Sciencemadness Discussion Board

US Hotplate/stirrer frequency conversion

BenZeen - 12-8-2010 at 20:14

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

densest - 12-8-2010 at 20:49

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]

BenZeen - 12-8-2010 at 21:11

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

psychokinetic - 12-8-2010 at 21:51

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.

BenZeen - 13-8-2010 at 00:50

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

zed - 13-8-2010 at 16:46

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.



densest - 14-8-2010 at 10:52

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%)).

peach - 14-8-2010 at 13:57

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.


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.


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]

zed - 14-8-2010 at 19:34

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]

densest - 14-8-2010 at 22:26

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.

peach - 15-8-2010 at 07:07

@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. :D



John

[Edited on 15-8-2010 by peach]