Sciencemadness Discussion Board

hotplate not working?

jwarr - 15-3-2010 at 18:16

So I'm a bit torn at the moment. It seems my stirrer hotplate isn't working as expected. The stirring works just fine, but even on high heat, it cannot even boil a beaker of water (only reaches ~92C). At first I thought maybe it was just taking a while to heat up, but after three hours an oil bath was staying at approx 90C. Now I'd hate to replace something that seems so simple.

Any suggestions on what could be wrong and/or how to fix it?

Eclectic - 15-3-2010 at 18:24

Perhaps you would like us to psychically repair your automobile also? :P

The heater has a high resistance from being burned out, or a bad wiring connection, or the thermostat is defective, contacts maybe corroded...... But if you could not come up with these failure modes on your own, you probably should not be messing inside electrical appliances...find a solder jocky?

hissingnoise - 16-3-2010 at 04:38

Yeah, a hotplate is a simple thing - just about anyone should be able to figure it out.
I agree with Eclectic; get someone with some expertise or study a bit!
It's that easy!

UnintentionalChaos - 16-3-2010 at 05:51

Did you buy a cheapo one? They're often very low power and thus basically useless. They can claim a max surface temp of 350C when nothing is on it, but fail to heat things up to anywhere near that temperature.

[Edited on 3-16-10 by UnintentionalChaos]

jwarr - 16-3-2010 at 09:04

Thanks for the info guys. OK so don't judge me here. I was testing it in the bathroom (tile floors, nothing flamable, etc). I think that was the problem. Perhaps some sort of voltage regulator on the outlet? Because when I plug it into an outlet in the garage, it heats up an oil bath to about 120 C in under 5 minutes. Anyway, it's not a cheapo one per se, but I did buy it used.

On to the next problem. Seems now the stirrer is working intermittently. I can see the magnet under it turning, but it slows down or stops every 30 seconds or so. I hope I'm not asking for a psychic evaluation :D But is it probably something related to the wiring once again? perhaps a blown potentiometer or something? Once again, any suggestions would be helpful.

I think I'll end up getting a new one, simply so I can try fixing this one without worrying about the outcome. Anything obvious to look for when I open this thing up? (Frayed wires, etc?)

Thanks again for all the help guys!

Eclectic - 16-3-2010 at 15:01

IF the stirrer is slowing down when the thermostat cycles the heater on and off it's likely a bad/corroded/high resistance connection somewhere.
That would account for both issues.

ninefingers - 17-3-2010 at 06:38

Mine heats well, but has the start/stop stirrer problem. I think the rheostat on the speed control is a bit dirty, Plus, remember motors need a big Oomph to start; so you may need to give it about 1/2 speed to start it in some viscous solutions; then throttle back. I also fund there is a "sweet spot" on the magnet's position where it starts/works best. Usually exact center of the plate. And, I use plastic ware when stirring unless I need to be above 90 C; as I've broken a flask with a stir magnet.

I doubt there was any "regulator" on your bathroom outlet. Possible a bad connecting or poor wiring or using a small extension cord. The garage was probably wired for a Real 15 amps, like for tools.

Contrabasso - 17-3-2010 at 11:27

Time I think for a competent person to look at the contacts and wire joints to see if there are dirty connections causing problems.

Also for a competent person to check whether the thing is internally wired correctly- could it be that the motor is controlled by the thermostat hence the cycling when the plate gets hot.

Also to check that the heater is wired in series with the stat but that the stirrer only runs off the speed controller (not the stat as well.

You may or may not be that competent person!!!

jwarr - 17-3-2010 at 20:02

Thank you once again for all the help guys.

Contrabasso, I am certainly not that competent person. So I guess the search is on :)

woelen - 17-3-2010 at 23:47

If you really are into home science and if you value your money for other things as well, then try to become that competent person! Just try to understand how such things work, take it apart to see its internals. The worst thing which can happen is loss of the device, but even then it has shown its value, because you have learnt a lot from it.

All apparatus in my home lab is second hand and some of these things were broken (e.g. my analytical Mettler balance). I purchased that balance for just EUR 40 and I managed to repair it (some switch was broken, a lot of dirt was in it). Of course I risked loosing EUR 40 with this act, but sometimes you just have to take such a risk. And indeed, I also once completely ruined a heating mantle of EUR 50 (the thin heating wires just fell apart when I removed the fabric mantle), but such is life ;) I now have another second hand heating mantle and if that happens to fail then I'll take it apart again, now knowing that I have to be more careful with the heating wires.