I was a student, we used a design that balanced the force between two rigid conductors in space; that experiment required the ability to adjust the current carefully.

    Susan, your experiment doesn't appear to have any special requirements -- a standard bench CV/CC supply would work fine. As others have said, I suspect the required voltage is quite low, because the supply is just putting current through a PC board trace. I've also been in your situation -- there's no "department", there's just a collection of stuff in a storeroom, and you need to make do with what you have.

    As russ_hensel said, if you can find a nice rheostat of the right value, that would work best with your 6 V 5 A supplies and be the most practical solution. Let's estimate some values. If the photo in retched's post shows a 0.6 A current at 1 volt, then the trace resistance is 1/0.6 or about 1.5 ohms. To get to 5 A, you'll thus need a voltage of 7.5 V. The 6 V supplies may not cut it, but then the students don't need to go all the way to 5 A. If you haven't done so, you can measure the resistance of the traces to help with the "design". Don't trust a digital multimeter if the resistances are under an ohm -- run a 1 to 5 A current through the thing and measure the voltage drop.

    The resistance needed is then

    R = \frac{6}{i} - 1.5

    where i is the desired current.

    Let's assume the students only need 5 data points; further assume the desired currents are 0.5, 1, 1.5, 2, and 2.5 A. Then you could buy or find five fixed resistors of values 10.5, 4.5, 2.5, 1.5, and 0.9 ohms, respectively. Make the last one a 1 ohm resistor, so the current will be 2.4 A. The power dissipations would be (i*i*R) 2.6, 4.5, 5.6, 6, 6.2 W, so you could almost get by with 5 W resistors. I believe you can buy 5 W sand resistors at Radio Shack for a dollar or two apiece, so $10 could get you the needed parts (of course, you use the values they have rather than what you've picked). Some creative heat sinking could make them work: have a fan blowing on the ones that are slightly over their power rating and see if that can keep them cool enough. The students only need to apply the current for 30 seconds or so to get a balance (it will probably go faster than that if they have an electronic balance to read the force).

    If you have an adjustable carbon pile rheostat with resistances between 1 and 10 ohms, that would be best -- and it would likely easily take the needed power. If you were local, I have the ideal rheostat I could loan you -- it goes from 0.35 to 17 ohms and it's rated at 1600 W! :)

    You might also call a local business that services car batteries, alternators, etc. They might have a carbon pile rheostat that they'd be willing to loan you.

    One other approach would be to us a power FET to continuously vary the current. There are numerous choices of suitable parts and the folks on this board could help with the design -- I suspect it would be mostly a simple pot and the FET connected to the output of the 6 V power supply, but adequate heat-sinking is a must.
     
  17. sspil

    Thread Starter New Member

    Feb 12, 2010
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    0
    I will look through the tangle of components in the lab tomorrow to see if there is something that will work for me.

    I found a couple of cheap 25W rheostats on Ebay, but the resistance is so high (1000Ohm) I don't think I can get the low end I need out of it. 4-5 points between 1-6 ohms doesn't sound practical for me to reach with a 1kOhm variable. I'll keep searching for something with a more reasonable resistance for my application.
     
  18. sspil

    Thread Starter New Member

    Feb 12, 2010
    11
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  19. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    You need a rheostat which will be at least 30W. The one you found is 25W and after about 4Amps it will overheat. Also, I recommend you to buy a 1.2 Ohm 30W resistor and connect it in series with the rheostat as to limit the current to 5Amps when the rheostat is set to zero ohms.
     
  20. sspil

    Thread Starter New Member

    Feb 12, 2010
    11
    0
    I was thinking that if I kept it under 4 amps for the test, the 25W would be O.K., but I can't count on the students to not crank the resistance down all the way and have it overheat. For student labs I need to make it fool proof.
     
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