Fixing a laboratory waterbath

[petehall347]

Re: Fixing a laboratory waterbath

if you got clever you could find a pcb mount transformer with one or 2 of the voltages ..might get luckier and find all 3 close enough .

[vrasp]

Re: Fixing a laboratory waterbath

I do have some chargers laying around but I dont think they have the right voltages. I might check anyways. If I understand correctly, if the amperage rating of the transformer is higher it s ok because the device will draw just the amount it needs?

[eccerr0r]

Re: Fixing a laboratory waterbath

Looks like the original is at least "20VA" or so, was staring at one of the spare 12.6V@300mA (3.7VA) transformers I have here and it's tiny, metal part is 38mm x 30mm, center core is 1cm x 1cm.

Remember this "20VA" would be the sum of the 8, 15, and 20V outputs. 8VA is part of the computation so the 12VA needs to apply to the 15 and 20V outputs.

[vrasp]

Re: Fixing a laboratory waterbath

This is new to me. Would it be a good idea to aim a bit higher to make sure? Lets say 25 VA. So assuming I get an 8v with 1 amp, there is 17 VA left so I could get a 15v and 20v with 0.5 amp each?

[eccerr0r]

Re: Fixing a laboratory waterbath

Physical size(and price) is the main problem.

[vrasp]

Re: Fixing a laboratory waterbath

Regarding the space, I think I could figure something out if it doesn't fit inside the case. This is a big lab device that likely will stay in one spot until it is replaced/thrown out. I could always have one or 2 of the transformers outside the case, in the back, hooked up with wires inside the case and isolated so it is not a hazard.

As for the price, I was going to find some old chargers and get the transformers from those(in case there is another issue I dont know about in which case I would have spent money on transformers for nothing). I m guessing these specs cant be that hard to find? This device is used in the lab I work at. They were just gonna get a new one for a couple thousand bucks, but I offered to see if I could fix it. They ll be more than happy if they end up spending hundreds instead of thousands.

[vrasp]

Re: Fixing a laboratory waterbath

Hey guys,

I m still looking at transformers. I found a store not too far from me that has a few options and would like to hear your thoughts. They have a toroidal transformers too. I found these ratings for one toroidal transformer:

Plitron 7695-B1-04

Output: Red -7.8V @ 0.83A
Orange - 9.2V @ 2.48A
Violet - 8.3V CT @ 1.2A
Brown - 18.5V C.T. @ 1.18A

It's also quite cheap.

If I hook up the violet, orange, and brown, I should get all the voltages required and over 50 VA. It's also much smaller. Would it work?

Thank you!

EDIT: It's not much smaller. Around 3.5" (Dia) x 1.25" (H). But still smaller than having 3 transformers.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

I thought you need these voltages “8” “12” “15” “20”

How are you going to be able to get all these voltages with the transformer that you are showing

[vrasp]

Re: Fixing a laboratory waterbath

I need 15 / 20 / 8. Based on eccerr0r, I can go from 9v to 15v for the "15", 14v to 20v for the "20" and 6.3v to 8v for the "8". The toroidal transformer I mentioned has several outputs within those ranges.

From my understanding, I dont need exactly 15 /20 / 8 because there are voltage regulators that adjust the voltage anyways.

The board with the transformer on it is probably better to do wiring hookups perhaps?
DNT = Do Not Touch
8V = 8 volt output, I would say 6.3VAC-8VAC is good
15 = 15 volt output, I'd suggest anywhere from 9VAC to 15VAC
20,20V = 20 volt output, I'd suggest anywhere from 14VAC to 20VAC

Do not go higher than the suggestions in my opinion.

BTW
only the 8V output needs to go to 1A, the other two outputs probably 200-500mA is sufficient.

and still... how did that trace on the transformer board burn out? Fed it 220V without changing from 120V or something?

[vrasp]

Re: Fixing a laboratory waterbath

Hey guys,

So I got my toroidal transformer with the specs I mentioned above. Below is the datasheet and a picture of it.

https://ibb.co/h89g9mR
https://ibb.co/JpkR1wH

I am currently doing some reading to try to understand how it should be wired. For example, on the primary, both wires are black. Which should be connected to 120v and COM? Does the dot polarity matter?

Any help would be appreciated.

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

The dot polarity does matter but for your case it shouldn't. Does the transformer actually indicate which one is the dot?

You probably should use the 7.8V output for the 8V output instead of the 8.3V.

I'm surprised, these toroidal transformers are typically quite expensive due to the human labor needed to make them... Unless if they're trying to get rid of them...?

[vrasp]

Re: Fixing a laboratory waterbath

The dot polarity does matter but for your case it shouldn't. Does the transformer actually indicate which one is the dot?

You probably should use the 7.8V output for the 8V output instead of the 8.3V.

I'm surprised, these toroidal transformers are typically quite expensive due to the human labor needed to make them... Unless if they're trying to get rid of them...?

I think this store I got it from has lots of second-hand stuff. This transformer cost me $4. Not sure why it's so cheap.

The transformer does not indicate which one is the dot, but I suppose (1) is the first black wire (which has the dot), far-right in the picture, and each wire to the left of that is 2, 3 etc.

So far, I wired (1) to 120v and (2) to neutral to see if it works. I am getting the correct voltages on each secondary so that's good (I was worried it was broken since it was so cheap).

When looking at the diagram below, how do I know where to put the neutral in each pair?

[vrasp]

Re: Fixing a laboratory waterbath

Ok so after some more reading, I now understand that with a bridge rectifier, the polarity doesn't matter as the output will be the same. On the board in the picture above, both the 15v and 20v have a bridge rectifier (the 20v bridge rectifier being on the other board in previous pictures). However, I don't see a bridge rectifier on the 8v. Does it matter which way the wires go in this case?

[vrasp]

Re: Fixing a laboratory waterbath

Guys, thank you so much for your help! I wired everything and... it works!






Now i m back to the original problem which was that the screen turns off when moved slightly. I am going to try to find if there is a bad connection somewhere.

I especially want to thank eccerr0r without whom I probably wouldn't have been able to fix it. If there is any way I can thank you please let me know!

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Varsp

Very nice repair job another device not going to in the landfill
Check ALL of the board soldering joints for cracking and if they are dull looking resolder them

eccerror very good advice on the transformer selection I was not completely convinced that it would work because of the difference between the two transformer taps

This has been a very good information about substitution of a transformer

Thanks again for the information

[eccerr0r]

Re: Fixing a laboratory waterbath

Lots of mishmash responses here:
AC has no polarity per se - but does have a phase. If even the phase doesn't matter then you can connect it any way you want. If phase does matter, then you can't.

Haste makes waste. Be careful of what you think is the problem - verify it to know actually understand the problem.

Now to debug the rest of the problem: things that need to be answered: When the display turns off, what else turns of, does the heater shut off? The pump? Neither?
Does the meter change as it turns off? Blinking? How about the other lamps and indicators?

Those contacts between the board are always suspect but since you did reseat them by disassembling them and reassembling them, it may not be the issue.

TBH I wish everything was less than 3-layer PCBs... and have electronic surplus stores near where I live...

[vrasp]

Re: Fixing a laboratory waterbath

I ve noticed some other issues. The knobs to change the temperature dont work. The water heats up to 35.8 celcius which is one of the preset setting. It seems stuck on that setting. At least the heater works!

I m going to check for the things you mentioned, take it apart, and try to understand how it works.

I ll probably be back with some questions unless I manage to fix everything on my own (unlikely).

sam_sam_sam thank you. Wouldn't have gotten there without the forum!

[petehall347]

Re: Fixing a laboratory waterbath

check the temp sensor

[vrasp]

Re: Fixing a laboratory waterbath

check the temp sensor

Thanks petehl347. I want to add that the temperature displayed is correct. In fact the water It heats up slowly from room temp to 35.7 celcius.

[eccerr0r]

Re: Fixing a laboratory waterbath

Make sure the knob pot is still connected properly, measure the voltage between the center wiper and one of the side terminals as you change it - does it also change? Linearly throughout the range?

Also what if you pour hot water into the system, does the system measure the temperature correctly?