Electronics > Beginners
Thermal dissipation.
<< < (3/3)
T3sl4co1l:
This is rather after the fact, I guess, but FYI -- metal-case resistors are typically a false economy, once you include the massive heatsink required, and machining and mounting steps.

Vitreous enameled types are only slightly more expensive (sometimes cheaper, almost always comparable), somewhat easier to mount, take up less space than the resistor-heatsink combo, and operate at much higher temperatures (= more power density, less airflow required -- assuming a high exhaust temperature is acceptable).

They're also much more tolerant of overload: they'll happily glow red hot for minutes on end before expiring.  The metal-case types can't tolerate much for long (e.g., 2.5 or 5x power for 5s), and overheating leads to cracking and voids in the epoxy fill, or complete blowouts.  That is to say: they are manufactured with magic smoke, while the other kind is not. :)

So, for modest overloads of 2-3x and longer durations, or higher overloads and shorter durations (<= 1s), I wouldn't worry about it.  I'd also rather have a series limiter than a shunt limiter, since supplies are usually CV-output (imagine what happens to the poor thing(s) when a beefy unregulated linear supply is connected?).

I've made a couple kinds of series limiter (protected switch), battery powered with reasonable life (>6 weeks continuous operation from a 9V battery), let me know if that sounds interesting. :)

Tim
sureshot:
Interesting to know about the enamel resistors, although I've not come across any high power variants. I will implement two thermal switches contacts NC opening on temperature rise. Just incase of a fan out peoblem, they will cut the input relay coil. Over voltage is unlikely to happen, but its just being sure its covered if it did happen. The main use for this unit is testing converted server power supplys, whilst also connected to HF radio gear and amplifier, this is to see how the host psu behaves under heavy loads. As in, as if i was using the equivelent of a high power amplifier with radio. I've a modest amplifier and radios. But want to see in real time if these power supplys can handle high current use with HF equipment. So as radio gear is connected to the same host psu, that's also the need to protect from over voltage conditions. That unit will also see lighter testing of linear power supplys. I'm hoping to see some wave forms of different units under different load conditions.
Tomorokoshi:
Something like this perhaps:

https://www.digikey.com/product-detail/en/ohmite/C300KR63E/C300KR63E-ND/1124873
JS:

--- Quote from: sureshot on June 05, 2018, 10:33:42 am ---Interesting to know about the enamel resistors, although I've not come across any high power variants. I will implement two thermal switches contacts NC opening on temperature rise. Just incase of a fan out peoblem, they will cut the input relay coil. Over voltage is unlikely to happen, but its just being sure its covered if it did happen. The main use for this unit is testing converted server power supplys, whilst also connected to HF radio gear and amplifier, this is to see how the host psu behaves under heavy loads. As in, as if i was using the equivelent of a high power amplifier with radio. I've a modest amplifier and radios. But want to see in real time if these power supplys can handle high current use with HF equipment. So as radio gear is connected to the same host psu, that's also the need to protect from over voltage conditions. That unit will also see lighter testing of linear power supplys. I'm hoping to see some wave forms of different units under different load conditions.

--- End quote ---

They are resistors, over voltage and over current go together, use a fuse for that case when something funny gets to its inputs and save the world.

JS
sureshot:
But if HF radio equipment was connected to the loaded host power supply, it could be damaged if the psu went above 14.4 Volts.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod