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Old 23rd July 2009, 07:28 AM   #1
N Brock is offline N Brock  United States
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Default Turning a power resistor into a power amp (sorta)

I would like to hollow out a power resistor to use as a case for the LM1875 amp I just built.

they sell very large 100W types at the local electronics surplus for about $5.

I need a safe way of removing leads as well as the "stuff" inside. I don't really want to start banging on the ends of one until I know what is filling the cavity.
After it is emptied the plan is to square up the insides using a mill so that the chips can be mounted effectively.

Thanks all,
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Old 23rd July 2009, 07:43 AM   #2
Zoodle is offline Zoodle  Australia
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My first thought: Asbestos.
Maybe I'm paranoid.
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Old 23rd July 2009, 11:29 AM   #3
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If they've been made in the last few years it's very unlikely to be asbestos in there, probably a ceramic of some sort. I'd still be inclined to have the vac going to suck up the dust whilst taking the innards out though, even ceramic dusts can be a bit rough on throats (let alone the mess). If you look on the manufacturers website you might find other info regarding safety. About the only other thing to be really careful of is berylium which is often used in RF transistors and is toxic. Once confimed safe a Carbide Burr might be hard enough to grind the contents out.
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Old 23rd July 2009, 06:32 PM   #4
N Brock is offline N Brock  United States
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Yes, I'll be careful, maybe the guys selling it will know what's inside.
I think you are probably right about the ceramic.
I'll post pics if it ever gets done.
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Old 24th July 2009, 12:29 AM   #5
lineup is offline lineup  Sweden
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I would use those resistors as GREAT power resistors.
For example single end Class A output stage load resistor.
But depends on if they are suitable in value: Say 3-10 Ohm
Probably, you can parallell to get down to such resistance.

I find it a waste to 'destroy' such nice power devices.
Can you not find something else to house LM1875 ...
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Old 24th July 2009, 01:24 AM   #6
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Default I wouldn't have thought

that it would have asbestos in it - the whole idea would be to conduct heat away from hte resistive element to the aluminium body, not contain it.

In all likelyhood, there will be a datasheet out there somewhere and if the contents include asbestos or other nasties, that should be on the datasheet.

"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 24th July 2009, 02:24 AM   #7
star882 is offline star882  United States
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Keep the resistors to use as test loads.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 24th July 2009, 03:04 AM   #8
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Thumbs up I've been inside one

While working on a motor controller, I had one of those resistors break with the electrode falling out of one epoxy end. I poked inside and it seemed as if the contents were just a carbon paste or some sort of resistive cylinder packed into the extruded aluminum body.

I didn't see or smell anything that seemed harmful in any way. Of course, I didn't drill it out or investigate it any further.

The funny thing is, when I shoved the contact back into the body of the resistor, it measured fine again. I still replaced it. Maybe it really is just a carbon pile epoxied into an aluminum housing.

I'd drill it out using proper ventilation and vacuum the contents as they're removed. Try anything once - twice if you like it.

As a side note, when I worked for Linear Power, we used 12 x 250W of those Dale resistors to form the 2 and 4 ohm loads to test our 500W and 800W power amps. After testing a couple dozen of those amps at full power, the whole side of the room would heat up. Those resistors were rock solid.
"Believers cling to the myth despite the evidence, reinterpret the myth to suit the evidence, or lie about the evidence to support the myth." "To err is human; to blame errors on external factors is even more so."
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Old 24th July 2009, 04:47 AM   #9
N Brock is offline N Brock  United States
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I will be going to the shop tomorrow but if I remember correctly the largest (size) resistor they had was rated 100 ohms, so not much to do there in terms of test loads but I could probably find some with a value better suited for that use.
I figure it is about the same price as an off the shelf case of that size and about ten times as cool.
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Old 25th July 2009, 01:05 PM   #10
gfiandy is offline gfiandy  United Kingdom
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I doubt there is anything dangerous in there but you never know and any dust can cause irritation so use a good dust mask to keep it out of your lungs.

Dusts are very invasive so if possible work outside, if you don't expect a thin film of dust over everything in the room you work in.

If possible use hand tools rather than grinding it out. I would try putting it in a vice and chipping it out with a old screwdriver and hammer. This will cause much less dust and you can always resort to a grinder if it doesn't work.

Best of luck. I think its an excellent idea should make a really interesting amp and its clearly a great heat sink so long as you can bond the chip amp to it well enough. Power resistors of this type are hardly a collectors item and $5 is less than most of the cases I have seen.

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