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Author: Subject: How to power a heating mantle?
International Hazard

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[*] posted on 4-10-2006 at 11:56
How to power a heating mantle?

I'll admit that electricity is one of my weaknesses.

I'm wanting to get a heating mantle eventually, and seen a fairly good deal on a variac which I'm thinking of purchasing.

The problem is I don't know squat about how to match loads with output power.

The variac is rated as being 0-140 V @ 10amps.

If this thing is cranked up to 100% output that means that 125V (houshold current) would be putting out 10 amps.

According to ohms law that is going to be around 1250W correct?

And if a heating mantle is rated at 400W then it would make sense that it should power it, and if anything would be able to give it too much power and so I shouldn't crank it up past 1/3rd of the way up right?

But since resistance and what not changes with voltage, then shouldn't I be able to crank it up more than that without burning anything up?

Someone please explain to me how this works before I continue to ramble on and spew more idiotic nonsense.
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lab constructor

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[*] posted on 4-10-2006 at 13:37

You're on the right track. But your Variac is capable of putting out 10 amps at 140VAC. Since P=EI, full power would be 1400w.

If your mantle is rated for 120VAC, then yes I would not turn up the Variac past (120/140)x100% = 86%. At 86% its output voltage should be 120VAC.

I wouldn't mind if someone verified my thinking on this.

FYI the amp draw at 120VAC for the mantle would be 400w/120v = 0.33 amps.

[Edited on 4-10-2006 by Magpie]

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Rosco Bodine
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[*] posted on 4-10-2006 at 19:18

Most variacs have jumpers so that you can connect
for 0-100% of line voltage output or 0-120% of line voltage output , if you want the 20% increased voltage
at the maximum setting .

If you have a mantle rated at 400W at 120V , then
you probably shouldn't put more than the 120V across
that load . Most of the ratings on equpment for ~120V are fine in the range of 110 to 125V ....but going to 140-150V is pushing it and depending on the equipment ruggedness , it could cause premature failure .

If your mantle is rated 400W @ 120V and you plug it into
a 120V wall outlet directly , then 400W is all it will draw .....even though that wall outlet may be rated to deliver 2400W , all that means is you could plug a total of 6 mantles into that one outlet before overloading
the supply circuit and blowing the 20A fuse .

The Variac is rated similarly according to its power output , and it doesn't matter if the load is smaller .
If you exceed the load rating , it will blow the fuse .
The only way you will put more power than 400W into
the mantle is by using the range of +20% increase over
normal line voltage that is available if you have the variac jumpered so that it will step up the line voltage ,
at its maximum range in which case the mantle would max out at 480W .

The 10A Variac rating only means the variac is sufficiently
heavily built to control nearly 3 times the load which will be presented by your 400W mantle at full power output .
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