Induction heater (Mini-Ductor II CE 230V, MD800) - Page 1
Forum section: electronics / repair

author: Signal Simon
author_rank: Contributor
timestamp: July 13, 2015, 11:06:42 pm
content: Got one of these to repair. It's from a local garage, they use these instead of oxy-acetylene torches to apply heat to free off rusted nuts.It's a return favour job - I fix this, they fix my aircon.I have found that the TVS diodes D11A, B and C have gone short, meaning there's been an over-voltage on the output (over 1100V).The reason for the over-voltage seems to be that one side of the work coil has had a poor connection, a few of the coils in the case have at least one leg discoloured.The main IGBTs seem OK (the TVS diodes did their job!), but the problem I've got is the fuse rating.It's a 10.1mm long by 3mm square ceramic body type - but the arc has been so violent that the side(s) with markings on have blown apart.Rated current of the unit is marked as 6A, so I'd expect the fuse rating to be in the range of 6.3 to 10A, but I don't know whether the fuse should be F or T type.Has anyone repaired one of these before? Even the US 120V version?Simon.

author: tautech
author_rank: Super Contributor
timestamp: July 13, 2015, 11:30:56 pm
content: Sounds like an interesting repair.You don't say if the fuse is on the incoming mains?Photos for the record would be nice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2015, 03:40:08 am
content: Probably not too tricky, but have you got schematics by any chance?

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: July 14, 2015, 05:55:01 am
content: The coils are available in a variety of sizes/styles including 'pads'.http://www.ebay.co.uk/itm/MD99-601-7-8-I-D-Preformed-Heating-Coil-for-Mini-Ductor-IDIMD99-601-/231361699529?hash=item35de3b5ec9I have not seen this tool before, but having just been cursing to remove a seized/frozen screw from an aluminium Hydro Drive unit, and using a lot of propane to achieve it, I would love to get a project on the go.Once again : schematics anyone ??BT

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2015, 08:40:35 am
content: Quote from: BurningTantalum on July 14, 2015, 05:55:01 amYears ago when I twisted spanners for a while we found TIG was the best for broken studs in alloy.A few thermal cycles to red hot normally loosened the most stubborn mongrels.Arc weld a nut to them sometimes will work too.

author: Signal Simon
author_rank: Contributor
timestamp: July 14, 2015, 11:36:25 am
content: Yes, the fuse is on the incoming mains.I can't find a schematic, the nearest thing I can find is a PCB layout from the design contractor's portfolio...http://www.mipcbdesign.com/design-photos.asp#miniductorThe replacement TVS diodes should be here tomorrow, will try powering it up through a large resistive ballast, ie 5kW of electric heaters.There's no PCB damage, the vapourised metal from the fuse element has been scrubbed off the PCB and the smoke stains cleaned off the nearby 10W dropper resistors.The work coil terminations will be cleaned up prior to testing.Simon.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: July 15, 2015, 02:17:14 am
content: When you have it up and running, could you give a brief update with your impressions of its performance. They are not cheap to buy.As I said- I had no idea that such a tool existed until I read the post, but found this:http://www.ebay.com.au/itm/12-48V-DC-1000W-Induction-Heater-Flyback-Driver-Zero-Voltage-Switching-ZVS-/141662539967?pt=LH_DefaultDomain_15&hash=item20fbbed0bfIt is not clear if this coil needs some form of cooling (not water...!!!!) and it doesn't look as if it could be too convenient to tote around once in a case with a PSU and made safe enough for a workshop.Regards, BT

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2015, 03:22:33 am
content: If you're going to be using it for more than a minute at that power level, you will need water cooling.I think the Miniductor doesn't (or claims not to), because it has that fiberglass weave stuff over the coils, and is only intended for intermittent use anyway. They'll get *very* hot under there.A "ZVS" or "Royer" driver puts +V on the coil, which is inconvenient for grounding against most power supplies. You can, of course, simply use a positive ground system for that circuit.My induction heater design, and most industrially, are made with isolated or grounded coils. Water cooling is trivial; water doesn't care about AC (specifically, for frequencies above the ionic drift rate in water -- probably something like 20Hz for flowing water).Tim

author: Signal Simon
author_rank: Contributor
timestamp: July 16, 2015, 09:09:58 am
content: Bit of an update...Replaced the TVS diodes, temporary 5A fuse fitted and.... Nothing.Good news is there was no pop / bang because the output is no longer crowbarred.Further investigation found that D9 and D10 (8.2V Zeners) were short.They are in the supply to the MC33151V gate driver ic, so will change that as a precaution, its absolute maximum supply rating is 20V which I guess has been exceeded just before the Zeners shorted.I'm also changing a thyristor MCR08BT1-D because I can't decide if it's case has a crack or just a scratch, but it tests OK.These components should arrive today....

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 16, 2015, 12:10:56 pm
content: I suggest you use an external (isolated) DC supply to power the control electronics and verify that the drive and feedback is working correctly before applying mains.

author: Signal Simon
author_rank: Contributor
timestamp: July 16, 2015, 05:21:49 pm
content: Well... It went POP.I did feed it through an isolation transformer but not an external DC supply.I forgot to connect my "ballast" arrangement.I had fitted an F10A fuse on the board and had a 5A fuse in the plug, as they are relatively slow. It took both of those out.It also took out the 16A breaker for my bench supply.One of the IGBTs has gone short all ways round, so that has likely killed the new 33151 driver. I have another one.It failed upon pressing the trigger button, so as you hint there may be something wrong with the feedback, unless the damage was done previously and the IGBT failed under load.The TVS diodes are still OK, as are the new Zeners.I'm starting to think that my first opinion on the cause was incorrect.I'm going to leave it for a few days now, while I wait for some IGBTs to arrive.Simon.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2015, 01:40:34 am
content: For future reference, it may be desirable to test with a load in/around the coil. Though whether this is best case or worst case for the switching circuit, depends on choice of topology (series/parallel).Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 17, 2015, 08:40:33 am
content: Power electronics is very unforgiving sometimes. It either works perfectly or destroys itself. That's why I always follow the conservative approach and test the control electronics with a bench supply and no mains. I then bring it up slowly on a variac while monitoring gate drives etc. More time consuming but generally pays off.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2015, 01:02:03 pm
content: Although tricky for full devices -- if they have UVLO detection, you can't use the variac without bypassing it somehow; and if it's really bad (bad drivers or logic, giving a shorted bridge condition?), still might not help enough. Then you have to dig really deep into the circuit or at least try to find some service points.Tim

author: Jogas
author_rank: Contributor
timestamp: May 03, 2019, 06:37:29 pm
content: Any new on this mini ductor2 plus .because I having something similar happening. Need help aswell

author: Signal Simon
author_rank: Contributor
timestamp: August 13, 2019, 09:32:40 am
content: This is still in pieces in a box above the bench. I just seem to have been too busy or not had the interest!