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Author: Subject: Powering a heating mantle with a computer PS
samm
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[*] posted on 28-7-2017 at 14:45
Powering a heating mantle with a computer PS


I searched a little but didn't see anything directly relevant. I have a 500w ATX PS, a wall dimmer switch, and a 380W mantle with no controller or cord.

The PS has both +12V and -12V wires. The dimmer switch has an obvious ground and then 2 other black wires. I have an idea of how to wire this, but I'm electrically challenged so I gotta run this by someone before I burn this place down. My thoughts are:

Run the two black wires off the dimmer in series from the +12V to one of the terminals on the mantle.

Run the other (not ground) terminal on the mantle through the -12V wire.

Ground all the grounds and short the PS where it needs to be shorted to work, that part I'm sure I can figure out.

Anyone? The two things I'm not really sure about are a) whether to run the dimmer wires in series, and b) whether or not I can/should use the -12V line. I realize that if this works, it's more power than the mantle was designed for, so I probably won't run it much over half power. Feedback welcome.

Thx!

[Edited on 28-7-2017 by samm]
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XeonTheMGPony
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[*] posted on 28-7-2017 at 14:49


first of is the mantle rated for 24V?

The one I got is rated for 120v at 300w at 24v it would put out 60W

and most dimmers are zero crossing trimmers, ie they only run on AC.

So we need a LOT more info to be of any help!

and at your mantles rated power I'd be very doubtful it is rated for 24V!

So first thing you need to do is verifie its voltage, easiest way is to measure the ohms and calculate the wattage at a given voltage in this case 24V Vs 120V

[Edited on 28-7-2017 by XeonTheMGPony]
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samm
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[*] posted on 28-7-2017 at 15:18


Shit, guess I'm glad I asked?

The mantle is rated for 115V, 380W.

The voltages off the PS are +3.3,+5,+12 & -12

No idea about amps.

edit: the +12v line is from the PCIe connector -- high performance graphics cards. IDK if that makes a difference but my guess is those lines might be higher amperage.

Again I'm electrically challenged so sue me if I'm way off base here :D

edit:
Quote:
The one I got is rated for 120v at 300w at 24v it would put out 60W


ok. This makes some sense. The dimmer switch won't work for DC though?

[Edited on 28-7-2017 by samm]
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[*] posted on 28-7-2017 at 15:35


The trouble is that the winding on the mantle will have a resistance matched to the mains supply.

a 12v supply with the same resistance won't give you the same power output.

And a dimmer is for inductive loads on a motor driven by AC, a computer powersupply is a switch mode supply which won't appreciate having it's input chopped up by a dimmer.

And if you thought about using the ATX to drive a dimmer then control a mantle, dimmers don't run on DC. They chop AC into pieces to reduce the amount of power the motor sees.

Stick with an arduino type controller or a PLC, and a relay.


Got delayed between writing and posting the response...others have said the same thing really.



[Edited on 28-7-2017 by Chemetix]
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[*] posted on 28-7-2017 at 15:39


You could hook the dimmer switch directly from the wall current to the heating mantle. I use a dimmer switch to regulate my heating mantle in this way. I don't recommend playing with the wall current if you don't know what you're doing, though. The principles behind it are not very complicated, but you can cause fires and electrocute yourself if you make a mistake.



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[*] posted on 28-7-2017 at 15:50


I would have thought using a dimmer would push it a bit, 380W at 115V should be drawing a current of ~3.3A ( P= VI) I guess If you get a decent one rated for a bit of power they'll cope.
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[*] posted on 28-7-2017 at 16:29


Most good dimmers will max out at 500w peak due to the use of halogen bulbs.

So a quality dimmer will handle 380w no problem, just make sure it says 500w max and good to go.

This is exactly how I plan to control my mantle as well, if you think about it incandescent lamps are really just heaters that happen to put some light out! and the average is 100w and you see plenty 4 bulb luminairs ! that's 400watts!

The formula given works in revers SO to get amps we devid the watts by volts so: 380/120(North american standard power) = 3.2amps (rounded up)

Now 12V + -12V = 24V*3.2A = 76.8w (Will get warm at least!)

and Samm, never hurts to ask! But it sure can kill you not too! So give your self a big fat star! as you did the smartest thing one can do! Know when ya don't know and ask! that's the secret to being a good any thing!

[Edited on 29-7-2017 by XeonTheMGPony]
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[*] posted on 28-7-2017 at 16:41


You'd be hard pressed to find a dimmer that *won't* handle 500 watts. Even dimmers rated for low inductive loads will usually handle much higher resistive loads (such as a heating mantle). But we don't need newspaper headlines like "Local Teen Electrocuted Following Instructions from Mad Science Website."



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[*] posted on 28-7-2017 at 18:11


You won't be able to run that much current through a -12V rail on the PSU, it's simply not designed to handle it. That limits you to using the 12V rail, which will most definitely support high current; most quality modern power supplies can supply nearly the entirety of their rated power at 12V. Unfortunately, the mantle's going to be basically useless since it won't draw enough current to provide a meaningful amount of heat at that voltage.

I agree with the others, a dimmer hooked up directly to wall power is going to be your best bet. Obviously a variac is ideal, but I do understand that they can be difficult to come by for a reasonable price.




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[*] posted on 28-7-2017 at 18:22


Ok, well at least I learned something, this all makes some sense. I was looking at Variacs but ya they aren't cheap, the 500W ATX PCIe PS was free.

I'll figure out how to run it from the wall, without zapping myself or burning the place down. I know someone who can help with that.

thx
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[*] posted on 29-7-2017 at 10:06


What about a solid state relay hooked to a PWM with a long period?


[Edited on 29-7-2017 by ficolas]
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[*] posted on 1-8-2017 at 02:32


There is one idea that nobody has mentioned - a boost converter. Essentially a switching power supply fed from the computer power supply.

You can find them on eBay for relatively cheaply. In this particular instance I found some ranging from $7-20 that could handle 400W. (Links omitted since they are time limited and will be irrelevant for future readers).

Now many of these units (boost converters) you get on eBay and related sites have adjustable voltage. This is a GREAT feature to have, actually. It acts similarly to a temperature control unit although it would put out constant POWER, not constant temperature. Best of all it is digital!

Another thing is that if you get an isolated boost converter they can be connected in series for higher voltages, too! This will only work for isolated topologies. Isolated in this case means ones that do not share a ground connection on the output with the input of the converter.

See the Wikipedia articles for "Boost Converter" and "Flyback Converter" if you want a graphical view of the difference between isolated and non isolated with the Flyback topology being isolated. There are other topologies that are isolated but these are omitted for simplicity.

On to a note about safety, and why I personally believe this might not be a great way to go:

1. Efficiency loss. - Less efficient method of doing things which just wastes power, takes up more space, more open circuits, wires, and clutter.

2. Safety! - Safety in the laboratory is paramount. Computer power supplies are not designed to operate in air tight, or corrosive environments as they rely on convection and air to carry away the heat from the circuitry. There is also the issue of exposed wiring, and the requirement of bypassing certain parts of the supply to get it to turn on to provide you with power. You could get around these with some modification such as making a nice case for it and properly terminating the wires though.

3. Variacs are awesome, and a 600W dimmer from one of the US chain hardware stores is $5.99! Get some proper insulated crimp connectors and a switch for extra safety. Maybe throw it in a junction box. You could also look into getting a PID controller that has a solid state relay in it and make it temperature controlled. I think the Chinese PID units were running ~$20.
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[*] posted on 21-8-2017 at 01:04


Here's two 2000-watt AC power controllers, for only $11!

http://www.ebay.com/itm/162449771354

That's from the US, but if you have the patience for the China Express, they're about $2.50 each.

Remember, the power they draw is based solely on the resistance at max power, NOT on the rated wattage for the power controllers, so you can use these controllers to power any resistive load with AC that's below 2000 watts.




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