Cannot get a working hot wire, anyone in North San Diego have one for sale?

General Discussion
21

Thanks, you’ve answered my question. I was just worried that a 20A variac would be too “hot” for cutting EPS. Also was worried about the output being dangerous relative to a lower amperage unit. Tingling foreheads don’t exactly inspire confidence (I realize that wasn’t a guy using a variac).

 

el-cocko…??? I guess I’d have to go digging around archives and brush up on my math and buy fire extinguishers to use your method, but if it works for you, great. I’m just lazy.

Thanks.

22

 Singrays variac of 10 amps is a good one as well and will do the job perfectly.

 

23

 

A rating of 12 volts at 3 amps (36 watts) sounds a bit weak to me. As best I can recall most of the hot wire cutting that I've done (with 22 or 20 gauge nichrome wire) works best  at a current of about 7 amps (Note: and is virtually independent of wire length). The voltage required to drive that current depends on the hot wire length as well as the alloy and gauge (but for most of the cutting I've done runs around 25 volts for a couple of feet of heated wire).   

 

 

24

[quote="$1"]

Thanks, you've answered my question. I was just worried that a 20A variac would be too "hot" for cutting EPS. Also was worried about the output being dangerous relative to a lower amperage unit. Tingling foreheads don't exactly inspire confidence (I realize that wasn't a guy using a variac).

[/quote]

I'm with ding on the use of the variac.  If you're concerned about the output, hook the variac output to a high amp DC converter.  Personally, I feel safer at 24v.

25

 

For a constant voltage (e.g. 12V) from your power supply, the wire will get hotter if it’s resistance is decreased, not increased (i.e. via the use of a larger diameter/smaller guage wire) since the power dissipated in the wire (i.e. as heat) is given by the equation:

W = V*V/R

…where W = watts of heat being dissipated, V = voltage, and R = resistance of the wire (ohms)

Of course one also has to be concerned with the limitations of the power supply (e.g. see post by MikeDaniel earlier in this thread… As the size (diameter/guage) of the wire is diminished, ultimately the current will increase to the point where the power supply can no longer provide it. Any further reduction in resistance will reduce the heat of the wire (and may burn out your power supply, depending on it’s design).

 

26

Is it overkill to use a 20A variac

20A refers to it's capacity not it's output. Variacs aren't current limiters, they are voltage regulators.

The amperage in a circut is determined by the voltage and the overall resistance in the circut. I=V/R (Ohm's Law)

So dropping a 100V through a 50 Ohm circut will net a 2A current.  Nichrome wire has about 0.8 Ohms/ft. If you round up for your leads resistance, that would be around 2-3 ohms.

So if you drop a 100 volts across a 2ft section you'll have anywhere from 30 to 50 Amps. But at 6V will get you 2 to 3 Amps.

Figure Watts as P=V x I or a wire dropping 6V at 3A is 18W.

End of basic electricity class. :-)

Harbor Freight has a fused 15A variable router speed controller for $13, basically a ghetto variac.

27

 

VARIAC OR TAKE UP ANOTHER HOBBY!

 

28

Okay, maybe the last question: What about the linear DC variable power supply on ebay? 3 to 12V at 2A. That sounds a little safer.

Thanks for all the info.

 

Peter

29

You can probably pick up a nice used variac in SD here for low $.  Good tool to have around the garage.
http://www.industrialliquidators.com

30

Hello OregonPeter.....

Go back and read the thread one more time...I took some classes years ago and struggled with this stuff....Marco and Surfding are spot on! Look at the photos of the Variacs. The 20 amp Variac looks good to me.

I'm working in the Low Tech Lab. I'm not throwing out insults. Lets see if we can understand all this stuff in a Low Tech fashion...

You have Juice comming out of the wall...110 volts...The juice goes into the Variac....The big black nob on the Variac lets you control the juice.....I like 17 volts....depends on lots of factors like wire size ect.........Better Variac = more control. Crappy wire ...waste of time... get the Nichrome or the Inconel....I think my Nichrome wire is 24 gauge...it's thin...

Different example.....

The electric current flowing through the  wire in your house is like a river.....The Variac is the dam....The dam operator controls the Flow of the river.....Do you have a Concrete steel reinforced dam or a earthen dam????

Variac or Harbor Frieght?

My 30 year old Concrete steel reinforced dam beats your earthen dam any day....

 

31

Well Said!