Help with variac

[barbasol]

I don't have a lot of experience with electronics so I hope you guys can help a brother out. I am using a small scale still, 4 gallon boiler 1 inch cooper column. My heating element is a 1000w 120v 60hz Aroma hot plate. I've had a lot of problems maintaining constant temperature (cycling of power created 20 degree temp surges) so I bought a variac transformer. Input is 110 to 120 volts, output from <4 volts AC to line, 4 amp fuse. It says it is rated 500 VA max.

Will this variac handle 1000w from the hot plate/provide enough power to the hot plate?
Is this setup generally safe?
Do I have to disable the thermostat in the hot plate or can I just leave it on high?

Gonna go for second run of UJSM this Friday and would love a little better control.

Here is a link to the variac I got
http://www.frys.com/product/4638758;jse ... IN_RSLT_PG" onclick="window.open(this.href);return false;" rel="nofollow

This is a second one they had that was more expensive
http://www.frys.com/product/5422799;jse ... IN_RSLT_PG" onclick="window.open(this.href);return false;" rel="nofollow

Thanks guys

[eternalfrost]

for a resistive load like a hot plate, you can just directly convert VA to watts. so the short answer is no, you wouldnt want to use it above 500W.

varics work great tho i use one. mine is 10 amp. it is slightly underrated for my element but works fine as long as you dont turn the power over 95% for too long or the fuse will pop. measure the resistance of your heater and use ohms law to find how much current youll draw and get a variac that can take it. the current is what frys things

you can get them pretty cheap used off ebay. you dont need ones with the dial etc etc, just the bare bones type. you can find them for $50-100 last time i looked for one

[barbasol]

Thanks eternalfrost. So it sounds like the second link I posted would do the trick as its rated to 1000w max? Otherwise I should try to find a different one on ebay.

Could I use the variac I have but only to 50% output, and do you think that would be enough power to get to temp?

[rad14701]

Based on the experience of others, you'll most likely be needing more power in the future so you might as well go for a larger variac from the get go... You may want a 1500W element some day...

[Hack]

and you'll likely want to upgrade to a larger boiler than four gallons.

[eternalfrost]

Could I use the variac I have but only to 50% output, and do you think that would be enough power to get to temp?

things are rated in terms of power but its a bit misleading in this case. "power" isnt an intrinsic property. P = I^2 * R = V^2/R. resistance is the intrinsic property of the element, it wont ever change. all you have control over is the applied voltage, which causes a current to flow through the element.

use P = V^2/R to calculate the resistance of the element without an ohmmeter.

it is this current that frys components and starts fires. so what you really have to look at is how much current your drawing. so just use ohms law V = I*R above and plug in the voltge your applying (a percentage of 120 from the wall) and the resistance of the element to find the current. this current must be under the rating on ALL parts of the circuit (includes all switches, wires,including those inside your walls)

[cemik1]

... resistance is the intrinsic property of the element, it wont ever change

It changes. Heater is unlinear element but generally the higher temperature - the higher resistance. It must me taken into account in precise calculation. But roughly to decrease current by twice (variac power is twice less than heater) you must take half of voltage. It gives you only 250W of power. So bigger variac is strongly recomended. Using variac you should bypass heater temperature regulator.

[eternalfrost]

It changes. Heater is unlinear element but generally the higher temperature - the higher resistance.

this is very true. but does not apply here. inside a boiler, the element wont get very hot. even at full blast it wont ever get much over the boiling point of the wash since it is submerged.

here is the maths...


basically resistance is generally linear with temperature for most materials over most temperatures (not so for superconductors!). in the equation, alpha is the temperature coefficient of the material, basically the slope of the line. common values of this are on the order of 0.001 K^-1 so given that the element wont ever heat up more then a few hundred degrees, the resistance change in going to be negligible, less then one ohm.

http://en.wikipedia.org/wiki/Resistivit ... istivities" onclick="window.open(this.href);return false;" rel="nofollow

on a side note, this is why light bulbs always blow out just as you flick them on. the filament is still cold and thus has a much lower resistance, and draws a huge current spike until it heats up.

[cemik1]

My heating element is a 1000w 120v 60hz Aroma hot plate.

I do not think this heating element is inside the boiler so its temperature can be much higher than 100ºC but anyway the resistance could change up to 5% (nichrome) so, as you said, it is negligible.