evil_lurker
National Hazard
Posts: 767
Registered: 12-3-2005
Location: United States of Elbonia
Member Is Offline
Mood: On the wagon again.
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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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Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
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Mood: Chemistry: the subtle science.
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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]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Rosco Bodine
Banned
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Registered: 29-9-2004
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Mood: analytical
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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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Texium
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Thread Moved
19-11-2023 at 11:24 |
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