Thermocouple welder teardown (DCC HotSpot), plus unconventional magnetics design - Page 1
Forum section: electronics / general

author: D Straney
author_rank: Frequent Contributor
timestamp: April 19, 2019, 01:37:09 am
content: Here's a bit of electronics with some interesting surprises inside: a thermocouple welder. You use one of these for creating your own thermocouples - take a dual-metal wire pair, strip the ends and twist them together, then weld the junction with something like this.These are the terminals (I think the pliers may have been an "aftermarket attachment"):There's no trigger button or footswitch: from what I understand, you just wait until it says it's ready, then grab the thermocouple with the pliers and touch the graphite block to the junction.Right away, you can see...* A board which hosts a piece of magnetics, some control circuitry, and a stud-mount SCR,* A giant capacitor, and* A sealed lead-acid battery.The battery, according to theofficial info, is for portable welding without AC line power.The SCR is a beefy fellow, a 50RAI20, rated for 200V and 50A under some condition I forget (80A RMS though).The capacitor, well, that's just huge:There's a control circuit, with an LM78S40 and a TIP41 power BJT. The LM78S40 is a "universal switching regulator subsystem", basically a set of building blocks like the UC3xxx series: it has an error amp, oscillator, PWM modulator, and so on.There's also a set of 4 diodes near the transformer (facing down on the board, and so hard to photograph clearly):It's possible that the capacitor is charged by a simple transformer-then-rectifier power supply (with the transformer leakage and resistance limiting the current draw, I guess?) and that the LM78S40 is just used for its pulse timing and "capacitor fully charged" detection. It's also possible that the LM78S40 and TIP41 actually form an off-line flyback supply to charge the capacitor. This feels a little less likely, as I don't see any other discrete transistors hanging around to drive the SCR's gate, but it's still an option I guess. The way the transformer is wound also has the windings side-by-side, rather than concentric/coaxial, which creates worse coupling between them / higher leakage inductance, and supports the first option.Here's the connections in the output circuit:Now how does it actually work? The giant capacitor, and the way it's discharged by the SCR into the thermocouple, just screams capacitive discharge welding - this is where a high-current pulse is sent through the metals to be joined; the resistive heating inside the junction (and at the welding-contact-to-metal interface) generates a high enough temperature to weld. In this interpretation, the series inductor on the output would be for pulse shaping - slowing down the discharge to turn a very short, very-high-current pulse into a longer, lower-current pulse.However, the official website claims the HotSpot creates an arc which welds the thermocouple. I can see a couple ways there could be an arc, because of the lack of a separate trigger switch (if it was "pure" capacitive discharge welding you might clamp the terminals onto the thermocouple, then hit a trigger, and there would be no arc):1. When the terminal approaches the thermocouple, at some point just before it actually touches, there'll be enough voltage to break down the air in between even if this only happens at an infinitesimal distance - if enough current can flow, it'll sustain an arc up until the terminal actually touches. You can see this effect when you plug a switched-on appliance into a wall outlet and see a spark just as the contacts touch.2. Even if there's not much of an arc (or the SCR hasn't triggered yet) when the terminal is approaching the thermocouple, just like with a mechanical switch there isn't going to be an instant transition from "not touching" to "permanently touching": some contact bounce will happen. If current flows when the terminal is touching, then as it pulls away slightly during the contact bounce, an arc will form, especially with the help of that series inductor (which will create a high output voltage in an "attempt" to maintain its current flow).So overall, there's almost definitely an arc involved somewhere. The real question is, is the arc itself or the high-current-pulse's resistive heating the dominant factor in actually welding the thermocouple? I've got no idea.Finally, here's the other interesting part of this device. Take a look at the weird piece of magnetics/output inductor in more detail:Notice the two sets of windings: this is an E-core transformer, with two windings on the center leg, but it also has the output-inductor winding (in much heavier-gauge wire) around the perimeter of the core.It's not obvious at first, but this is actually an extremely clever way to put two independent pieces of magnetics on one core. The transformer windings use the E-core in the expected traditional closed-magnetic-circuit way, while the inductor winding uses the E-core as a rod-core. The magnetic fields created by the windings are orthogonal to each other, so the transformer and inductor don't interact at all, and are completely decoupled from each other. A diagram will explain better - the solid arrows show the magnetic field path for the transformer windings, while the dashed arrows show the magnetic field path for the inductor winding:Hopefully this makes sense. A rod-core style, where most of the magnetic path length is air, is an ideal choice for the output inductor; it must handle very high currents, which would be more difficult to do in a closed-magnetic-path inductor design (like the E-core) without saturating the core material. In a mostly-air inductor like this, the actual magnetic field strength seen by the core material is comparatively very low. With the transformer, on the other hand, a high inductance is more important and the current-handling requirements are lower, so have a closed-magnetic-path E-core design makes the most sense.Let me know if you have any questions or insights.

author: David Hess
author_rank: Super Contributor
timestamp: April 19, 2019, 10:27:37 am
content: The inductor is operating as a magnetic amplifier to control the peak current. A low DC current through the winding inside controls saturation of the core so the inductance is variable. Greater inductance lowers the peak current without changing the capacitor's voltage. The orientation of the windings prevents the higher current from coupling into the control winding making it an inductor instead of transformer.

author: D Straney
author_rank: Frequent Contributor
timestamp: April 19, 2019, 02:51:24 pm
content: Now that's an interesting theory, I like it! My only problem with it is I'm not sure where the AC step-down would come from in that case - I'd been assuming that the center-leg windings were a 60 Hz step-down transformer (with the windings intentionally placed side-by-side to increase leakage inductance enough to limit current), and there's no other active devices or magnetics I could see.Thereiswhat looks like a high-voltage ceramic cap and a power resistor in series at one corner, though, so if the mag-amp part is true, that series RC could be a capacitive-dropper supply for the AC line to 12V for the battery/cap; I don't see a step-up for when using only the battery, so I'd assume the cap gets charged to 12V (which seems like a waste for a 75V cap). In that case, the 12V probably charges the cap through a resistor: 17,000uF at 12V is only 1.2J, which over 10 seconds charging time is an average of < 1/8W dissipation in a charging resistor.I'll have to pop this thing open again sometime next week and see if I can trace out the circuit to confirm the mag-amp part. There's so little circuitry in this for the features it has that I'm expecting some hackishly simple shortcuts. This whole welder gives off a strong "80's hobbyist in a garage" vibe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2019, 03:25:15 pm
content: Hah, cool!I don't think it's a saturable reactor. Back-of-envelope calculations suggest a saturation current in the 25kA range, easily into the range of fragmentation for that SCR. A few kA would be realistic, and also more consistent with the capacitor (which probably has 10s mohm ESR?).It could be saturable the other way (as David noted), but the control range is small. The effective mu_r of that short core (as you've correctly drawn) is maybe 2, so it would only have a small effect on peak dI/dt.It also seems strange to me that dI/dt should need to be varied at all; the capacitor's ESR should dominate in terms of energy delivered.It may just be a cute shortcut, and not actually quirky at all. What better place to stow your enormous air-cored inductor, than over another inductor already in the circuit? And hey, we can reduce turns by 30%, win-win?Tim

author: D Straney
author_rank: Frequent Contributor
timestamp: May 03, 2019, 07:05:27 pm
content: So for what it's worth, I traced a bit of the circuit and it looks like Tim's (and my) initial guesses were correct: the transformer windings on the center leg of the E-core step down the AC line voltage, which then seems to get diode-ORed with the output of the battery. There must be a battery charging circuit in there somewhere, but I didn't finish following the traces (an annoying amount was hidden under the giant cap or other components so didn't seem fully worth it). The IC seems to control a switching regulator (probably boost converter?) based on the TIP41 and a small axial inductor not visible in these photos; that boost converter is probably what charges the capacitor.

author: TopLoser
author_rank: Unknown
timestamp: May 03, 2019, 07:59:22 pm
content: I’ve got another thermocouple welder from a different manufacturer, doesn’t look like a rebadged model. I’ll take the top off and see what’s inside tomorrow.

author: Yansi
author_rank: Super Contributor
timestamp: May 03, 2019, 08:19:05 pm
content: di/dt needs to be controlled due to slow SCR turn-on, where the turn-on "ionisation" of the SCR gate propagates slowly across the area of the die.If not limited, too high current will burn through the die, because only passing through small active area.So the coil is just simple coil - limiting the rise time of the current.

author: NorthGuy
author_rank: Super Contributor
timestamp: May 03, 2019, 08:29:51 pm
content: I needed to weld some termocouples about a year ago. So, I used a very old primitive 30V AC/DC converter - just a 1:5 transformer followed by full rectifier followed by a capacitor. I used an old motor brush as a contact. It worked great, so 30V is quite enough for the purpose. I even think 12V car battery might have worked. Some of the PCB traces in the rectifier overheated, fortunately after I've been done with termocouples. I think adding a 100 Ohm resistor between the rectifier and the capacitor would limit the current enough to prevent any damage to the rectifier. I've got an impression than the capacitor alone (I had 100 mF) would produce enough current to weld the termocouple.