BenZeen
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US Hotplate/stirrer frequency conversion
Hi everybody,
I have become the recent proud owner of my first hotplate/stirrer which was purchased of Ebay from the USA. I live In Australia so i had to buy a
step down converter to change the power from 240v to 110v to run the appliance. I thought this would be all i had to do, but when i fired up the
stirring function there was no variation between low - high, it seemed to just have one setting - full blast. turns out this is due to the frequency
of the Ac being 50Hz in Aus and 60Hz in the USA.
So my question is how can i solve this issue? It would be nice to have control over the speed of my stir bars. Thanks
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densest
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This has nothing to do with line frequency. What kind of step down converter did you use? Is it a transformer, i.e. does it weigh 500g or more? If
not, you've unfortunately burned out the control electronics. It is probably repairable. The main triac has probably been fried. You can either
replace it with one rated 2X the voltage (not recommended because other components might die) or get a transformer and replace the triac with an
identical part.
There is a "converter" for lamps and some small motors which is just a rectifier. It removes 1/2 of the AC waveform so the average is 1/2 as much,
which will prevent an incandescent bulb or a small "universal = AC/DC" motor from burning out. It is pretty much guaranteed to destroy any electronic
device. It is easily identified because it weighs almost nothing.
[Edited on 13-8-2010 by densest]
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BenZeen
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Ok, I thought it was the frequency because i called an electrical repair business and thats what he said.
I have a 300W step down transformer, yes its a small heavy box.
I have a feeling that some component has burnt out and needs replacing so i will look into that first. Thanks for the feedback
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psychokinetic
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Excuse the potentially silly question, but are you sure it converts the right way?
Sounds silly, but I've seen an old US PS2 get turned into a doorstop by simply having the conversion switch in the wrong setting.
“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found
the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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BenZeen
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Hi psychokinetic
my step down transformer is a small box that has a cord that plugs into the wall (australian socket) and two female (American) sockets that my
hotplate plugs into. I was unsure if 300W would be enough for my unit, but it seems to be enough power. I am pretty sure the triac has been burnt out
like Densest said, because the knob for setting the stirring speed doesnt work even though the thing stirs just fine (albeit at max power!)
hopefully wont cost too much to fix
Benzeen
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zed
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Often, used items purchased via e-bay, have minor problems.
There is some possibility, that the stirrer rheostat is malfunctioning. Likely, it is a wire-wound variable resistor, that over time, has become
crudded up with conductive particles. As such, it has become an "invariable non-resistor".
Open up your stir-plate, and thoroughly clean out the spaces between those wire windings. Put it back together, and test the RPMs.
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densest
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Were life so simple - a rheostat fails "open", passing no current. The symptom of the motor running full speed can't happen from an open circuit. A
simple series rheostat hasn't been used for this sort of speed control for many decades. The standard 2 or 3 semiconductor motor speed controls all
fail to "off" if the control potentiometer (rheostat) is open. I suspect the safety agencies would not like controls that easily fail to full speed
from a control failure.
Power semiconductors (SCRs, thyristors, triacs, FETs, BJTs) fail shorted 95%+ of the time. Anything which disturbs the impurity diffusions will do
that - overheating (including locally by static discharge) is usually the cause. You can make one fail open - you can melt a bonding wire if you can
drive 100s of amperes through it for a tenth of a second or so.
So if the stirrer was made in the last 40 years, it's got a semiconductor speed control. They can fail either off (control circuitry failure or solder
joint, usually) or full on (power semi failed (90%) or conductive crud on the control circuit (10%)).
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peach
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Quote: Originally posted by zed  | Often, used items purchased via e-bay, have minor problems.
There is some possibility, that the stirrer rheostat is malfunctioning. Likely, it is a wire-wound variable resistor, that over time, has become
crudded up with conductive particles. As such, it has become an "invariable non-resistor".
Open up your stir-plate, and thoroughly clean out the spaces between those wire windings. Put it back together, and test the RPMs.
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I'm genuinely embarrassed to ask you this again, and am assuming you've seen it, but I've not seen a rheostat (wire wound variable resistor) in any
piece of consumer electronics for loooooooooooong time, particularly ones with triacs in them. I now know you do actually mean a rheostat, as you've
mentioned the coils and them clogging up, but can you give a specific example of what you've seen one in, preferably the most recent? That's not a
loaded question, I'm genuinely interested to know who's still using potential dividers over PWM and such.
| Quote: Originally posted by densest | Were life so simple - a rheostat fails "open", passing no current. The symptom of the motor running full speed can't happen from an open circuit. A
simple series rheostat hasn't been used for this sort of speed control for many decades.
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Thank science someone else mentioned that, I thought I was missing something there. However, I'm willing to believe Zed may have seen one in a
modernish plate, still.
But myself, that's 'before my time' technology really; the world I know is tiny black plastic things. Personally, I do enjoy the mechanical feel and
look of the older electronics (mercury train station rectifier, awww yeah!).
I'm going with option C, and the simplest and most likely (in my opinion, of coarse), there's crap on the tracks of the potentiometer or it's
corroded. Open it up, stick a ohm meter (lend one from a neighbour if you don't have one, take it to school / uni [electronics lab] if you're still
there) on one terminal and the test probe on the wiper. Twirl the dial and watch for it rising and falling smoothly. If it's feeding a simple PWM
circuit, it'll go up and down in a linear fashion. Check that the maximum excursion of the wiper goes from around zero to the full (static) reading
you get between the two ends of the pot.
If it's skipping all over the place, or not changing, the pot is dirty or ruined.
I have largely ruined mine (I suspect) after HCl(g) entered the plate and has messed with the tracks. I can heard crackling if I set the plate to it's
maximum. Whilst everyone else hunts for explosion proof methods, mine's like it's got it in for me.
The stir motor on plates that don't have closed loop feedback on the RPMs will also go ape shit if they're not connected to a load (a flask with a bar
and some water in it). It's like running your car and pressing the accelerator down as far as you would normally, but with the wheels off the floor.
The RPM's will rocket up, because the engine isn't pushing against anything to slow it down.
If mine decouples from the bar mid reaction, the motor will spin up to it's max (even set to 1) and the whole setup will shake like a washing machine
on a spin cycle.
So, first test, get a beaker and bar on there with some water in it.
Then, check the pot inside. If it's wonky, blow into it, hard. If that fails, try an air duster or air compressor. That fails? Buy a new pot (not
expensive at all, I might even have one you can have for free).
Never underestimate the possibility for loose connections and corrosion. There are many times engineers will sit there, baffled for hours, then
realize one connection just isn't screwed into the board properly. The terminals on battery powered things will corrode up and need a quick sand to
get going again.
One impressive example I had was a contactor (basically a big switch controlled by a solenoid to connect and disconnector the mains). It was working
absolutely fine before I unscrewed it from the board it was on. Then, nothing. Turned out an invisible layer of oxide typically forms on them, so
(over two decades of being sealed away) it had insulated it's self to only accept the wires the way they were screwed in. Had someone not pointed out
he'd had this problem, I'd probably still be wondering about it now, years later.
The frequency thing is probably just someone trying to block a potential refund. You didn't buy the converter from there did you? If yes, I think we
have an answer
John
[Edited on 15-8-2010 by peach]
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zed
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Cole-Parmer, model 4658.....I bought a couple of Hotplate/Stirrers, very inexpensively on e-bay last year. The 4658 had a hitch in its get-along. I
cleaned up the windings on the stirrer controller, adjusted the motor mounts a little bit, and now it purrs.
Nice clean unit. Mechanically simple. Repairable. I don't know how old it is, but it seems to be in fairly newish condition.
Manufacturers aren't doing us any favors by using solid-state controllers. Cheaper to build, harder to fix.
I also acquired a VWR 370, at the same time. Seems like I got both units for less than a hundred dollars total, including shipping. I just cracked
open the VWR, and it also has a rheostat....but it is integrated with some solid state stuff.
The Cole Parmer appears to have more quality than the VWR, so....maybe I should sell off the 370, and pick up another Cole Parmer 4658.
Both units probably hail from the 90s.
Jeeze, don't you guys use variacs to control your heating mantles?
http://shop.ebay.com/?_from=R40&_trksid=p3907.m570.l1313...
[Edited on 15-8-2010 by zed]
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densest
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Sort of on topic: I find that stirrers always "want" to go too fast. What would be useful is something that went from 20 RPM to 300 RPM. Most top out
at closer to 900 or 1800.
If the malfunctioning stirrer is still going too fast with a load of a stir bar and water, before messing with potentiometers, check for a short
between the motor leads and the wall plug pins. A cheap multimeter is $US 10 - the Powers That Be in the British-influenced world can't get that to be
more than $30 (I hope). If so, check to see if www.harborfreight.com will ship to you. You can buy better tools than they offer, but you would have an extremely hard time getting them for less
money.
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peach
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@Zed
I both agree and disagree with that. Solid state can do incredible things (far beyond what most people would understand, but that's not saying much
really considering they don't know the difference between a volt and amp) compared to vintage gear for next to no money, and it can be as or more
reliable depending on how it's designed. I strongly agree with the repair issue. I had an IKA plate blow a single surface mount resistor. Replacement
cost of said resistor, around $0.01, and something I could identify and solder myself.
I asked them for the nominal resistance on it and they flat out refused to tell me, saying they didn't give out schematics, despite this being for a
single resistor (and me saying that's all I wanted). They then got quite shirty with me and started going on about it being my fault and not their
problem, and that I needed to buy a few hundred pounds worth of complete PCBs to fix it, on a plate that retails on the painful side of four digits.
Unnecessary, even with solid state. It' a single resistor, not a custom blown, proprietary FPGA chip.
If you like old electronics, you'll love this 30 second video
As with science, people will see my glassware and immediately assume 'illegal', 'drugs', 'bombs', 'toxic', 'precise'. Not knowing it could just be
water in there or how organics will spew out all kinds of rubbish besides product. With solid state, similarly, it goes beyond what they can assess,
so they assume it's all the same, precise, perfect, unquestionable. When, in reality, two people can design a circuit to do the same thing for the
same price and using nearly identical parts (or identical), one will barely function, the other will never fail. It can even be as simple as who's
laid the PCB out. And, of coarse, digital does make mistakes, that's why checksums, package fail flags and latency appear.
In terms of science, particularly physics, that vintage gear still has a place, even over solid state. You're not squeezing 15kV for ionization work
out of a chip. I've been watching videos and reading about Microwave Oven Transformer (MOT) stacks recently. They can output up to 15kV at around
an amp; far more than an Neon Sign Transformer (NST), and free. The voltage can be continually varied by driving them with a variac,
but I'm going a step further and looking at sticking a simple PWM or microprocessor controller on the front end, for a digitally controlled, fully
variable (current and voltage), possibly computer controlled and logging, 15kV 1A supply, for dumpster diving prices. Put a resonant tank on the
output and you have hundreds of thousands of voltages and huge amperages, albeit until something pops.
Variacs for mantles. Given the abuse of the heating controls some people seem to engage in, they may as well plug it directly into the wall. "It isn't
on fire yet, so I can turn it up". I expect there are numerous labs still buying and using the variac / mantle method over IKA's plates. MIT are using
them in their digital lab manual videos.
Thanks for the names on those plates, as I say, I was genuinely curious, not saying you're wrong.
@Densest
Indeed, you've mentioned this before, and you are entirely correct. High stir speeds are good for water neutralizations and such, where I want lots
and lots of stirring to get a smooth, accurate, stable pH. But, as soon as the solution thickens (which will happen even with neutralizations),
vvvvvvVVVVVVVVV, the bar's jammed and the motor's off into it's own vibrating world.
Can't be bothered spending a few dollars more on a pulley? Darn...
I believe, having spent arguably too long on forums about high end audiophile turn tables for LP's, there is actually a simple equation for working
out the ideal drive ratio for a given RPM to produce the maximum input / output transfer efficiency across the mechanical impedance; as there is for
electrical impedances in load matching transformers and the gearbox in a car.
I have a QuickFit overhead, vacuum capable stir shaft in the post actually. 99p from a liquidation. WIN! That can go on my QuickFit flanged, bottom
take off, jacketed reaction vessel from the same liquidation. Win, win, win! My stirring is moving up in the world, quite literally. No one can accuse
me of not putting some dedication into these endeavors now.
I'll be building my own stir setup. I'm not paying IKA £1k for some AC powered, brushed, sparking piece of shit in a blue die cast for it to go
mental and have them tell me they want to borrow my credit card again.
We can also get multimeters dirt cheap, around that price or less. I mentioned school and uni as, whilst I was at York in the UK, I could just wander
into the electronics labs and get on with something. 50Vdc / 5amp / no mains limits. The staff were more than happy to use the SMD gear for things
nothing to do with the university; they helped bypass the SMD boot loader on my PSP without even asking why. And they helped with the IKA plate.
The only time they complained was when I managed to borrow a 1kV supply from one of the molecular beam epitaxy staff in the research department. They
didn't like that being in there, despite the 40mA current limit.
The biology labs, where I was at the time, were all keycoded and we needed swipe cards to get into the ones we were allowed in. Due to the benches
being lined with bits of gear costing £k's a piece. I also made it through the more secure swipe card doors by just hanging around them like a bad
smell, waiting for a graduate to come through, and wandering in. Inside, there was the departments engineering workshop, where I discovered a single
lonely engineer who turned down my B24 PTFE tapers, again, without even asking what they were for; because he was bored out of his mind and annoyed
that all the other staff were on holiday.
I've had two Fluke Scopemeters. They are nice. 
John
[Edited on 15-8-2010 by peach]
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