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Old 06-18-2021, 05:17 AM   #121
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Default Re: Fixing a laboratory waterbath

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Originally Posted by eccerr0r View Post
The resistor may still be good. Measure it.
I don't understand why it smoked, sure you didn't short something else by mistake?
Pretty sure it s fried. It s complerely black and has no resistance. The capacitor below it is fine. I checked it. Also ignore the black stuff on the top part of the black connector on the right. I melted some of the plastic when I resoldered the capacitor above it.

The only thing I did was hardwiring the one of the outer metsl to the center metal of the rotary switch. That s when it got smoky.


Last edited by vrasp; 06-18-2021 at 05:25 AM..
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Old 06-18-2021, 05:52 AM   #122
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Default Re: Fixing a laboratory waterbath

Could it be that the center of the resistance was toucheing the capacitor pin below it and got a big discharge of current? It does look pretty close.

I have to go to the store to get a new one. I m still trying to figure out what the ohm might be.

By the way, in red is what I wired together before the resistor blew. This is the other side of the rotary switch.


Last edited by vrasp; 06-18-2021 at 06:14 AM..
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Old 06-18-2021, 08:13 AM   #123
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Default Re: Fixing a laboratory waterbath

I don't believe the jumper caused the resistor to fry. 12V doesn't go to the display board, it's solely to power the op amps and other analog circuitry and should be very low current.

Sorry for the blunt words but I don't know what you're doing with the device, seems you're causing random shorts here and there likely due to sloppiness that started with the track on the transformer. Need to rectify this right away before going onward.
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Old 06-18-2021, 08:35 AM   #124
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Default Re: Fixing a laboratory waterbath

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I don't believe the jumper caused the resistor to fry. 12V doesn't go to the display board, it's solely to power the op amps and other analog circuitry and should be very low current.

Sorry for the blunt words but I don't know what you're doing with the device, seems you're causing random shorts here and there likely due to sloppiness that started with the track on the transformer. Need to rectify this right away before going onward.
No offense taken! I appreciate anyone taking their time to help me.

Before this resistor fried I soldered each pin to wires from the green/blue board to the display board to fix the connection issue. It fixed the display issue. I m going to check everything i did to make sure each pin is isolated from the other ones.

What I need help figuring out is what to replace this resistor with.
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Old 06-18-2021, 11:51 AM   #125
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Default Re: Fixing a laboratory waterbath

It's probably no more than 10Ω or so, but without a good initial photograph it's tough to tell what it was.

And here again I listen like a scientist. When someone says "has no resistance" I think ...

What has no resistance?

A SUPERCONDUCTOR has no resistance! Therefore 0 ohms... so it's conductive.

If it's conductive, the resistor is still somewhat working.

NO PROBLEM!

So do you really mean "open" or infinite resistance or do you actually mean something low resistance, which would mesh with a guess of a something between 1Ω to 10Ω resistance?
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Old 06-18-2021, 12:44 PM   #126
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Default Re: Fixing a laboratory waterbath

data sheet for b40c suggest 1.0 ohms minimum protector resistor .

https://www.vishay.com/docs/88533/800dm.pdf

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Old 06-18-2021, 02:53 PM   #127
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Default Re: Fixing a laboratory waterbath

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Originally Posted by eccerr0r View Post
It's probably no more than 10Ω or so, but without a good initial photograph it's tough to tell what it was.

And here again I listen like a scientist. When someone says "has no resistance" I think ...

What has no resistance?

A SUPERCONDUCTOR has no resistance! Therefore 0 ohms... so it's conductive.

If it's conductive, the resistor is still somewhat working.

NO PROBLEM!

So do you really mean "open" or infinite resistance or do you actually mean something low resistance, which would mesh with a guess of a something between 1Ω to 10Ω resistance?
Quote:
Originally Posted by petehall347 View Post
data sheet for b40c suggest 1.0 ohms minimum protector resistor .

https://www.vishay.com/docs/88533/800dm.pdf
This is gonna sound like I m not measuring properly but my multimeter now measures 77 ohm across that blown resistor. I swear yesterday it said 0 ohm. Anyways.



Ok so I need something between 1 ohm and 10 ohm.

Does it matter if I dont get the exact same one that blew since there is a bridge rectifier? Could I just use a 5 ohm and be done?

Also, about that hardwiring, did I do it right? I connected those pins to force the manual temperature option but I wasnt 100% sure about the pins to hardwire. That option is the fifth one so I wired rhe fifth pin to the center pin.
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Old 06-18-2021, 03:14 PM   #128
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Default Re: Fixing a laboratory waterbath

you need to find the rest of the switch or draw out the schematic .
what value resistor supplies the other bridge rectifier ?

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Old 06-18-2021, 03:25 PM   #129
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Default Re: Fixing a laboratory waterbath

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you need to find the rest of the switch or draw out the schematic .
what value resistor supplies the other bridge rectifier ?
There is no resistor with the other bridge recitifer.
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Old 06-18-2021, 03:49 PM   #130
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Default Re: Fixing a laboratory waterbath

i see it has a fuse .
you can roughly work out the correct resistor from the voltage and expected current draw from the circuit after the bridge .
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Old 06-18-2021, 05:14 PM   #131
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Default Re: Fixing a laboratory waterbath

The resistance I offered was a ranged guess, it may very well be up to 77 ohms(highly doubt it, however, but who knows what the original is designed at), but also are you measuring out of circuit? Measuring in-situ might make it confusing.

As this circuit appears to be overspeccing the transformer and using the resistor to compensate (once again, a guess!) this may be a significant resistor here and 5Ω is too low of a resistance.
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Old 06-18-2021, 06:38 PM   #132
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Default Re: Fixing a laboratory waterbath

all it has to do is protect the rectifier and be capable of passing enough current for the circuits to work properly with a bit of overhead . without a schematic i am a bit lost .. i would take a chance and sub in an ammeter with say a 300ma fuse and briefly power it up and see what sort of current is being used . this like anything has its risks . some things just like to take whatever current is available .led.s for example .no idea about the display and if it has its own protection resistors or not .
i have done things like this by taking a guess at the resistor value and trying it to see how hot it gets . trouble is not knowing if there is now a problem that might burn the correct value resistor .
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Old 06-18-2021, 06:56 PM   #133
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Default Re: Fixing a laboratory waterbath

Sounds like a use for another lightbulb!
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Old 06-18-2021, 07:25 PM   #134
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Default Re: Fixing a laboratory waterbath

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Sounds like a use for another lightbulb!
good plan .
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Old 06-19-2021, 08:38 AM   #135
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Default Re: Fixing a laboratory waterbath

Thanks guys! I was going to try your suggestions. However, another bad news. I spent the day yesterday moving to my new appartment. While bring the waterbath to the new place, one of the pins on the Triac broke. This waterbath is very heavy, over 100 pounds easy. The pins of the triac must have been bent somehow and broke off. This is a picture of it. You can still see the pin and I have tried to put solder on it so I can hook up a wire but the solder is not staying on.



I did some reading about triacs. I m not sure what it s used for. Apparently it's used to switch the heating control? The good news is that I know it's a IT715 which is 800VRM and 15amps TO-220. Can I buy another 800v 15amps or are these other specs to look at?

Since I m not sure of the importance of the triac, I want to ask: Can I bypass it while I conduct the test with the lightbulb to figure out the resistance of the blown resistor? Or Do I have to install a new triac first?

This is quite frustrating. I really tried to be careful during transport.
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Old 06-20-2021, 08:12 AM   #136
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Default Re: Fixing a laboratory waterbath

Hey guys,

The triac has been replaced with a NTE5671 800VRM 20 amps.

I measured the amperage by connecting my multimeter where the resistor used to be and I am getting about 10 microAmps. Is this possible? In the picture I had removed the lightbulb but it was there as a precaution the first time I measured the current.

If it really is 10 microAmps, then based on the voltage formula V=R*I, if I want to drop the voltage by 4v (18v from the transformer to 14v to reduce load on the bridge rectifier), then:

4 = R * 0.000010
R = 400K

Seems a bit much...Am I doing this right?




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Old 06-20-2021, 09:01 AM   #137
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Default Re: Fixing a laboratory waterbath

No, I'd expect the proper 'on' current for the 12 and 20V lines to be measured in milliamps, if it's measured in microamps, something is wrong.
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Old 06-20-2021, 09:06 AM   #138
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Default Re: Fixing a laboratory waterbath

I want to add that when I turn on the waterbath without connecting those wires together, the display displays 0s, and when I connect the wires, it shows the water temperature (which is correct). If I hardwire the switch (i just pinched the center and outer metal with tweezers) the temperature goes up. It seems like everything is working as it should. This current is confusing.
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Old 06-20-2021, 09:08 AM   #139
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Default Re: Fixing a laboratory waterbath

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No, I'd expect the proper 'on' current for the 12 and 20V lines to be measured in milliamps, if it's measured in microamps, something is wrong.
Maybe it s not on. Let me see if pushing a button changes anything
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Old 06-20-2021, 09:37 AM   #140
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Default Re: Fixing a laboratory waterbath

I ve tried a few things. The highest current I m getting is 0.x milliAmps, so in the hundreds of microAmps.

I hardwired the manual temperature switch and I m able to adjust the temperature setting. The water temperature goes up as it should. It looks like everything is working as it should.
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