Mislabelled Power Resistors - Page 1
Forum section: electronics / beginners

author: Dave Turner
author_rank: Frequent Contributor
timestamp: September 10, 2021, 06:41:35 pm
content: Please point me in the right direction.I've received a few 8 ohm power resistors which I thought were going to be rated at 10W.They are labelled at 100W. They are approx 60x15 mm approx 11mm diameter encapsulated resistor the rest an alloy. Scratching the thin brass/copper appearing coating reveals probably an aluminium substrate.I don't really care whether they are rated 10W as advertised or 100W as marked. What I would like advice on is how to non-destructively test them to find out what their rating actually is.Clearly I'll have to pass a constant current through them at various voltages in free air and measure the temperature rise; which I can do with with my Brymen 869s's temperature probe.What are the limits that I should look for to determine their rating?Thanks

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2021, 06:51:43 pm
content: Are they the types you can bolt onto a heatsink?Then the 10 W is probably "free air" dissipation and 100 W at some certain case temperature on a heatsink.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: September 10, 2021, 09:41:57 pm
content: Yes they are able to be bolted to a heat sink.However that is beside the point.I still need to understand what the safe power is that they can sustain. 10 or 100 Watt and how I can sensibly determine which is which.Somehow (old age) I'm missing the obvious.

author: The Soulman
author_rank: Super Contributor
timestamp: September 10, 2021, 09:51:54 pm
content: Quote from: Dave Turner on September 10, 2021, 09:41:57 pmBeing bolted to a heat sink is the point.In free air the resistor will heat up faster and to a higher temperature than it would be bolted to a heat sink,being fed the same power.Thus when properly cooled down (mounted to a heat sink) the resistor can handle more power before a critical temperature is reached.

author: TimFox
author_rank: Super Contributor
timestamp: September 10, 2021, 10:10:58 pm
content: If you know the manufacturer, you can find out what case temperature allows 100 W, which will determine the allowed thermal resistance of the heat sink for full power.

author: thm_w
author_rank: Super Contributor
timestamp: September 10, 2021, 10:31:43 pm
content: Here are some examples:https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=1773035&DocType=DS&DocLang=English- with heatsink 25W- without heatink 12WIts more like 2-4x dissipation with a heatsink.Anyway, I would hook them up to the thermocouple as you said, run some power into them until they stabilize at around 100C. That would be a safe power level to run them at. They can take up to at least 130-150C or more.

author: Jwillis
author_rank: Super Contributor
timestamp: September 11, 2021, 12:34:09 am
content: Uxcell 100W wire wound aluminum case come in those dimensions 60mm X 15mm But they're usually green . TE and ARCOL are have a 65mm body length . Vishay RH100 are 88mm long and the NH100 is 114mm long. Even the Liwang 100W have a body 98mm long .Might be tough to find manufacturer if they are rebranded .Well as we say here , you might have got hosed.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: September 11, 2021, 06:04:35 am
content: That's what I'm missing! Just what is the critical temperature - or at least the maximum temperature that one should permit a resistor to maintain in the long term?Note the above reply crossed with the forum. 100C is what I'll aim at as a maximum.Thank you all.