Fast Resistance/Joule Heating

MaxHeadRoom

Joined Jul 18, 2013
22,374
The concept is that you want to keep volt drop in the conductors as low as possible, hence a large cross section conductor, when this culminates at a small dia point, i.e. higher resistance, this results in the heat concentrated at this point.
Max.
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
If you take a look at the picture I've included, you'll see the geometry of the tip I'm trying to heat rapidly. We want to heat just the "domed" area so that we can melt plastic (and then cool it rapidly to solidify it).

There's a debate whether our problem lies in the electrical part of the system or the geometry of the tip to allow the heat (resistance) to focus where we want it. You can see the slot in the tip that helps to focus the energy (only top side is shown, but it is on the opposing side as well).

Also, this is a stamping, so the thickness of the material is pretty much consistent throughout.

Any thoughts on how to make this happen or somewhere to start?

Max: Can you elaborate on the voltage drop you mention and how it would relate to this tip?

Thanks!

 

ronv

Joined Nov 12, 2008
3,770
Are you sure the slot is not to let the cooling air out after the weld is complete?
In the picture below is the single turn secondary (copper)
Many turns for the primary (yellow covering)
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Hi ronv,

That's actually a great question! We've all thought that the slots are for directing the current, but have not considered the airflow requirements. Some sort of air escapement would be necessary without a doubt.

Now I'm wondering if the tip material is the crucial factor. We want to centralize the heat to the "dome" of the tip where it will contact the plastic. Any ideas on how to develop/test this?

Thanks again!
 

ronv

Joined Nov 12, 2008
3,770
Hi ronv,

That's actually a great question! We've all thought that the slots are for directing the current, but have not considered the airflow requirements. Some sort of air escapement would be necessary without a doubt.

Now I'm wondering if the tip material is the crucial factor. We want to centralize the heat to the "dome" of the tip where it will contact the plastic. Any ideas on how to develop/test this?

Thanks again!
Can you cut thru one and see what it looks like?
 

MaxHeadRoom

Joined Jul 18, 2013
22,374
It would be my guess also that the slot is for air escape.
The examples you show seem to point to the simple resistance welding method, do you have the unit that feeds the conductors and electrode end?
Max.
 

ronv

Joined Nov 12, 2008
3,770
Hi ronv,

That's actually a great question! We've all thought that the slots are for directing the current, but have not considered the airflow requirements. Some sort of air escapement would be necessary without a doubt.

Now I'm wondering if the tip material is the crucial factor. We want to centralize the heat to the "dome" of the tip where it will contact the plastic. Any ideas on how to develop/test this?

Thanks again!
Here is one thought:
Maybe a copper stamping inside similar to this.
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Hi Max,

We just purchased a unit that is used with the special tips. I was able to dissect one about two years ago, but my understanding of electronics was even worse than it is now. I can get the tips for a reasonable cost.

ronv: I will send an illustration of the tips cross section. The one in my most resent picture is stamped and is about 0.010" thick throughout.
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
The answers to those questions depend on how you are doing the heating. If you are doing it inductively, then DC isn't going to work worth a darn. But if you are doing it conductively, then it won't make much of a difference and the primary driver will be the effective power delivered to the element. But if the heating path involves reactive elements (such as a heating coil's inductance) then using an circuit that has a resonance and operating it at an AC waveform tuned to that resonance can have a very significant impact.

There are also techniques you can use to get very high power for short durations, such as charging a coil with current and then dumping it into the heating element or doing something similar with a capacitor. Which would work best would largely depend on the resistance of the element.
Hi WBahn,

Can you explain a little more about how the tuned AC waveform would increase the speed at which a tip is heated? I know for sure that the system is sending AC to the tips, I just need to find a way to deliver it properly. We just have each leg hooked up directly to the secondary of the transformer (switched on and off by a relay). Does the resonance help to sync the voltage and current for a better delivery of power? Is there a way to actively tune this resonance based on the tip?

Any help would be great! I think you may be on to something.
 

ronv

Joined Nov 12, 2008
3,770
Hi Max,

We just purchased a unit that is used with the special tips. I was able to dissect one about two years ago, but my understanding of electronics was even worse than it is now. I can get the tips for a reasonable cost.

ronv: I will send an illustration of the tips cross section. The one in my most resent picture is stamped and is about 0.010" thick throughout.
That might help. So the wires are crimped to this part?
What does your current transformer look like?
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
The wires are brazed onto the tip. They are typically solid copper leads.

The current transformer we are using is a toroidal style with seven to ten turns on the secondary. The wire is very high gauge on the secondary (about 4ga).

What's interesting is that the model I dissected has your everyday laminated core transformer. I'm not sure of the turns, but there are four taps coming off of it that allow for different output power (for different tip sizes). Another interesting thing I just realized is that the wire gauge on the secondary taps is substantially smaller. Somewhere around 12-16ga.

I ran our designed unit and I was getting about 1.5v from the transformer, but when I measured the leads that go to the tip, it dropped drastically down to 0.4v. My thoughts are that I need to have enough voltage to carry the current around the tip, but I'm not sure how to calculate this. Any thoughts?
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Here are some pictures of the unit we've been looking at. I'm curious to know if anyone can tell me the type of transformer used. I've assumed it's a laminated, but I'm wondering if it could be a resonant transformer. You can see the large gauge on the primary and the multiple taps coming from the secondary.

DSC_0133.JPG DSC_0135.JPG
 

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Thread Starter

amspurge

Joined Jan 15, 2016
48
One quick correction: After taking a second look at the photos, it appears there are four taps on the primary and a single tap on the secondary of the transformer.
 

ronv

Joined Nov 12, 2008
3,770
Here are some pictures of the unit we've been looking at. I'm curious to know if anyone can tell me the type of transformer used. I've assumed it's a laminated, but I'm wondering if it could be a resonant transformer. You can see the large gauge on the primary and the multiple taps coming from the secondary.

View attachment 99002 View attachment 99003
So can you measure the voltage going out to the tip? Then measure the resistance of the tip?
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Yes, we have another unit coming in this week and I will measure it as soon as it arrives. I will also measure the tips we have in house.
 

hp1729

Joined Nov 23, 2015
2,304
Hello everyone,

I'm trying to find different methods of resistance heating (metal) and how AC vs DC changes the outcome. My goal is to heat metal as fast as possible and I'd like to know what techniques I can use to achieve this (AC waveform, frequency, voltage, material, etc.).

I'm also curious to know if there is a special type of transformer that could help me with this rapid heating of metal.

Please let me know if you have any thoughts. Any help would be great!
Do any of the arc welders already on the market fit your needs?
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Do any of the arc welders already on the market fit your needs?
Well, we need to heat the metal tip so that we can make contact and melt plastic. I'm not sure if an arc welder would allow us to do such a thing.

The idea is to heat the tip quickly, melt the plastic, and then cool the tip rapidly to allow the plastic to cool and form.
 

ronv

Joined Nov 12, 2008
3,770
One quick correction: After taking a second look at the photos, it appears there are four taps on the primary and a single tap on the secondary of the transformer.
I have an old Weller soldering gun. If you want to mail me a tip I'll try it.
 

Thread Starter

amspurge

Joined Jan 15, 2016
48
Hi Guys,

Looked around the pictures some more and noticed this relay which appears to be controlling power on the primary side of the transformer. Upon further research, this has a "zero cross" function. Anyone know how this could play into the system? Omron part G3NA-210B

Is it looking to only send power when it is at the zero cross of the wave?

I've attached a picture of this relay.
 

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MaxHeadRoom

Joined Jul 18, 2013
22,374
That is a solid state relay (SSR) it is in place of a switch to control the transformer, zero switching means it only turns on at the zero point of the AC instead of any random point.
Are the four primary taps fixed on one are are they switched in for different power level out?
Max.
 
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