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HT-2008: €90 SMD hot plate & death trap: teardown & review.
Posted by BreakingOhmsLaw on 20 Mar, 2021 16:59 -
So, i bought this HT-2008 SMD hot plate on ebay. It's a 400W plate with a 80mm x 80mm heating element which is set into a larger phenolic HLT plate.
I bought this because i like the design, a smaller work area with some protective plate around it where you can rest your hand while working - or at least not get 3rd degree burns right away when you accidentally touch it. The price is around 90 Euro, which is around $100 US or £77 GBP
It's chinese made and from the outside it looks to be pretty well made. There were however some scratches in both case and top plates fresh out of the box. Nevertheless I decided to open it before even connecing it to mains, and boy, that was a good idea.
The bottom is made of another phenolic plate. That opens right up by removing the feet and we have access to the guts.
Immediately, we see some serious issues. First of all, there is no fuse whatsoever. The mains is a standard IEC plug and there is no filter. While this is mostly a resistive load, there will be a SMPS in that regulator, and I'm not sure that solid state relais will just switch on the zero passes, so that is not good.
All of the spade connectors are not insulated. That's bad, especially since the metal case is not connected to ground. Funny enough, the heater plate is, and that is probably the part you will be making an effort not to touch when this is in use.
The solid state relais is a Taiwan jobby by Fotek, and it has an UL mark. It's mounted to the case with a rather roughly machined piece of aluminium. No thermal compound, at the rated 400W it's propbaly fine. Still going to apply some before remounting for good measure. Peculiar: The relais has a hole on the back that looks like someone accidentally hit it with a drill. Might be a hint at that these units might be second quality parts or even rejected.
I checked all screw terminals, some of them were not tightened correctly. Also, some of the unused terminals were loose, so the screw can get lost over time and make short-circuits in the unit.
The heating assembly is mounted on yet another yellow HLT plate, and here comes the shoddy work. They didn't bother cleaning it, it has saw dust all over it. The brass screws look like the have been cut with a dremel after screwing, and they didn't bother to fully remove the plastic protective film entitrely. That would undoubtely molten during first use.
The heater is made of a solid block of aluminium with two heating elements embedded inside that are held with a grub screw. There is some play between the plate and the heating elements, so heat transfer will be affected somewhat.
Well, first things first, i connected the case to earth and put heat shrink on the spade connectors. I have a combined filter/fuse IEC connector on order which i will be mounting soon.
While reassembling, I discovered a stamp on the backside of the bottom plate. Apart from ISO9001 it's all chinese. Google translate doesn't make any sense of it.
So, all assembled and time to fire it up. I was surprised to be able to set the desired temperature to 999°C. Given that aluminium melts at 660°C, that's borderline stupid.
Wanting to see if it would actuallys go that high, I let it heat, but when it started smoking at 450°C displayed temperature, i aborted the test. The actual temperature is lower - see below. Now, the seller does state that there can be some smoke on first use due to residual oil burning. However, I did not feel like setting my lab on fire on a saturday night.
Magic Smoke
The overall heating seems to be quite uniform throughout the plate. The temperature was a good 100°C below what is set as target, but I'm not sure how accurate the FLIR camera is.
The unit is advertised with a 400W rating, but my power meter says 312,8 Watts peak.
The HLT plate surrounding the plate does get hot, i measured it at 74°C when running the plate at 350°C target temperature. However, the edges remained relatively cool.
Picking parts of a heated board worked well, so it can be used for SMD work. The last set temperature is stored.
To sum up its issues:
Safety:
- no mains fuse
- no thermal fuse
- no insulation on spade connectors
- case not grounded
- thermal setting not limited
Use:
- Temperature readout very inaccurate. Haven't found out yet if this can be calibrated.
Pro's:
- Well. It's cheap and the mechanical construction is okay.
All these issues can be fixed or accounted for by the user, but still, the total lack of safety in the design make this a unit to pass unless you are on a budget and are willing to adress the issues yourself.
I hope you found this useful, if you have any questions, go ahead. -
Did you set emissivity on the FLIR to match the aluminum? It should be around 0.4. Or you can put a piece of black tape and measure that, problem is finding one with high temperature rating.
Good photos, and yeah, you'll want to ground that thing with the potentially fake Fotek SSR. -
Did you set emissivity on the FLIR to match the aluminum? It should be around 0.4. Or you can put a piece of black tape and measure that, problem is finding one with high temperature rating.
Thanks for that remark. This is a FLIR Camera in a Cat S61 Smartphone. It just has emissivity settings for the surface structure, which is set to "matte". It doesn't tell you which numerical value that correspons to, but it appears to be the lowest setting.
Good photos, and yeah, you'll want to ground that thing with the potentially fake Fotek SSR.
I have another IR thermometer that spits out nearly the same value as the FLIR camera, so I expect this to be not far off the actual temperature.
The Fotek SSR is likely a counterfeit one, it does not have the cut corners in both plastic and label that is intended to prevent the label to be applied in reverse. The seems to be one of the giveaways of a fake one. Not taking any chances, I will fit a new unit bought from a local distributor.
After modification, that puts the price here:
€90 base unit
€36 SSR (25 Amp Gavazzi or Crydom should do)
€13 Netfilter Schurter 4300.5054
€ 3 Fuse holder Schurter 3101.0110
€ 2 small parts.
€145. Still a decent deal if you are willing to fix the safety issues yourself.
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Interesting product; I find it a bit odd that a hot plate would be surrounded by phenolic material, as it tends to decay quite rapidly with temperature. Perhaps is a different material from the old PCBs...
Good that you opened up the product before any potential problems. -
the watt rating might help if voltage is too high so it does not easily get destroyed
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You take a good, pragmatic approach to the testing and upgrading of your unit. I bought a similar hot plate wth thermostat and i still have not opened it up or attempted to stress test it.
I have tested the grounding and that the case is not electrically hot.
My hoptplate was an incredibly good deal (around $45) and looks externally to be of decent construction. It has a larger aluminum plate than yours but looks quite similar. Its blue and yellow and does not have any clips for holding the board as some do. (I use a very old large Panavise that I got at an estate sale)
So, even though it was purpose built for electronics, Its just a plain square digital hotplate.
The hotplate itself appears to be an approximately 200x200 mm square of aluminum with a pattern of concentric circles etched into it. (presumably to prevent metals from sticking to it) Inside it likely has rock wool or asbestos - the manual mentions this, and that one should exercise caution with it if one needs to replace the heating element.
I have a hot tub timer switch that is plugged in to the 3 power outlets that I use with it and my soldering irons.
It allows me to turn them on for a maximum of one hour with a single twist when I am using the hot plate or my soldering irons. When it runs out while I am still soldering I just give it another twist. This is a small price to pay for the peace of mind that its not going to be sitting there baking and using up electricity when I'm not actually using it for any more than the set time.. I also have a house wide fire-burglar alarm, with a sensor nearby. -
Well, it's all done now.
The Schurter filter was not a mere drop-in replacement. The hole in the case was chamfered, the Schurter however needs a square hole. It also was ~2mm wider than the old IEC plug.
Nevertheless, 5 minutes with Mr.Dremel fixed that.
The thread holes are identical, so no worries there. Some spade plugs needed to be changed from 4.7mm to 6.3mm.
A step drill worked fine to create the holes for the fuse holder and the earthing screw.
This is now properly safe to use.
I might add a thermal runaway protection, but regular thermal fuses don't go that high. Not sure yet how to do it in a simple way.
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Re. the calibration, check the thermistor is making good contact. On mine the way it was screwed into place (the thermistor was on a lug) it was loose so some washers were needed.
Yours looks to be stuck in a hole? Some thermal grease will help to thermally couple it to the hot plate.
Also, if your controller isn't completely stripped of features (as my original one was, I replaced it with a cheap ebay unit with a full feature set) you should be able to find a manual online to find the calibration setting. I have a manual for the common REX-C100 if you need it, it also has auto PID tuning too which helps prevent too much temperature over and undershoot.