Fixing a laboratory waterbath

[petehall347]

Re: Fixing a laboratory waterbath

i suppose you could replace the heating element with a bulb .

[vrasp]

Re: Fixing a laboratory waterbath

Should the potential between GATE and MT1 be the same? There is no information of that in the datasheet. Right now it's 1.1v. Maybe due to the 1N4148.

This is my plan:
1. Solder the new 800v 15A TRIAC.
2. Solder a 15A fuse on MT1 and a 0.1A fuse on GATE.
3. Plug the waterbath in series with one or a few 100w bulbs.
4. Measure the voltages between GATE and MT2 and MT1. It might allow to understand where the issue is coming from.

Something I noticed when I had 300w of bulbs to limit the current (before I blew it) - the TRIAC and heatsink never even got warm. At the time I thought it was due to the current being limited, but now I m thinking maybe the TRIAC was not working at all.

I dont know where the issue might be coming from. The circuit is basically just a current activating a TRIAC and closing a high current circuit to the heating element. Not sure what is causing the TRIAC to blow. I ve been ignoring the input side of the transformer. It is connected to the control board. Could a wrong voltage/current on the input cause a current high enough on the ouput to blow the TRIAC?

I d love to hear some suggestions on how to get to the bottom of this.

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

Nope
N-I-H-ism if I have to spell it out.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Eccerror

Was this the word you trying to spell

“nihilism”

Just curious

[eccerr0r]

Re: Fixing a laboratory waterbath

Definitely not DIAC, DIACs do not have bands on them.
Yes that transformer is there solely for isolation. The whole design probably could be made one heck of a lot simpler with a MOC3041 but probably designed with NIHism...

[sam_sam_sam]

Re: Fixing a laboratory waterbath

What would the signal transformer purpose be in this circuit
Is this a way to isolate the triac from the rest of the circuit
Or am I under the wrong assumption of what it’s purpose is for

R_J
Quote

“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.”

I did not see this until just now

How common is it to use a signal transformer with a triac setup

Can you push more current with it pluse-ing the signal is stead of just using a turn on signal to the triac

[R_J]

Re: Fixing a laboratory waterbath

That is just a 1N4148 small signal diode, neither a zener or a diac

[vrasp]

Re: Fixing a laboratory waterbath

That other one might actually be a zener. There is 0.5v across.

[vrasp]

Re: Fixing a laboratory waterbath

Based on what you said, this seems to be the configuration. You say 2 zener? are there 2 zeners? or only the one?
It is possible it is not a zener but a diac

Yes I think that's exactly it!

This is what there is between the transformer pin and Gate. It looks like it's touching that terminal in the photo but its not. It connects to the gate on the other side. There is another one between the two primary pins too ( so where you wrote "to trigger circuit").

[R_J]

Re: Fixing a laboratory waterbath

Based on what you said, this seems to be the configuration. You say 2 zener? are there 2 zeners? or only the one?
It is possible it is not a zener but a diac

[vrasp]

Re: Fixing a laboratory waterbath

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

Could it be that these voltages are for when the TRIAC is in circuit?

I bought a new TRIAC, 15A fuses to place before MT1 and 0.1A fuses to place before the Gate. I hope to be able to troubleshoot the issue without having to replace the TRIAC again. Also maybe seeing which fuse blows will help me determine whether the issue is at the Gate of the TRIAC or at MT1 and MT2.

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.
The triac connection which is connected to the transformer should likely be connected to neutral, and the element should connect between line (120vac) and the other triac connection.

Nice explanation. Thank you!

The line with MT1 has a capacitor connected to neutral. I don't know if that counts as being connected to neutral. The heating element is connected to MT2 and MT1 is connected to 120v. MT2 and MT1 are connected within the TRIAC (when activated) but not on the PCB. I hope that makes sense.

Unless the short messed up with the control PCB via the transformer (if that's possible), everything on this circuit should work. It's only made up of the TRIAC, 2 zener(I m still not sure those are zeners but they look like it), the heating element, and a capacitor which is new. The heating element has a resistance to neutral that is within range of what I read it should be (around 10-15 Ohms I think).

[R_J]

Re: Fixing a laboratory waterbath

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.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

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

[vrasp]

Re: Fixing a laboratory waterbath

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://cdn.badcaps-static.com/pdfs/...3966fef995.pdf

[eccerr0r]

Re: Fixing a laboratory waterbath

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]

Re: Fixing a laboratory waterbath

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]

Re: Fixing a laboratory waterbath

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]

Re: Fixing a laboratory waterbath

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]

Re: Fixing a laboratory waterbath

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]

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.

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.