Help With Variable Power Supply

[sspil]

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

[mik3]

Give us more information about the experiment.

[SgtWookie]

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.

[ifixit]

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

[John Luciani]

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 *)

[sspil]

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/sh...7&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.

[John Luciani]

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 *)

[mik3]

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?

[russ_hensel]

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.

[SgtWookie]

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.