stopgo
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Registered: 24-12-2011
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Digital VWR Hotplate Problems!
I recently purchased a digital hotplate/stirrer off of ebay. The item was described as "does not power on". Once I examined the unit I could see
that the power leads connecting the a/c power to the pcb were melted and the board was bunt. After repairing the board, cleaning up all tracers and
doing a rework I thought it was fixed. After doing a test run I found out that the hotplate would start to warm up but after about 3 minutes it
would shut off. When it shuts off the digital temp readout goes from the temp setting to "E1".
A few things to consider:
1. When I unplug the thermostat from the board the plate still shuts off.
2. I wanted to check if it was a faulty thermal overload switch so I unplugged the unit and plugged it back in while still hot.
- This allowed me to continue heating the plate for about the same amount of time as a cold start.
3. When heating it up by unplugging a few times the stirrer no longer allows variable speed. It just reverts to full speed.
- From a cold start the stirrer works as intended.
Any help would be great! These things are about $500 and I'd love to have a working hotplate at the price I paid.
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peach
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Registered: 14-11-2008
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If it has fuzzy / PID logic control of the temperature (and it likely does have a PID controller), the three minutes until the error message may be
the lag on the fuzzy logic eventually giving up and an error message. My fuzzy logic thing follows the same path, the thermometer has to be reasonably
well connected to the heat coming from the plate or, after a while, it'll give an error and shut the element off. That is normal. However, yours doing
it all the time is not.
The fact the heating circuit is still coming on is excellent. The fact you can unplug the plate, plug it back in and it will give another 3 minutes of
heating is also good, as it means it likely is a feedback problem and possibly not too expensive to fix, versus the boards (integrated circuits)
themselves actually being burnt out (those are expensive to replace).
Find the thermostat and follow the leads back.
Is there any sign of overheating or damage anywhere around there? Does anything seem loose? Have you plugged in an external thermometer and are you
100% sure this particular model does not require one if you haven't? A number of the newer digital plates need the external
thermometer to be connected. And it needs to be thermally coupled to the element somehow for the plate to remain on (e.g. dunk it in
a beaker of warm water sat on top of the hotplate).
If that isn't the case, the next thing to think about would be, is the internal and / or external thermometer damaged? You may only be able to tell by
replacing it / them. If you know what kind of sensor it is using (a thermistor, resistive or thermocouple element), you can try replacing it with a
cheap generic part if only to test it prior to buying a VWR part.
The stirring going out of control, I can't think of much for that beyond it perhaps having a feedback from a sensor detecting the speed, and that
sensor is also damaged. It is worrying that both it and the temperature control are going funny. As that tends to suggest it is not the sensors
themselves, but the controller.
How badly burnt are we talking with the boards? That (or whatever caused it) is obviously the main candidate for the failure of the other things. And
it may even reoccur if not corrected. You say it didn't power up when you got it and you had to do some rework, you mean solder had actually undone or
bridged? Sounds like something kind of drastic has happened in it's past (e.g. wrong fuses put in, liquid's been spilled on it, it's shorted and
roasted it's own lead). Have you got some photos?
[Edited on 24-12-2011 by peach]
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Texium
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Thread Moved
20-11-2023 at 12:00 |
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