Fixing a laboratory waterbath

[vrasp]

Re: Fixing a laboratory waterbath

Well you're right it is MT1 MT2 Gate, but sorry, no additional clues why it's frying. You're on your own.

That's ok. You ve already helped a lot!

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Like I said earlier in a post you need to know if you are getting a signal from the controller to the triac and it is controlling the triac if it not then you need to figure out what is not working

Or get something like this

https://www.ebay.com/itm/22210532801...53.m1438.l2649

And manually control the temperature and simplifies this setup if you can not figure out how to fix it

Or get a temperature sensor controller and control the temperature that way

Also you can do the compressor controller the same way use a temperature controller for this part as well and do way with the controller it has now but it going to take some work on your part and Google for the different controller that you need for this project

If I can help you figure out what might work for you I would gladly help you as much as I can

[vrasp]

Re: Fixing a laboratory waterbath

Like I said earlier in a post you need to know if you are getting a signal from the controller to the triac and it is controlling the triac if it not then you need to figure out what is not working

Or get something like this

https://www.ebay.com/itm/22210532801...53.m1438.l2649

And manually control the temperature and simplifies this setup if you can not figure out how to fix it

Or get a temperature sensor controller and control the temperature that way

Also you can do the compressor controller the same way use a temperature controller for this part as well and do way with the controller it has now but it going to take some work on your part and Google for the different controller that you need for this project

If I can help you figure out what might work for you I would gladly help you as much as I can

I haven't worked on it since last time. I ll check tomorrow or Friday if there is a signal from the controller. I do like the SCR voltage regulator idea. I might do that if I really cant figure it out. There is already a temperature sensor that works so I just need to control the heating.

Thanks for the help! It's greatly appreciated.

[vrasp]

Re: Fixing a laboratory waterbath

I think I finally got to the bottom of this.

I plugged the waterbath to the wall (no bulb in series) and got a short on MT1. The 15A fuse didn't even blow but the circuit breaker of the laboratory did. I thought the fuse would blow before the circuit breaker. I don't know why that was not the case. Anyways, I decided to remove the TRIAC from the heatsink and left it hanging without touching anything. Then I turned the waterbath on and this time there was no short and the lamp in place of the heating element turned on.

I then put the TRIAC back on the heatsink, this time with two mica. Now everything seems to be working fine (lamp turns on). I have no tried it with the heating element yet.

I had measured the resistance of TRIAC with the heatsink and screw and got max values even at 2000k Ohms. Obviously the TRIACs that blew must have had some continuity with ground. This surprises me. How do I know I can trust the measurements? My leads are clean and I was careful when taking those measurements.

Next step is to connect the heating element and make sure the water heats up.

[petehall347]

Re: Fixing a laboratory waterbath

obviously it was shorting to the heat sink by the way its burned the aluminium check the earth connection as it looks burned . i would drill and tap a new screw hole and move the triac to a flat bit .

[eccerr0r]

Re: Fixing a laboratory waterbath

Sigh...so it turns out something silly... If we can't trust measurements, what can we trust...

dirty probes? Probes have sharp points for a reason - to dig through thin oxide layers.

Using two insulators, though better electrically, is significantly worse thermally. Usually TO-220s can handle at most about 2W without a heatsink. For TRIACs this means you can pass about 2A before you fry it when not attached to a heatsink.

Also are you using conductive thermal paste? Don't, or don't bother with thermal paste at all. Wish I could trade you this ZnO heatsink compound I have, I'm still waiting for "gray" heatsink compound to see if it's better for my CPUs...

[vrasp]

Re: Fixing a laboratory waterbath

That transparent mica that comes with the TRIAC is not enough, or no good. I ll try with just the other mica that I bought separately instead of both.

I didn't check the tube but I m pretty sure it's the regular kind. It was $4. The metal based ones are more expensive I believe. I ll have to double check the label.

Like I said in a previous post, even putting both probes on the heatsink gives me no continuity. But everything else gives me proper measurements. That's what I find confusing.

I didn't realize it could fry with so little. I guess if I had tried with the heating element right away I would ve fried it.

[eccerr0r]

Re: Fixing a laboratory waterbath

If you perhaps created some shrapnel that could have pierced through the insulator, but I've never had this happen. Never had a TO220 short with the heatsink when I have all the correct hardware installed.

As petehall347, you might have to end up selecting a new, clean site. But try with no heatsink compound unless it's known to be insulating (like "White" ZnO) first. Use the sharp probes to scratch into the paint and oxide coatings on metals to ensure you get a good contact.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

I have found that some plastic spacer have an issue with the way you install them the plastic is very thin and if you use the wrong type of screw then you can cut the plastic and short out to the heat sink and like what was mentioned earlier about the heat sink compound you have to use the right type and if you only paid $4.00 for it and it more than a couple of ounces then you got some real crap compound because when I buy heat sink compound I pay about $20.00 for a tube

Here is the one I use

https://www.mpja.com/Heatsink-Compou...info/30389+MG/

I have use this product for years and not had an issue with it

[vrasp]

Re: Fixing a laboratory waterbath

What if I clean off the surface and sand it with some 3000 grit sandpaper? I d like to avoid drilling a new hole.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

You need to use a scotch-brite-heavy-duty-scour-pad pad that you can find in the grocery store it one that is green in color do not use sand paper ( unless you use a block of wood that has a very flat surface and it must be very fine grade ) it will make it uneven and you will not have the heat transfer

https://www.zoro.com/scotch-brite-he...28/i/G4676353/

They also make polish pad that you can put on your drill or die grinder

https://www.harborfreight.com/pack-o...scs-99985.html

You must make sure that you have NO connection to the heat sink from the device pins or the mounting tab this a must or you keep burning up triacs

Just in case you are not sure how to do this the correct way I am not trying to be an a** about this

When you put the plastic spacer to the triac it is the smaller end that goes through the metal tab of triac the head of the screw must be on the plastic flat part of the spacer you must use the nonconducting thermo pad under the triac metal tab and you put thermal heat sink compound on the thermal pad to the triac and you put on the heat sink as well one note you only need a small amount of heat sink compound on the metal tab to the thermal pad only you do not need to smear it on the thermal pad on the side of the triac the side that is the heat sink side needs to cover the thermal pad completely

[eccerr0r]

Re: Fixing a laboratory waterbath

new hole is unlikely going to fix the problem.
BTW that plastic shoulder washer... if you're screwing the screw in, the screw is too big. The screw should slide into the plastic.

The screw screws into the heatsink so it will be grounded. The heatsink should have an small air gap or the insulator material between it and the metal tab.

[eccerr0r]

Re: Fixing a laboratory waterbath

TBH the only thing that makes sense ASSUMING you assembled the TRIAC properly is that your heatsink compound is conductive or you somehow have shrapnel on the screw.

Actually I'd have to say: since you're using an insulator sheet (mica, kapton, or silicone), don't use any heatsink compound at all, especially if you don't know if it's conductive or not. These materials are soft enough to expel air bubbles, just do not overtighten the screw (5-10 inch-lbs is sufficient). Make sure it's cleaned off of all old compound. Clean off any metal debris you can see.

Mica is clear, brittle, and cleaves little shards. Kapton is orange and flexible. Silicone is gray and rubbery.

Again use the sharp points of the multimeter in the highest resistance range to check the dielectric. Make sure your meter is actually working too... Scrape the top side of the metal tab of the TRIAC and check against the screw.

[vrasp]

Re: Fixing a laboratory waterbath

That thermal compound had no label so I have no idea if it's conductive.

I just got back from the store with some NTE ZnO thermal compound, a few mica (they re thicker than the one provided with the TRIAC), and while I was there I also got some more washers. I love this store - they have everything!

The plastic washer I have works fine, it slides into the screw like you said. The screw didn't touch the TRIAC at all. I think everything was correctly assembled so it has to be something with the thermal paste or a damaged mica.

With all of this, I should be good to go. I'll clean the heatsink of the old thermal compound, scrub it with the scotch-brite pad, and assemble it all.

[eccerr0r]

Re: Fixing a laboratory waterbath

You probably could just stick your multimeter probes into the heatsink compound and see if there's any conductivity.

No need to scrape or scrub the heat sink, just need to make sure the surfaces are flat and rid of any shrapnel that might be large or sharp enough to damage the insulator. Again don't bother with thermal compound for now. And the main reason for not torquing down the screw more than a foot pound or so is to prevent warping, along with not damaging the insulator.

[vrasp]

Re: Fixing a laboratory waterbath

You probably could just stick your multimeter probes into the heatsink compound and see if there's any conductivity.

No need to scrape or scrub the heat sink, just need to make sure the surfaces are flat and rid of any shrapnel that might be large or sharp enough to damage the insulator. Again don't bother with thermal compound for now. And the main reason for not torquing down the screw more than a foot pound or so is to prevent warping, along with not damaging the insulator.

Sounds good. I ll do that this coming week.

[vrasp]

Re: Fixing a laboratory waterbath

I was going to mount the TRIAC with the new thicker mica I got. However, I can peel the layers off. I initially thought that there was a sticky part to stick to the TRIAC or something like that but below each peeled layer was another layer that is easily peeled. I added a photo below with a "full" mica and one peeled into it's layers. Anyways, not knowing if this was normal, I decided to use the other isolator I have which I believe is Silicone or Kapton.

I cleaned off the surface of the TRIAC and heatsink and used the new washers I got. Checked for resistance, everything was good (max resistance between the metal tab and heatsink/scew). Turned everything on and it worked!

I then proceeded to resolder the heating element instead of the lamp. Turned on the waterbath and the water heated as it should. It took about 20 min to go from 20 °C to 40 °C. I then proceeded to measure the TRIAC temperature with an infrared thermometer. After 20 min of heating, the TRIAC temperature was approx. 80 °C to 90 °C depending on where I point the laser. The datasheet mentions temperatures from -40 °C to +125 °C so it should be good. I assume that the TRIAC reaches it's operating temperature quickly and that this temperature stays more of less constant over time (assuming constant room temp) because the current going through is always the same. In other words, leaving the waterbath on for 20 min or 24 hours should not have a significant effect on the temperatures I measured. Can someone confirm that?

There are two other minor issues that I believe are due to bad soldering. The first one is that the display is missing some LED bars that show the numbers and also the dot for decimals (it's still readable so no big deal if I cant fix that). I had changed the ICL a while ago and have to check each pin is properly soldered on its pad. The other minor issue is that I sometimes have to turn the waterbath on - off then on again for the display to show the temperature. So the first time I turn it on the display turns on but doesnt show numbers (just a some LED bars are on) then if I turn it off and on again it shows the proper temperature and it's good to go. Again, it probably has to do with bad soldering. I ll have to double check all the connections.

Thanks for your help guys.

[eccerr0r]

Re: Fixing a laboratory waterbath

No you don't want to peel mica, it's one of the characteristics that it can be split very thinly. And no there's no adhesive layer. Though thinness helps heat conduction, it also encourages shorts so you want to use full thickness. And peeling it, as you assumed, ruins it - so don't use the peeled version.

Question is how hot is the heatsink getting versus the TRIAC. At 90°C I'd think it's on the warm side though it's within limits. Rule of thumb is that if you can't keep your thumb on it, it's too hot. However don't do that as it's electrically live and you could get electrocuted. If the heatsink is also getting warm then you can probably leave it; if the heatsink stays cold then you might want to tighten the screw a bit more or look into heatsink paste.

[vrasp]

Re: Fixing a laboratory waterbath

I did do that. I can keep my thumb on the heatsink but it's definitely hot. I did a quick measurement but didn't write it down. I think it was around 60 °C. What I m worried about is reducing the TRIAC lifetime if it gets too hot. Do you think 90 °C is ok or should I try with some ZnO thermal paste? There isn't much else I can do. The TRIAC is tight on the heatsink.

[eccerr0r]

Re: Fixing a laboratory waterbath

It's probably the way it is. 90°C is hot, but it could be hotter.
Also have to consider, is it hot when the heater/TRIAC is "off"? Should get some reprieve when the heater is off as it shouldn't be on 100% of the time.

Is the heater heating as fast as it was before? Perhaps the TRIAC you got is a fake?