Author Topic: Metcal not just heat conduction?  (Read 8025 times)

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Offline Poe

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Metcal not just heat conduction?
« on: January 31, 2013, 04:44:53 pm »
So I recently got two Metcal PS2Es with a dozen good tips from an industrial auction ($140).  Both work wonderfully. 

Performance is so impressive that I wonder if there's more than heat conduction going on here. 

First of all, for those that are unfamiliar with how the Metcal works, here's a video:



So instead of a typical heating element and control circuit, this one utilizes high power RF, coil and ferrite. 

It takes less than ten seconds to warm up thanks to the lower tip mass and large power output.  This allows me to keep the power off most of the time.  Combined with the metal tip cleaner I use, tips last forever. 

I mentioned performance is impressive because with a relatively thin tip I can effectively solder surprisingly large copper pours.  Much larger and faster than I could with any traditional Hakko or Pace setup we have.  While only touching a DIP lead with the very end of a spike tip, I'm able to melt solder on the lead within a couple seconds.

Now, is this a fluke?  Could my units be defective and inducing currents in the part I'm soldering?  Do Metcal units heat parts up TOO quickly?  If this is normal, it's very impressive.  Although it looks like there's more than just heat conduction going on.  Almost like at least some of the heating is happening in the lead/pad I touch.  Could this be happening? 

*Disclaimer... I know very little about RF and only have a very basic understanding of magnetic fields and induced currents. 



 

Offline cyr

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Re: Metcal not just heat conduction?
« Reply #1 on: January 31, 2013, 05:08:12 pm »
Welcome to the world of proper soldering tools, sounds pretty normal to me...

I use JBC at home, and we have metcals at work. They work equally well I would say, and JBC doesn't use any "exotic" RF stuff.
The key is having the heater element integrated into the tip...
 

Offline mikeselectricstuff

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Re: Metcal not just heat conduction?
« Reply #2 on: January 31, 2013, 06:08:57 pm »
The two key things are :
a) The heat is produced in a piece of metal which is welded to the tip, so there is no electrical insulation, with its inherent thermal resistance, between heat source and tip.
b) The temperature control is inherent in the material, so they can pour a large amount of energy in with no risk of overshoot.
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Offline Poe

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Re: Metcal not just heat conduction?
« Reply #3 on: January 31, 2013, 06:59:58 pm »
The two key things are :
a) The heat is produced in a piece of metal which is welded to the tip, so there is no electrical insulation, with its inherent thermal resistance, between heat source and tip.
b) The temperature control is inherent in the material, so they can pour a large amount of energy in with no risk of overshoot.

Sorry, are you saying there is relatively little thermal resistance and no electrical insulation between heat source and tip?  So the currents induced in the skin of the metal core might not be confined to only the core and cause current and therefore heat to be generated in the 'skin' of the tip?  I can only assume they have sufficient shielding so the field is confined to the area behind the tip?  I'm just imagining those flux lines extending out  beyond the tip to the lead/pad and can imagine them acting like the metal core.  If there was some kind of shield to focus the field around the copper core then it's just a matter of heat conduction right? 

So long as this performance is normal I'm happy.  I guess my only concern was that the units themselves are damaged (they're second hand) and causing damage to the devices I'm soldering... by overheating or somehow inducing currents and heat... or some other thing my anxious mind could cook up.
 

Offline mamalala

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Re: Metcal not just heat conduction?
« Reply #4 on: January 31, 2013, 08:49:48 pm »
Sorry, are you saying there is relatively little thermal resistance and no electrical insulation between heat source and tip?

That is somewhat the case, except for electrical insulation. But it is a bit more sophisticated: the tip basically _is_ the heat source.

I don't know the PS series, they use a much lower frequency (455 kHz, iirc), but only the MX series at 13.56MHz. But the principle is basically the same, i think.

What you see as soldering tip extends a tiny bit inwards as a copper slug. Around this slug is a cladding of a special alloy. Into that alloy you can induce a magnetic field, but only if it is below the Curie-Point temperature. Once it reaches that temperature, no more power can be induced into it. Around that is a coil (insulated wire). The RF is applied to that coil.

That construction basically is a transformer. You feed RF into the primary, while the secondary is just a single-turn short. Feeding in energy heats up that secondary, of course. Since that cladding is in direct contact with the copper slug/tip, the heat is transferred (basically) instantly into the tip. Curie point reach -> no more heat produced. Dropping below Curie point -> induction starts again.

So the temperature control here is based on the physical property of that special alloy. Since all that is tightly thermally coupled, and very small in physical dimensions, heat recovery is almost instant. The real trick in these units is to control the power that the RF final is generating. RF has the nasty habit to be reflected back into the output amplifier if there is a mismatch between source and load impedance. Naturally, as long as the tip is cool (and thus RF energy is consumed), little is reflected back. But once the temp is reached, there will be a huge mismatch. In normal circumstances it would destroy the RF output stage. Basically the same as transmitting with a HAM/CB radio but no antenna/dummy connected. The Metcal (and Thermaltake) units detect the amount of reflected RF energy and regulate the supply voltage of the RF amp accordingly.

So, what they do is to include the amount of reflected RF into the regulators feedback-loop.

[shameless plug]
In case you are wondering, i have just recently designed my very own RF supply for the 13.56 MHz system from Metcal: https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply. But please note that this is _not_ compatible with the PS series.
[/shameless plug]

Greetings,

Chris
 

Offline Poe

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Re: Metcal not just heat conduction?
« Reply #5 on: January 31, 2013, 10:39:42 pm »
Just to put it in my own words to understand.. correct me if I'm wrong...

When the alloy heats up, the "transformer" coupling is reduced, so more of the RF energy is reflected, causing the controller to reduce power output.

The induced current and therefore heat is mostly limited to the outer portion of the copper slug due to high frequency, right?  The alloy isn't where the current flows (heat originates)?  If the heat originates in the skin of the copper slug, wouldn't there also be heat generated in the skin of the tip, or is the field very weak here due to some shielding? 

 

Offline notsob

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Re: Metcal not just heat conduction?
« Reply #6 on: January 31, 2013, 11:09:03 pm »
Sorry, are you saying there is relatively little thermal resistance and no electrical insulation between heat source and tip?

That is somewhat the case, except for electrical insulation. But it is a bit more sophisticated: the tip basically _is_ the heat source.

I don't know the PS series, they use a much lower frequency (455 kHz, iirc), but only the MX series at 13.56MHz. But the principle is basically the same, i think.

What you see as soldering tip extends a tiny bit inwards as a copper slug. Around this slug is a cladding of a special alloy. Into that alloy you can induce a magnetic field, but only if it is below the Curie-Point temperature. Once it reaches that temperature, no more power can be induced into it. Around that is a coil (insulated wire). The RF is applied to that coil.

That construction basically is a transformer. You feed RF into the primary, while the secondary is just a single-turn short. Feeding in energy heats up that secondary, of course. Since that cladding is in direct contact with the copper slug/tip, the heat is transferred (basically) instantly into the tip. Curie point reach -> no more heat produced. Dropping below Curie point -> induction starts again.

So the temperature control here is based on the physical property of that special alloy. Since all that is tightly thermally coupled, and very small in physical dimensions, heat recovery is almost instant. The real trick in these units is to control the power that the RF final is generating. RF has the nasty habit to be reflected back into the output amplifier if there is a mismatch between source and load impedance. Naturally, as long as the tip is cool (and thus RF energy is consumed), little is reflected back. But once the temp is reached, there will be a huge mismatch. In normal circumstances it would destroy the RF output stage. Basically the same as transmitting with a HAM/CB radio but no antenna/dummy connected. The Metcal (and Thermaltake) units detect the amount of reflected RF energy and regulate the supply voltage of the RF amp accordingly.

So, what they do is to include the amount of reflected RF into the regulators feedback-loop.

[shameless plug]
In case you are wondering, i have just recently designed my very own RF supply for the 13.56 MHz system from Metcal: https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply. But please note that this is _not_ compatible with the PS series.
[/shameless plug]

Greetings,

Chris

I have a PS2E in pieces at the moment, (looking for a a 240V tranny for it), it has a 13.56MHz Crystal,so obviously the PS2E does not run at 455kHz.
 

Offline mamalala

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Re: Metcal not just heat conduction?
« Reply #7 on: January 31, 2013, 11:36:11 pm »
The induced current and therefore heat is mostly limited to the outer portion of the copper slug due to high frequency, right?  The alloy isn't where the current flows (heat originates)?

Wrong. The alloy is the place where the induced current flows. After all, that special alloy is exactly the part that acts as a shorted winding. Only after it reaches the Curie point the induction will stop. Of course there will still be "losses" in the copper slug itself, due to eddy currents. But they are not really losses, since that energy is converted to heat, which is what we want here. However, those "losses" are minimal compared to what happens in the alloy below the Curie point.

Greetings,

Chris
 

Offline mamalala

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Re: Metcal not just heat conduction?
« Reply #8 on: January 31, 2013, 11:38:15 pm »
I have a PS2E in pieces at the moment, (looking for a a 240V tranny for it), it has a 13.56MHz Crystal,so obviously the PS2E does not run at 455kHz.

You are right. When reading "PS2E" i was thinking of the new PS series from Metcal, like the PS-900. But here it was STSS-PS2E that is discussed, and that is indeed a 13.56 MHz system.

Sorry for making that mistake ;)

Greetings,

Chris
 

HLA-27b

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Re: Metcal not just heat conduction?
« Reply #9 on: February 09, 2013, 01:30:01 pm »
Simple question: Why hasn't anyone thought of using diesel glow plugs as heating elements? 



Tough as nails, cheap as chips and at soldering temperatures they would last practically forever.
 

Offline robrenz

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Re: Metcal not just heat conduction?
« Reply #10 on: February 09, 2013, 02:14:52 pm »
Not trying to be negative, just curious.  ;D What is the drive current and voltage?  Aren't they only energized momentarily for startup then combustion temps keep them hot?  Is there any temperature feedback?

HLA-27b

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Re: Metcal not just heat conduction?
« Reply #11 on: February 09, 2013, 02:40:43 pm »
Not trying to be negative, just curious.  ;D What is the drive current and voltage?  Aren't they only energized momentarily for startup then combustion temps keep them hot?  Is there any temperature feedback?

Might have hijacked the thread slightly  :-// sorry about that.

Acorrding to these

http://www.boschautoparts.com/BAP_Technical_Resources%2FDiesel%20Parts%2F201232_GlowPlugTechGd08.pdf
http://www.w124performance.com/docs/general/glow_plug_info1.pdf

Drive voltage is the rail voltage of the vehicle, 12V or 24V.
Some cars energize momentarily, others provide "post-glow" at reduced power which lasts 3 minutes or so.
No temperature feedback, just a regulating coil to limit the current.
Working temperature around 1000C
Guaranteed for 20000km in practice they last around 50000 (2 years or so)
No info on current consumption, got to measure I guess.
 


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