Building a Variable Power Supply

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  #1  
Old 12-29-05, 07:04 PM
StinkFish
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Question Building a Variable Power Supply

I need to build myself a SAFE and economical (-$50) variable power supply. I need it to be capable of either 30v @ 5 amps or 42v @ 2.5 amps and either would need to be variable to those max figures to keep it reasonably safe as I'm allergic to 110v. This is to power a hot wire foam cutter, which is why it needs to be variable... higher output for larger cuts and lower for smaller cuts.

I've researched this through modeler forums, but they lacked the electrical expertise to judge the safety of their designs. Here are the suggestions I've heard so far, please comment.

Suggestion #1:
Use a 120v 3 amp Variac conected to a 48v transformer and then output to the hot wire.

Suggestion #2:
Use a 48v transformer connected to a rheostat and then out to the hot wire.


Any ideas?
 

Last edited by StinkFish; 12-30-05 at 12:13 AM.
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  #2  
Old 12-30-05, 01:49 AM
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I'm not sure about your $50 target,

but I think the best way to go is to use a step-down transfomer (120V input, [email protected] 5amps output), a fuse, switch and two resistors. (You can include another switch for the AC power, as an on/off switch.) I'm assuming the foam cutter doesn't care if it gets AC or DC power.

Fuse the 120V transformer input (my math says you should draw about 2 amps, so make the fuse a little larger, say 3-5 amps.) Ground goes to the transformer's case. Add the on/off switch if you'd like.

On the other side of the transformer, you'll need a 1.2 ohm, 30 watt resistor. This will drop the 48 Volts to 42 Volts. (5 amps current, need to drop 6V =1.2 ohms. 6 volts x 5 amps= 30 watts). This resistor will always be in the circuit.

Wire the 2nd resistor in series with the first resistor, which will be 4.8 ohms at 30 watts. (42 volts in, want 30 volts out = 12 volt drop. 12V @ 2.5 amps=4.8 ohms. 2.5 amps x 12v = 30 watts).

The switch will need to be rated at 5 amps constant current. Wire the switch so that each side of the switch is across the 4.8 ohm resistor.

The idea is that the first resistor will give you your 42 volts with the switch closed, removing the 2nd resistor from the circuit (shorting it out). When you open the switch, you add the extra resistance, and the voltage should drop to 30 volts.

You may need to build and experiment a little, you'll probably need to adjust the resistor's values. Wire it up on the bench and see how close you can get. Make sure that you are careful with the 120 Volts side. Make your voltage measurements while under load, as a transformer's voltage will be much higher with no load applied.

CONCERNS:
1) Heat-- your going to be trying to dissapate 30-60 watts of heat constantly. You may need to include a fan to keep this from bursting into flames.

2) Cost -- the transformer and switch you need may not be cheap, unless you already have a source in mind.

3) Finding the resistors -- I have no idea where you can find these value of resistors in the wattage you want. You could use multiple resistors in series and parallel to increase the wattage. You could also try using light bulbs at different wattages (different wattages will have different resistances) to replace the resistors, especially the smaller value resistor. Also, a light bulbs' resistance increases with temperature (current flow through it), which may work to your advantage if your load isn't constant. Lastly, they won't mind running at a lower voltage-they may actually last longer. Wire them the same as you would for the resistors.

4) This design has no adjustment features. You won't be able to boost the voltage a little if you need to cut some more stubborn material. I'm not sure of how to add an adjustment either-- maybe a rheostat in parallel with the first resistor to give you a little adjustment? It would still need to be a high -wattage rheostat though. If the light bulb idea works, then maybe change bulbs for a little adjusment - or add another switch to change between two different bulbs.

This idea comes with no warranties expressed or implied - use at your own risk. In theory, it should work, but a lot will depend on the parts you find and your foam cutter's needs. This is just an idea I had to try and help you. If you have little or no electrical/electronic background, then I highly suggest you don't even attempt this. Ask a friend to look this idea over and see what they think. Maybe ask them to help build a test setup. Use caution and keep a fire extingisher handy!
 
  #3  
Old 12-30-05, 03:28 AM
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Are you using nichrome wire for the knife ?
Are you making the knife ?
How did you come up with the voltage and current for your requirements ?
A 48 volt, 7amp transformer will be hard to find.
 
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Old 12-30-05, 05:52 AM
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just a wild thought, you might be able to "mcgyver" one from a heavy duty soldering gun.
 
  #5  
Old 12-30-05, 06:47 AM
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You need one of these

http://www.variac.com/staco_Variable...former_Map.htm

That is what I use. Perhaps you could find one used or on ebay.
 
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Old 12-30-05, 12:34 PM
StinkFish
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Originally Posted by GWIZ
Are you using nichrome wire for the knife ?
Are you making the knife ?
How did you come up with the voltage and current for your requirements ?
A 48 volt, 7amp transformer will be hard to find.
The wire will be either .020 Inconel or .030 SST Aircraft Saftey Wire.

The cutter is a homemade hotwire bow similar to the Tekoa FeatherCut.
http://www.tekoa.com/gravity.htm

The voltage I came up with is just what I've been told what has worked for others. I was told I need something variable (0-30v @ 5amps or 0-48v @ 2.5 amps) so it can be adjusted for wire type and length needed for the cut.

I just found one of these for less than $50 used (http://www.audio-warehouse.com/web/m...LAB/detail.asp)
but I didn't know if it would cut out when connected to the hotwire due to some sort of safety circuitry. Any thoughts?
 
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Old 12-30-05, 02:11 PM
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The link is for a 12V power supply.
Choose a supply that has the voltage you need with the worst case power consumption. With that in mind, when you set the voltage, the power supply will deliver the amperage based on the resistance in the circuit.
Your worst case power consumption is 30V at 5A. Remember that V=IR, so if you choose 30V, then there needs to be 6 ohms of resistance in the circuit, capable of dissipating the power.
 
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Old 12-30-05, 03:44 PM
StinkFish
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Originally Posted by HandyRon
The link is for a 12V power supply.
Here are the specs right off the web page:
Output Current: 0-5A Adjustable
Output Power: 0-30V Adjustable

Doesn't that mean that it is capable of 30V @ 5A?
 
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Old 12-30-05, 04:13 PM
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Series resistance is the worst way to set voltage. It consumes lots of heat energy, and the voltage will vary in proportion to current.

A variable transformer of a suitable current rating would work, but may be expensive and hard to find. A regulated power supply is fairly simple and straight forward. Google for some schematics.
 
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Old 12-30-05, 04:14 PM
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Power is measured in watts. I don't know if it is 30V or 30W.
On the top of the page it says 12V power supply.
 
  #11  
Old 12-30-05, 06:39 PM
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Now a days, you cant tell what your getting.

The specs don't state continuous output, but state its a 12 volt supply.
It maybe 12 volts at 5 amps continuous output.
Maybe less current at 30 volts.

I had a conversation with a person that sold me some parts to a PYRAMID power supply
Not the same one, it was a fixed supply.
Told me the ratings were not for continues use.

I don't like the wording in the link on the power supply.

" Output Current: 0-5A Adjustable "
That implies to me its a constant current supply, but more then likely that adjustment sets the current/amp trip of the alarm.
You set it at 3 amps and if you draw more it will trip the alarm.
I call that an adjustable trip point.
 
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Old 12-30-05, 06:45 PM
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First of all any amount of voltage can be unsafe and dangerous.
No one can say how safe, because some people are unsafe with anything.
--------------------------------
" Suggestion #1:
Use a 120v 3 amp Variac conected to a 48v transformer and then output to the hot wire.
Suggestion #2:
Use a 48v transformer connected to a rheostat and then out to the hot wire. "
===
Both your Suggestions can work, I think 48 volts is to HI
The rheostat will need to be large, rated to handle over 100 watts. not easy to find.
48 volts sounds hi to me, but I never built a hot knife.

Without knowing the exact current the wire needs I am only Guessing.
I would assume that the resistance of your wire is lower then nichrome wire.

I would go with a power supply like the one in your link, and add a 2 ohm 100 watt resistor in series with the knife
that should-should help prevent the power supply from shutting down if the resistance of your knife it too low.

The resistance of the wire changes as the wire gets hot. The ideal power supply would have a constant current control.
that will keep the heat constant.

You may want to look at this link, the voltage sounds to be more in line with my thinking.
BUT the current maybe from 2 amps to 10 amps.

http://wolfstone.halloweenhost.com/H...FoamTools.html
 
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