Fixing a laboratory waterbath

[eccerr0r]

Re: Fixing a laboratory waterbath

Rule #1 of resistance measurement : You cannot measure resistance of a component when it's in circuit. Thus you cannot make any conclusions of resistance values taken when devices are in circuit.

Yes effectively MT1 and MT2 will be connected together in a TRIAC when gate is triggered and will remain so until the current going through drops below the threshold.

[vrasp]

Re: Fixing a laboratory waterbath

I wrote in my previous post that it was fixed (I removed that part). That's not the case. There was just a bad connection with the heating element. The resistance between MT1 and MT2 is 17 Ohms.

[vrasp]

Re: Fixing a laboratory waterbath

It goes GATE MT2 MT1. GATE is connected to the transformer that is connected to the control PCB. MT2 is connected to the heater. MT1 is connected to 120V. I m not sure my understanding of the TRIAC is correct. My understanding is once the GATE is activated by a current (from the control PCB), MT2 and MT1 are activated. When the TRIAC is activated, MT2 and MT1 become connected in the TRIAC, forming a closed circuit and allowing current to flow from MT1 to MT2 to the heating element. Maybe someone can confirm this.

The TRIAC is an NTE56010. 800v and 15A. It s the same as the original one.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

What is the resistance between MT1 and MT2 with out the triac and the heater element installed because this is what going to matter as far as the triac output is concerned

Correct me if I am wrong but is MT2 is the output of the triac if this is the case then you also need to check the resistance between the output side of triac and the neutral side of the power supply cord

Because either you have an issue with the heater element or something in this controller output from the triac output that is causing this issue blowing up the triac

Here is something I would do but be very careful doing this you need to check how much current the heater element is drawing by itself

Also put a incandescent light bulb on connections for the heater element and see what type of amperage you have on MT1 a 100 watt incandescent light bulb should be about 2 amps or so but check how much amperage your incandescent light bulb is drawing current because the two should match other reading

What is the gate voltage for this triac because if this is not correct you could be not turning the triac completely ON or the voltage is to high on the gate pin you need to look at the data sheet for this triac

[vrasp]

Re: Fixing a laboratory waterbath

I read that the TRIAC should have a high resistance between MT1 and MT2. Does that apply when it s in the circuit as well? I was measuring the resistance between where the terminal of MT1 and MT2 connect, so I m measuring on the board and trying to see why there is a low resistance. If this statement only apply to the triac off circuit then maybe it doesn't matter that the resistance between MT1 and MT2 is low on the PCB.

I dont know why the TRIAC shorted again and this was the investigation I was doing because I cant think of any other reason. The TRIAC was not grounded. I checked many times and with each pin and the metal plate too.

[eccerr0r]

Re: Fixing a laboratory waterbath

not sure what you're trying to measure here...

All that matters is that you don't exceed its rated current/power dissipation/voltage limit. And when you measure resistance of the TRIAC it needs to be measured out of circuit.

[vrasp]

Re: Fixing a laboratory waterbath

I removed the primary of the toroidal transformer which supplies 8v, 20v, and 15v on its secondary to the board below and the display board. Now the resistance between hot and neutral is ~20-23 Ohms.

This means that if I were to install a new TRIAC, it would have about 23 Ohms between it's MT1 and MT2. Is that ok for a TRIAC?

[vrasp]

Re: Fixing a laboratory waterbath

The triac is rated at 15 amps. However it worked before. I think the resistance between hot and neutral should be higher.

I have a very high resistance between the heating element and ground and ~13 Ohms between hot and neutral of the heating element. From what I read this is a normal value for a heating element.

I ll try removing each element on this board until I get a high resistance between hot and neutral. There is the transformer and some smaller components I havent tried removing yet.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

What is the specifications of the triac that it has

Really 120 volts @ 5 ohms = 24 amps if your triac is not rated for that then you would fry them

Check the heater element by it self

[vrasp]

Re: Fixing a laboratory waterbath

I m getting ~5 Ohms between hot and neutral with the heating element disconnected (and no triac since it has been removed). There isnt much left on that line so it should be easy to find.

[R_J]

Re: Fixing a laboratory waterbath

One thing that can happen with heating elements is they can short to ground when they heat up, the internal element expands and in some cases breaks through the insulation and shorts. This can burn a hole in the element but not always. You can try insulating the element from ground and disconnect its ground wire if it has one to test this.

[vrasp]

Re: Fixing a laboratory waterbath

I actually had already checked the heating element with ground and it was fine. All three terminals had high resistance with ground.

I m thinking of everything thats on the board. I ve already removed the capacitor. I m pretty sure it s not the transformer. I can try removing it. The rest are part of the circuit that feeds the pcb below it so not related to the actual heating.

Isn't a heating element just a thick tube of copper/alloy with a high current going through it? How could it be faulty?

Below is a photo of the board I ve posted before.

Next to the number "12" is in order

Gate
MT2
MT1

I made a mistake in my previous post. MT1 terminal is connected to the heater element and MT2 terminal to 120V. Either way, there is continuity between them where there shouldn't be.

[vrasp]

Re: Fixing a laboratory waterbath

Thanks. I ll check that.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

If the heater element is like the type in a water heater then you need to check the outer case to the terminal of the heater element and you should NOT get any resistance if you do have any resistance then there is an issue with the heater element

What is the resistance of the heater element because if it is for 120 volts then the resistance can not be extremely low because if this might be your problem ( it should be around 8 to 10 ohms )

This also depends on the tirac amperage rating if it lower than 15 amps then your resistance would be higher than 10 ohms

[vrasp]

Re: Fixing a laboratory waterbath

Or there is a very low resistance between neutral and live. I ll check that.

[vrasp]

Re: Fixing a laboratory waterbath

I plugged waterbath straight into the wall without the bulbs in series. The TRIAC blew again. It was not grounded, I checked.

These are the observations I made:

The resistance between where MT1 and MT2 connect have a low resistance (even with no TRIAC connnected). Based on my reading, it is supposed to have a high resistance. From now on I ll refer to where MT1 and MT2 connect as MT1 and MT2 to make things easier. MT1 connects to the heating element. It's the live of that heating element and then there is a neutral wire that goes from the heating element to the PCB. MT2 connects to 120V.

I removed the heating element cable from MT1 and the resistance between MT1 and MT2 became very high, which makes sense since MT1 is only connected to that. Could a faulty heating element cause that? I don't see how but I don't know.

Any ideas?

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

It's up to you to decide what's going on... especially when dealing with heaters and other things that draw a lot of power.

[vrasp]

Re: Fixing a laboratory waterbath

I tried with 3 bulbs this morning. They all light up. I dont think they re at their full brightness but I m not 100% positive. They do get very hot.

I noticed the voltage drop across the primary of the transformer is about 65v. Is it normal for these bulbs to drop the voltage by that much?

The waterbath temperature increased by 0.5 degrees in 20 min. So it slowly heats up which would make sense with the limited amps and lower voltage.

I m not confident everything is working well enough to plug it without the bulbs.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Yes unless you have to many lights bulbs then would almost as though you were using a fuse

[vrasp]

Re: Fixing a laboratory waterbath

btw how the heck do you get a water bath to 120°C, is it pressurized water?

To keep it liquid at 120°C, you must increase pressure to at least 200 kPa (so around 100 kPa gauge pressure).

No pressure here. We use a mixture of water and anti-freeze. It allows us to get to temperatures below 0 and a bit higher than 100 °C. We almost never use it at such high temps anyways. If we wanted to we d have to use another mixture. We do use it at temperatures below 0 though.

Yesterday I made myself a small current limiter setup with a 100w bulb. I tried it today with the waterbath. The waterbath starts and runs but the display is dim and the pump fan turns slower than it should. Also the bulb is bright. I suppose it needs more amps to run properly.

Tomorrow I ll try with 2 bulbs or more. If I understand well, if I can get to a point where the bulbs are dim and not at full brightness, I can confidently assume that there is no short. Otherwise, if after adding a few bulbs they re still really bright there is probably a short. I don't know how many amps the load normaly takes but since there is a heating element I think it's likely at least a few amps. What do you think?