Fixing a laboratory waterbath

[R_J]

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]

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.

[sam_sam_sam]

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]

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.

[vrasp]

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?

[eccerr0r]

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]

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.

[sam_sam_sam]

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]

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.

[vrasp]

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.

[eccerr0r]

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]

Originally Posted by eccerr0r

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.

Got it. So I should ignore the fact that there is a relatively low resistance between MT1 and MT2 terminals (I keep calling them that but it's not actually MT1 and MT2 Because there is no TRIAC there atm, they re the pads where MT1 and MT2 would wire into). I will shift my focus to the resistance between the Gate and neutral to see how much current goes through and report back. I ll have to wait until Monday to do that.

I wonder if it has something to do with that transformer I thought was a relay or the control PCB below since it sends the signal to activate the gate.

[eccerr0r]

BTW TRIAC terminals are MT1 and MT2 because they're pretty much symmetrical. Either way the metal tab is one of the two terminals, and neither are GND.

Brings back to the question why was the first triac replaced, nevermind the oops with the second one.

[vrasp]

Originally Posted by eccerr0r

BTW TRIAC terminals are MT1 and MT2 because they're pretty much symmetrical. Either way the metal tab is one of the two terminals, and neither are GND.

Brings back to the question why was the first triac replaced, nevermind the oops with the second one.

That was my mistake. The bath stopped heating after being on all day. I opened it up and noticed one of the cables that goes to the display was not on its pad anymore (these connections have been good since then, I redid them). I soldered it back then turned the bath on. That's when a short blew the TRIAC. I realized later that one of the terminals of the TRIAC was touching the case. The boards are not held by anything when I work on them so I have to rest them on the metal parts where they normally screw in and one of the terminal was touching the case. I always triple check now.

I dont fully understand the transformer there. One of its pins is on the 120v line. It s a big trace also going to MT1. Makes sense since it supplies high current to the heating element. The other pin is on a thinner trace that goes to what I think is a Zener (it s on the other side of the board so we cant see it but it s in the previous video) then to the gate. The gate takes max about 2v - 2.5v and about 50-70 mA. These 2 pins are on the same side. Wouldn't the 120v (and high current) go to the gate trace? That would blow the TRIAC.

You cant see the transformer on the photo below, it s on the other side of the board, but you can see the traces. Red is the transformer pins. Black is the gate. Yellow is what I think is a zener (I ll double check).

[eccerr0r]

If you think that end of the transformer as the "output" side of the transformer, it would make more sense... because it is the output side...

[vrasp]

Originally Posted by eccerr0r

If you think that end of the transformer as the "output" side of the transformer, it would make more sense... because it is the output side...

I understand it's the output but what about the 120v? The 120v is connected to the Gate trace (or rather the zener ) through the winding inside the transformer. What prevents the current from that thick trace to go into the Gate trace? I think of one side of the transformer as just coils around a core and connects 2 pins. If the 2 pins are connected, current can go through. I know it doesn't go to the gate pin because then jt wouldn't work. But why doesn't it?

[eccerr0r]

Question to ask yourself is not the absolute voltage at the gate but what's the voltage difference between MT1 and Gate?

Thought experiment/example: what if you take a TRIAC and connect all three pins together, and then plug the shorted pins into the hot pin in in a wall socket. Despite doing this is quite useless, why does the TRIAC not fry, the gate is connected to 120V too!

[vrasp]

Originally Posted by eccerr0r

Question to ask yourself is not the absolute voltage at the gate but what's the voltage difference between MT1 and Gate?

Thought experiment/example: what if you take a TRIAC and connect all three pins together, and then plug the shorted pins into the hot pin in in a wall socket. Despite doing this is quite useless, why does the TRIAC not fry, the gate is connected to 120V too!

I think it wouldn't fry because all pins are at the same potential. I think I got it!

I measured (no TRIAC) the resistance between the Gate pad and neutral : 68 Ohms.
Gate and MT1 pads Voltage: 1.1vac
Gate and MT2 pads: 114vac
Between pins of the input of transformer: 0.5vac

Again, there is no TRIAC atm. These are the values at the pads. Maybe these values are useless without a TRIAC in the circuit as it might affect those values.

This is the datasheet for the TRIAC>
https://www.farnell.com/datasheets/202004.pdf

[sam_sam_sam]

Okay your voltages are are not what the data sheet says it should be so you have some real issues with this

[R_J]

The triac is pulse controlled. There are control pulses on the primary side (from the control board) the triac has the gate and one MT connected to the secondary of the transformer. With no pulses from the transformer, the gate is effectively turned off, when there are pulses present the triac turns on.
The pulses are likely to control the temperature of the element and provide zero cross switching to prevent noise on the ac line.