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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Same style as those but a LOT smaller (about the size of the coin)
They would blow out if you started the amp three times in a row without letting the resistor cool. It was a huge overload, since the resistor was 47 ohms, rated about 5 or 10 watts, and I was soft-starting a heavy load off a 240V line. For the half a second it took the 40,000uF worth of filter caps to charge to 50V, it would be dissipating about 100 times its rated power.
I changed to a 50 watt pulse-rated thick film resistor, screwed to the metal chassis, and it has worked fine for years now. The amp is here:
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Whoa! Be careful about power numbers and resistors. Especially since you brought up those aluminum housed resistors. An aluminum housed resistor (like the one Doug shown) rated for say 50W, is only rated to 50W under specified conditions. In the case of these resistors, that 50W rating (or whatever the rating is), only applies when mounted to an appropriate heatsink at an ambient temperature of 25degC. For unmounted, the power rating is usually around 50-60% less at 25degC. Similar goes for other power resistors.
Read the datasheet before using to see just how much power can be dissipated across the resistor.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
I would derate them even more than Dan does -- without a heatsink, those power resistors aren't much good for even a quarter of their rating. They must be heatsinked! =)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Does it need saying that if you have a series string of metal cased resistors, and heatsink them all to a grounded piece of metal, then the wire to case voltage for the ones at teh high voltage end will be the full voltage of the string, not just the voltage across the resistor?
That's a good argument for not heatsinking such resistors, but for keeping the dissipation down to what can be lost in free air (air, not high performance, but the cheapest coolant and insulator). IIRC, metal cased resistor data sheets do tend to give two ratings, for heatsunk and for free air.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's why we tend not to bother using those gold cased Dale resistors for high voltage at all. They cost more than a sugar cube or Globar type resistor with the same non-heatsinked rating, so you only use them in applications where you want to bolt them to a heatsink.
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Resisitors, question on wattage.
(about the size of the coin) 
