Fixing a laboratory waterbath

[eccerr0r]

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.

[petehall347]

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 .

[vrasp]

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 ?

There is no resistor with the other bridge recitifer.

[petehall347]

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 ?

[vrasp]

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?

data sheet for b40c suggest 1.0 ohms minimum protector resistor .

https://cdn.badcaps-static.com/pdfs/...dfc8538c4d.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.

[petehall347]

Re: Fixing a laboratory waterbath

data sheet for b40c suggest 1.0 ohms minimum protector resistor .

https://cdn.badcaps-static.com/pdfs/...dfc8538c4d.pdf

[eccerr0r]

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?

[vrasp]

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.

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.

[eccerr0r]

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.

[vrasp]

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.

[vrasp]

Re: Fixing a laboratory waterbath

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.

[eccerr0r]

Re: Fixing a laboratory waterbath

The resistor may still be good. Measure it.
I don't understand why it smoked, sure you didn't short something else by mistake?

[vrasp]

Re: Fixing a laboratory waterbath

tell us what it connects to ?

It's getting 20v AC from the transformer and on the output it goes to the bridge rectifier which goes to the voltage regulator.

appears to be a kind of jumper... which could explain the need for 20V as there'll be a drop across this resistor to reduce heating of the regulator...

The voltage regulator is a 7812. If I am able to find out more or less how much current is in this circuit I can calculate the resistance needed to lower the voltage from the 18v of my toroidal transformer to about 13-15v. Is that a good approach? I could connect my multimeter in series to measure the current where the resistor was, and calculate a resistance that would drop the voltage by, let's say, 5v.

[eccerr0r]

Re: Fixing a laboratory waterbath

appears to be a kind of jumper... which could explain the need for 20V as there'll be a drop across this resistor to reduce heating of the regulator...

[petehall347]

Re: Fixing a laboratory waterbath

tell us what it connects to ?

[vrasp]

Re: Fixing a laboratory waterbath

go through your pictures to find a good close up of the resistor .

I did, it s really difficult to see.

The bands look like Red brown something (grey?) brown red. Of course the blown resistor had to be the one that is the least visible in the pictures.

Is there a trick to figure out the resistance of a blwon resistor before it blew?

[petehall347]

Re: Fixing a laboratory waterbath

go through your pictures to find a good close up of the resistor .

[vrasp]

Re: Fixing a laboratory waterbath

The capacitor is fine. It is the resistance that blew.



Are you able to see the lines? I can't tell what ohm this is.

[vrasp]

Re: Fixing a laboratory waterbath

if you're inclined to look for the small metal piece lost, it probably looks like a bent leaf.

The unfortunate thing is that getting an exact match will be hard...mainly to fit in the board. Rotary select switch is pretty much is all you can look for, and it appears to be a 5 throw 2 pole (DP5T) rotary (of which it really just needs 1 pole but the original designer appears to have cheesed the switch to make it easier to route PCB tracks?) perhaps if you can't find the leaf it's easier to just hardwire it to the adjustable setting?

How do I hardwire it? I thought I just had to wire the fifth outer pin (that corresponds to the manual temperature selection) to the inner pin (golden metal ring). Things started getting smoky and I turned it off immediately. The smoke came from one of the capacitors or the resistance next to it. I think I reacted quickly enough. If I have to replace it it's no big deal. However, I don't understand what pins to wire.

Basically, I did this:

[vrasp]

Re: Fixing a laboratory waterbath

if you're inclined to look for the small metal piece lost, it probably looks like a bent leaf.

The unfortunate thing is that getting an exact match will be hard...mainly to fit in the board. Rotary select switch is pretty much is all you can look for, and it appears to be a 5 throw 2 pole (DP5T) rotary (of which it really just needs 1 pole but the original designer appears to have cheesed the switch to make it easier to route PCB tracks?) perhaps if you can't find the leaf it's easier to just hardwire it to the adjustable setting?

I didnt think of that! That would save me the trouble of looking for a proper rotary switch. I m gonna see if I can do that right now.