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Old 7th January 2012, 06:43 PM   #1031
EUVL is offline EUVL  Europe
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Let me perhaps explain why I spotted your problem straight away.
And what Andrew said is true is any case.

The power dissipated by the MOSFETs are taken care of by the large heatsinks. That's fine.
For the CRC, you calculated 1.5W per resistor, or 6W in total. That is also quite OK.
Had you used normal metal film power resistors, or ceramic resistors as I suggested, you would have had NO problems.
These are not thermally coupled to anything, and they give off their heat to the ambient.

The problem is that you used metal cased power resistors which are normally thermally coupled to heatsinks.
By mounting them onto a metal plate, you now coupled them thermally to that plate, and thus the plate becomes the heatsink.
Which it can for 6W, but it will still get quite warm.
Still OK if you do not use that at the same time as your Gnd bus bar and connect it to the capacitors.

I presume you don't want your caps too warm.
So the easiest thing you can do is to thermally decouple those resistors from the Gnd bus bar.
And the easiest way to do so is , as I suggested, use free-standing 5~10W ceramic resistors.

Now why ceramic resistors ?
You want to filter off high frequencies from your power supply.
So in addition to resistance, a bit of inductance is actually beneficial.
On top of that, they are cheap and easy to get.
So why not ?




QED,
Patrick
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Old 7th January 2012, 07:14 PM   #1032
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Quote:
Originally Posted by EUVL View Post
The simplest solution to the Gnd plate "problem" is to use free-standing ceramic resistors instead.
Simpler still would be to use insulating spacers or washers (4-5mm thick Teflon?) at the mounting holes of the existing resistors.
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Old 7th January 2012, 07:33 PM   #1033
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Without a heatsink, their temperature will likely be 50% higher than now.

(judging from the size of the metal plate they are currently mounted on)
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Old 7th January 2012, 08:28 PM   #1034
reytnz is offline reytnz  New Zealand
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Thanks for all the inputs, much appreciated. I kind of like the look and feel of these power resistors it matches the " heavy duty" feel of the whole amplifier. What i'll do is thermally decouple the resistors from the gnd plate - but before that i want to listen to the F5 some more - to lazy to go back to the bench.
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Old 7th January 2012, 09:07 PM   #1035
evanc is offline evanc  United States
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but before that i want to listen to the F5 some more

after all that is the reason for all the fun......
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Old 7th January 2012, 10:02 PM   #1036
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Yep. Those "metal jacket" power resistors are intended for heat sink mounting, and should be significantly derated if not used that way. In my F5, I added another RC stage to the RCR PSU, and used power resistors (photo). What's obvious, is that I have both the bridge rectifiers and power resistors firmly mounted to the 3/16" chassis base plate. What's less ovbious is that I had masked off pads for the rectifiers and power resistors on the base plate, prior to powdwercoating and used thermal grease between the rectifiers and the resistors and the base plate when I mounted them. (And the entire internal chassis is actually a forced-air plenum that gets circulation from two low-speed, low-noise fans.)

Bottom line..... Don't go for the "industrial look", if you circumvent the proper engineering to make practical use of it.

Ceramic resistors are a better choice, unless you want to "go the distance" with the finned metal jackets.
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Old 8th January 2012, 02:14 AM   #1037
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Yep. Those "metal jacket" power resistors are intended for heat sink mounting, and should be significantly derated if not used that way.
CanAm Man, that's not how I remember it on several metal Dale's like this. The 50W (on mine) rating is in free air I believe, and at 50W they're hella hot, but I have added additional hs and gone way above 50W.

Last edited by Andersonix; 8th January 2012 at 02:20 AM.
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Old 8th January 2012, 02:26 AM   #1038
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Andersonix....... Yeah..... You've got me wondering about that, and I think you're right. I know in my case, after my PSU calculations, I decided that using the chassis plate as a heatsink was a very good "safety margin" for my resistors. Let me dig out some Dale spec sheets and convince myself..... In any case, my resistors run a little warm, as is the chassis plate surrounding them......
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Old 8th January 2012, 11:29 AM   #1039
AndrewT is offline AndrewT  Scotland
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Heatsink mountable resistors are rated as described in the datasheet.
I can virtually guarantee that the gross rating ONLY applies when mounted on an effective heatsink as described in the datasheet That may be an infinite heatsink @ 25degC or @ 70degC.
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Old 8th January 2012, 01:06 PM   #1040
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Quote:
Originally Posted by Andersonix View Post
I believe
A 50W Dale RH series in free air does 20W at 25C ambient.
The 50W rating at 25C requires a heatsink surface that shouldn't be underestimated.

(btw, the Dale RH-50 and RH-100 have been around for many decades. When i first used them in the mid '80s, Dale was the single ready available industrial power resistor brand available overhere)
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