how to heat treat copper wire with dc current?

Thread Starter

dreamer44

Joined Nov 19, 2015
36
Hi all,
I wonder if someone here can help me with a small project of mine. I need to heat treat a piece of very thin copper wire with a DC current, namely, by using the wire resistance to make it hot. I thought it could be done with a 12V DC dimmer, so I bought one, connected the thin wire (45 AWG), and slowly turned the knob on. The wire got a few degrees warmer (really negligible temp) and towards the middle of the turn, something on the dimmer started to smoke (it is rated 12V 8A). Obviously, I was doing something very stupid... Can someone tell me what would be the right way of doing this? The idea is to take a 3 feet piece of 45AWG wire up to about 500 Fº. The wire MUST be copper, not nichrome, etc.
Thanks a lot for any help with this.
Marcel
 

MaxHeadRoom

Joined Jul 18, 2013
19,006
Use a transformer and use the dimmer on the primary, the secondary can be anything from 6v-12v, you can also experiment with your own few turn secondary.
Max.
 

GopherT

Joined Nov 23, 2012
7,983
Hi all,
I wonder if someone here can help me with a small project of mine. I need to heat treat a piece of very thin copper wire with a DC current, namely, by using the wire resistance to make it hot. I thought it could be done with a 12V DC dimmer, so I bought one, connected the thin wire (45 AWG), and slowly turned the knob on. The wire got a few degrees warmer (really negligible temp) and towards the middle of the turn, something on the dimmer started to smoke (it is rated 12V 8A). Obviously, I was doing something very stupid... Can someone tell me what would be the right way of doing this? The idea is to take a 3 feet piece of 45AWG wire up to about 500 Fº. The wire MUST be copper, not nichrome, etc.
Thanks a lot for any help with this.
Marcel
It was my understanding that pure copper can only be work hardened. What makes you think copper can be heat treated? What alloy do you have? What property are you trying to improve with heat treatment?


http://www.copper.org/publications/pub_list/pdf/a1360.pdf
 

Thread Starter

dreamer44

Joined Nov 19, 2015
36
Use a transformer and use the dimmer on the primary, the secondary can be anything from 6v-12v, you can also experiment with your own few turn secondary.
Max.
you mean a regular transformer with 110V primary and 12V secondary?
 

Thread Starter

dreamer44

Joined Nov 19, 2015
36
I thought the dimmer was the way to go, thinking of the piece of copper wire would emulate the bulb filament. I know, there are different materials and variables involved, so it's not that simple... Result, something in the dimmer starts to smoke before the copper gets even warm (again, we're talking about a very thin wire...) But perhaps there IS a simple way of doing it. Someone here got to know how...
 

Dodgydave

Joined Jun 22, 2012
8,373
If you can use AC, try using microwave transformer converted to a mini welder, that will give you low voltage at high current, loads of info on youtube...
 

wayneh

Joined Sep 9, 2010
16,102
I don't think this will turn out well. The heat loss from a 45 gauge copper wire at 500°F will be a very steep function of all the various variables - conduction, radiation, convection, and current flow. Oh, and let's not forget oxidation of the surface and the effect that might have on all the above. The slightest deviation will not be ... slight... in outcome. The thinnest nick in the wire will get much hotter than the rest of the wire and will act as a fuse.

How would you even know the wire was at 500°F? You might be able to use the conductivity of the wire itself as a measure.

I'll ask it again - why not cook it in a temperature-controlled oven?
 

Thread Starter

dreamer44

Joined Nov 19, 2015
36
What you need is a power supply with an adjustable current limit.

Are you trying to anneal the copper?
Yes I am.

I don't think this will turn out well. The heat loss from a 45 gauge copper wire at 500°F will be a very steep function of all the various variables - conduction, radiation, convection, and current flow. Oh, and let's not forget oxidation of the surface and the effect that might have on all the above. The slightest deviation will not be ... slight... in outcome. The thinnest nick in the wire will get much hotter than the rest of the wire and will act as a fuse.

How would you even know the wire was at 500°F? You might be able to use the conductivity of the wire itself as a measure.

I'll ask it again - why not cook it in a temperature-controlled oven?
The experiment is to heat treat with a DC current, not in an oven.

If you can use AC, try using microwave transformer converted to a mini welder, that will give you low voltage at high current, loads of info on youtube...
I need a device that would increase the heat on a 45AWG piece of wire up to around 500F. Not a welder.

Yes.
Max.
as I said, I need to use DC, not AC on the wire.

you didn't actually use 3ft of wire in your test did you?
what kind of question is that, man?

What you need is a power supply with an adjustable current limit.

Are you trying to anneal the copper?
can you be more specific about the PS?
 
Last edited by a moderator:

DickCappels

Joined Aug 21, 2008
5,876
In response to mcgyvr's question "you didn't actually use 3ft of wire in your test did you?" dreamer44 wrote:
what kind of question is that, man?

My comment on the above: You are more likely to get cooperation if you reply to a question by somebody trying to help with an answer rather than a throw-away comment.

Here is one way that might work:

Take a length of wire.

Suspend the wire you want to treat so that it does not touch potential heatsinks.

Connect an adjustable constant current source to the ends of the wire. Include a means of monitoring the current.

Connect a digital multimeter across the wire, a cm or two toward the middle (away from the ends).

Measure and note the resistance of the section of the wire.

Using the resistance vs temperature graph find the expected resistance for the length of wire between the voltmeter connections.

Switch the digital multimeter to measure volts.

Monitor the current and voltage and calculate the resistance as the current is slowly increased until the resistance that corresponds to the desired temperature (500° F) is reached.


This chart was found at:
http://www.spiraxsarco.com/Resources/Pages/Steam-Engineering-Tutorials/control-hardware-el-pn-actuation/controllers-and-sensors.aspx
 

Thread Starter

dreamer44

Joined Nov 19, 2015
36
Thanks a lot for this. If the voltmeter is also multimeter with an amp meter I suppose the current monitoring can be done through the same device, right?
This is pretty much what I have tried with the dimmer, which I thought was precisely an adjustable current source.
This is the dimmer I've used: http://www.ebay.com/itm/331601522755?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

It didn't work, probably because it's not what you have suggested above. Can you point me in the right direction for the adjustable constant currant source?
Would something like this work: http://www.instructables.com/id/Adjustable-constant-current-source-4mA-to-3A/
 

DickCappels

Joined Aug 21, 2008
5,876
You can get by with one multimeter if you put a current sense resistor in series with the current source, then you can leave the meter in the voltage mode and move the probes between the sense resistor and across the wire. I think I would use a DPDT switch if I had to do this more than once.

No way to tell how that dimmer works, but it is a good bet that is varies the voltage rather than the current -though there are few clues in the photograph, it does not appear to merely be a rheostat or pot.

The current source shown in the instructables article is the right sort but since he is using a 10 ohm pot and the LM350's reference voltage is 1.2 volts the minimum current would be 120 milliamps. You don't have a feel for how much current you are going to need. Fortunately #45 copper wire is inexpensive. The wiring diagram in the instructables article is terrible. Just so there is no misunderstanding, take the output from the ADJ terminal.
 

Dodgydave

Joined Jun 22, 2012
8,373

Thread Starter

dreamer44

Joined Nov 19, 2015
36
You are better using an LM338 regulator in constant current mode, page 10 datasheet,

https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://www.uni-kl.de/elektronik-lager/416059&ved=0ahUKEwj5s4-k8J7JAhVMExoKHXB7CgkQFghSMA8&usg=AFQjCNFy8x6dLp9vWmCYI1HakR9ZkIWVlw

this will give you upto 5amps, so your going to need a dc power supply that can maintain this current to start with, like a car battery, do you know how much current is needed to heat the 3ft of cable,?
no idea...
 

Dodgydave

Joined Jun 22, 2012
8,373
Ok, going from the chart by Dickcappels in post#13, the resistance at 500F(260C) would be Approx 160 ohms, so a 1 amp current will need 160V minimum.
 

mcgyvr

Joined Oct 15, 2009
5,394
what kind of question is that, man?
as shown above copper wire has a specific resistance per foot (and that changes based on temperature too)..
Hence I was inquiring about how long your wire was to show why the dimmer overheated..
There is a big difference in trying it with 2 inches of wire vs 3ft of wire
 
Top