Author Topic: How hot is too hot for a resistor ?  (Read 7550 times)

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Offline ez24

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How hot is too hot for a resistor ?
« on: May 15, 2015, 02:40:34 pm »
Hi

I used 2 - 8 ohm 100 watt resistors in parallel  at 4.3 amps at about 19 v (laptop power supply) =  about 74 watts


I did not use them long because the temp of the resistors got to 200F very fast

So I was wondering how hot should I allow a metal 100 watt chinese aluminum (with small fins) power resistor get to ?   I was using them at 50 watts (2 - 100 watters in parallel) , but I do not know how hot I should allow them to get to

Similar to
http://www.ebay.com/itm/1PCS-High-Power-1-0-Ohm-1R-100W-Watt-Aluminum-Shell-Case-Wirewound-Resistor-5-/381091958208?pt=LH_DefaultDomain_0&hash=item58bad9cdc0

Anyone know hot is acceptable for a 100 watt Chinese metal power resistor?  This would be without a fan.

thanks

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Online coppice

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Re: How hot is too hot for a resistor ?
« Reply #1 on: May 15, 2015, 03:04:08 pm »
How big a heat sink is it attached to? Without a fan you need a pretty big sink to get rid of 100W, if you want things to stay cool. As for maximum temperature, that should be part of the device's spec. Some of the all ceramic designs can be run like an electric fire, with the resistive element glowing red. It reduces the lifetime, which may or may not matter to you. The resistance will also change quite a bit, which also may or may not matter to you.
 

Offline rickselectricalprojects

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Re: How hot is too hot for a resistor ?
« Reply #2 on: May 15, 2015, 05:56:07 pm »
i would try high quality power resistors rather than crappy eBay Chinese resistors ;)
 

Offline PeterFW

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Re: How hot is too hot for a resistor ?
« Reply #3 on: May 15, 2015, 06:25:43 pm »
Anyone know hot is acceptable for a 100 watt Chinese metal power resistor?  This would be without a fan.

May i ask about the application were you need this?
There might be a easy solution to your problem, slim change but might.
 

Offline tron9000

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Re: How hot is too hot for a resistor ?
« Reply #4 on: May 15, 2015, 06:27:36 pm »
http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188087/?searchTerm=HS100+1R+J&relevancy-data=636F3D3126696E3D4931384E4272616E644D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5B5C707B4C7D5C707B4E647D5C707B5A737D2D2C2F255C2E5D2B2426706F3D3526736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D455249432677633D4E4F4E45267573743D4853313030203152204A26&sra=p

The resistor above gives you some quick specs, though for you ebay special I'm not sure if they'd be identical, but how hard is it to copy a 100W alu clad resistor?

this one has 1.0degC/W, and a max operating temp of -55 to 200 deg C

36.98W per resistor, call it 37W

temp rise above ambient at that power per resistor = 37deg C = 1.0degC/W * 37W

call ambient 24degC therefore surface of resistor = 61degC

However your getting 93.33DegC? which means your chinese knock off probably has a worse thermal resistance than that above. Seems my question has been answered - guess it is hard to copy a 100W resistor

I would not exceed 150DegC (302F) for this one. I wouldn't trust it seen as there are no specs given on thermal resistance. If in doubt, bolt it to a big chunk of metal! But for the extra couple of bucks, I'd buy the branded ones from a reputable supplier
« Last Edit: May 15, 2015, 06:31:18 pm by tron9000 »
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Online coppice

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Re: How hot is too hot for a resistor ?
« Reply #5 on: May 15, 2015, 06:33:42 pm »
http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188087/?searchTerm=HS100+1R+J&relevancy-data=636F3D3126696E3D4931384E4272616E644D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5B5C707B4C7D5C707B4E647D5C707B5A737D2D2C2F255C2E5D2B2426706F3D3526736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D455249432677633D4E4F4E45267573743D4853313030203152204A26&sra=p

The resistor above gives you some quick specs, though for you ebay special I'm not sure if they'd be identical, but how hard is it to copy a 100W alu clad resistor?

this one has 1.0degC/W, and a max operating temp of -55 to 200 deg C
Its doesn't have 1 deg C/W. They say its only rated for 100W when mounted on a 1 deg C/W heat sink. Few of these power resistors are designed to work at their rating without being bolted to a substantial heatsink.
 

Online wraper

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Re: How hot is too hot for a resistor ?
« Reply #6 on: May 15, 2015, 06:34:33 pm »
http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188087/?searchTerm=HS100+1R+J&relevancy-data=636F3D3126696E3D4931384E4272616E644D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5B5C707B4C7D5C707B4E647D5C707B5A737D2D2C2F255C2E5D2B2426706F3D3526736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D455249432677633D4E4F4E45267573743D4853313030203152204A26&sra=p

The resistor above gives you some quick specs, though for you ebay special I'm not sure if they'd be identical, but how hard is it to copy a 100W alu clad resistor?

this one has 1.0degC/W, and a max operating temp of -55 to 200 deg C
Where did you get this 1.0degC/W from? All what written there is:
Quote
Power At 25°C   100W with 1.0°C/W Heatsink, 50W without Heatsink.
Read: it needs 1.0°C/W heatsink to operate at full power with 25°C ambient temperature.
 

Offline T3sl4co1l

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Re: How hot is too hot for a resistor ?
« Reply #7 on: May 15, 2015, 07:18:03 pm »
Metal case resistors are potted with epoxy or something like that.  150C max internal temperature.  They have pitiful ratings when used without a huge heatsink.

Ceramic core (silicone and vitreous enamel coated) resistors are capable of operating at incendiary temperatures, at least for short periods (say, a few minutes).  Ratings of 225C and up are typical.  The high surface temperature gives them a much smaller profile for the same power dissipation, and they're usually not much more expensive.

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Offline tron9000

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Re: How hot is too hot for a resistor ?
« Reply #8 on: May 15, 2015, 07:39:52 pm »

Where did you get this 1.0degC/W from? All what written there is:
Quote
Power At 25°C   100W with 1.0°C/W Heatsink, 50W without Heatsink.
Read: it needs 1.0°C/W heatsink to operate at full power with 25°C ambient temperature.

my bad. Still it says not to exceed 200degC (392F) in the datasheet. plus give him something to think about if and when he does decide to bolt to a heatsink.
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Offline PeterFW

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Re: How hot is too hot for a resistor ?
« Reply #9 on: May 15, 2015, 08:47:26 pm »
Ok... i will just say it.
If you only need the resistor as a dummy load for test purposes, or just for a short time, a bucket/bottle of water and you old toaster will make a great 500W power resistor with no danger of overheating.

Rip the heating wire out of your toaster, stuff if in the bucket till you reach the right resistance and pour water on that.

Your lead wires have to connect to the resistance wire under water, if you feel fancy make yourself a jig out of some old wood to wrap the wire around so it will not shift and change resistance.
« Last Edit: May 15, 2015, 08:58:22 pm by PeterFW »
 

Online Psi

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Re: How hot is too hot for a resistor ?
« Reply #10 on: May 15, 2015, 08:55:28 pm »
Quote
How hot is too hot for a resistor ?

If you hear any sizzling when touching it with a wet finger, its too hot.
If you smell burning, its too hot
If you see flames, its too hot
If you hear fire trucks, its too late.
« Last Edit: May 15, 2015, 08:58:34 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline tron9000

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Re: How hot is too hot for a resistor ?
« Reply #11 on: May 15, 2015, 09:32:02 pm »
or just solder some wire to it and when the solder melts again - then its too hot!
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Online PA0PBZ

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Re: How hot is too hot for a resistor ?
« Reply #12 on: May 15, 2015, 09:58:02 pm »
or just solder some wire to it and when the solder melts again - then its too hot!

Yes, just like this:

Keyboard error: Press F1 to continue.
 

Offline tron9000

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Re: How hot is too hot for a resistor ?
« Reply #13 on: May 15, 2015, 10:15:29 pm »
Yes, just like this:



Is that a sort of thermal cutout? like a bi-metallic strip?
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Offline bktemp

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Re: How hot is too hot for a resistor ?
« Reply #14 on: May 15, 2015, 10:23:26 pm »
Yes, just like this:



Is that a sort of thermal cutout? like a bi-metallic strip?
The solder at the top end works as a thermal fuse: When it melts, the springy contact moves away and disconnects as depicted in the drawing.
 

Online PA0PBZ

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Re: How hot is too hot for a resistor ?
« Reply #15 on: May 15, 2015, 10:23:47 pm »
Is that a sort of thermal cutout? like a bi-metallic strip?

No, it is just the solder melting and the spring action. Found a lot in old color TV's, and fixable with a soldering iron.
However, I have no idea if special solder/low melting point was used. Fixed a lot of them in the 80's.
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Offline tron9000

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Re: How hot is too hot for a resistor ?
« Reply #16 on: May 15, 2015, 10:26:49 pm »
never seen them like that before, I suppose the type of solder would dictate the trip temp. I suppose not many around now: limited applications, possible RoHS?
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Offline SeanB

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Re: How hot is too hot for a resistor ?
« Reply #17 on: May 16, 2015, 02:44:02 am »
Was common until fusible resistor technology advanced to the point where you could get a unit that would reliably ( as in more than 99.9% would) handle the rated power, yet would fail open circuit with no or minimal smoke emission with about 3 times the power, but with a lifetime that would give years of service at rated power.

As to the OP, you MUSt run a metal clad resistor on a heatsink ( and a pretty big one for high power ones) to get rated power. Otherwise you are derated to about 10% of the rating, for the 100W unit it is about 15W in free air without a heatsink.
 


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