A decade ago, I bought some 200W power resistors. Aluminum extruded, 165mm long, rather heavy, to build a dummy load with.
This month, I bought four more of what appeared in the listing to be the same thing (even their chart showing different wattages and sizes showed the 200W being 165mm long). What I received however was only 100mm long. According to the chart, these are 100W units. But they are marked exactly with the same part numbers as my 10 year old resistors.
Either the laws of physics have changed in 10 years (doubtful), or somebody made an error in labeling.
The seller investigated and this morning informed me that this is correct. I don't believe them. The parts I received don't even match specs with their own resistor chart.
I'm reluctant to build the dummy load with these resistors, but if I try to buy them again, I'll probably get the same thing all over again.
So what do ya'll think? Labeling error, or improved physical materials?
This month, I bought four more of what appeared in the listing to be the same thing (even their chart showing different wattages and sizes showed the 200W being 165mm long). What I received however was only 100mm long. According to the chart, these are 100W units. But they are marked exactly with the same part numbers as my 10 year old resistors.
Either the laws of physics have changed in 10 years (doubtful), or somebody made an error in labeling.
The seller investigated and this morning informed me that this is correct. I don't believe them. The parts I received don't even match specs with their own resistor chart.
I'm reluctant to build the dummy load with these resistors, but if I try to buy them again, I'll probably get the same thing all over again.
So what do ya'll think? Labeling error, or improved physical materials?
While I doubt this is the case for you there are some resistors that can be very small for there power rating that use phase change materials (braking resistors) however once the internal material has changed phase they are 'used up' and the power rating plummets.
From my experience of building dummy loads a lot of resistor power ratings are very optimistic even from big brands like Vishay and TE. Particularly bad where a batch of TE flat panel resistors that only had a single mounting hole in the center. Over the course of minutes the edges of the resistor lifted off the heatsink and burnt out. When we inquired why this was the case we where informed that one of their engineers had been running the same resistors at 150% of rating for months... we ended up using them but at 25% rating as this was the only way of making them last a decent length of time. The Vishay resistors claimed that if the back panel of the resistor was below a certain temperature the resistor element was within its thermal envelope. We monitored them with a very accurate thermal camera and found this was not the case as resistors would fail (below rating) and with the back panel far below the maximum temperature. Since that point we just started de-rating any power resistor by 75% and haven't had anymore problems.
From my experience of building dummy loads a lot of resistor power ratings are very optimistic even from big brands like Vishay and TE. Particularly bad where a batch of TE flat panel resistors that only had a single mounting hole in the center. Over the course of minutes the edges of the resistor lifted off the heatsink and burnt out. When we inquired why this was the case we where informed that one of their engineers had been running the same resistors at 150% of rating for months... we ended up using them but at 25% rating as this was the only way of making them last a decent length of time. The Vishay resistors claimed that if the back panel of the resistor was below a certain temperature the resistor element was within its thermal envelope. We monitored them with a very accurate thermal camera and found this was not the case as resistors would fail (below rating) and with the back panel far below the maximum temperature. Since that point we just started de-rating any power resistor by 75% and haven't had anymore problems.
changing reference to peak value,
changing wire composition
...
some bench tests may help to verify seller assertions.
changing wire composition
...
some bench tests may help to verify seller assertions.
The resistor can’t dissipate 100 watts. It needs a heatsink ( or sufficent chassis surface area to dissipate the heat.
The physical properties of the bobbin, on which the resistive wire is wound, play a large part.
The chosen material for the bobbin obviously has to be heat resistant, but the higher the heat conductivity the better. A higher conductivity material such as beryllia (beryllium oxide) is superior to the likes of porcelain.
The only way that I can conceive of smaller resistors handling the same power is if the internal insulating materials have better thermal conductivity to the outside flange. Of course these resistors are intended to be used with a heat sink. I bought a massive heat sink last month on ebay, weighing about 8lbs and having 2-1/2" deep fins. I sized the heatsink around the original 165mm size resistors I used in my other dummy load.
Then there's their own resistor chart, which defines the dimensions of each power rating and clearly it shows the 200W resistor measuring 165mm long, while the 100W is listed as 100mm long.
Then there's their own resistor chart, which defines the dimensions of each power rating and clearly it shows the 200W resistor measuring 165mm long, while the 100W is listed as 100mm long.
I would send them back for a full refund. If necessary file a credit card false charge form,
or a Paypal refund request. Order the right ones from somewhere else. These are ancient
commodity items, and their specs don't change, even if the markings do.
Of course you could attach it to the heat sink, apply rated power, and check the temperature,
which would tell the story. I would derate parts such as these by at least 50%, regardless.
or a Paypal refund request. Order the right ones from somewhere else. These are ancient
commodity items, and their specs don't change, even if the markings do.
Of course you could attach it to the heat sink, apply rated power, and check the temperature,
which would tell the story. I would derate parts such as these by at least 50%, regardless.
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Do they look the same as the old ones? Better conductivity and higher max temperature can decrease the resistor size.
Show a reference to the datasheet.
As with power transistors, the power rating can often be specified at a specific case temperature. Up to the user to ensure this is maintained:
heatsink, even forced cooling etc.
Maximum dissipation cannot be specified without environmental conditions. A 1w resistor dissipating 0.5w will soon be toast if wrapped in thermal insulation.
As with power transistors, the power rating can often be specified at a specific case temperature. Up to the user to ensure this is maintained:
heatsink, even forced cooling etc.
Maximum dissipation cannot be specified without environmental conditions. A 1w resistor dissipating 0.5w will soon be toast if wrapped in thermal insulation.
Obviously, ambient conditions change the thermal capacity of a device, but in this world of constants, a smaller part is going to have less area to dissipate heat.
These dummy loads are heat sinked, but I don't trust these new resistors to be up to the capacity of the older ones. And their size is not in agreement with the seller's resistor size vs wattage rating chart. They sold me a 100W sized resistor.
Unless they changed their environmental conditions from, say 25°C to 18°C to get the smaller package to "meet spec", I don't see how these little parts will handle 200W for any duration.
My other dummy load can tolerate 1200W for 15 seconds without degradation or damage. I much doubt these will handle even short term over limit tests.
If the seller doesn't agree to correct the matter, I'll be filing a claim to get it refunded. But I have no intention of paying freight to ship these back to China!
Mouser has power resistors of this nature, but curiously, Ohmite makes them smaller still:
Length:89.7 mm
Width:72.5 mm
Height:41.8 mm
And $60 a piece, vs. $8 from eBay.
Starting to wonder if a change in materials is causing them to be smaller.
These dummy loads are heat sinked, but I don't trust these new resistors to be up to the capacity of the older ones. And their size is not in agreement with the seller's resistor size vs wattage rating chart. They sold me a 100W sized resistor.
Unless they changed their environmental conditions from, say 25°C to 18°C to get the smaller package to "meet spec", I don't see how these little parts will handle 200W for any duration.
My other dummy load can tolerate 1200W for 15 seconds without degradation or damage. I much doubt these will handle even short term over limit tests.
If the seller doesn't agree to correct the matter, I'll be filing a claim to get it refunded. But I have no intention of paying freight to ship these back to China!
Mouser has power resistors of this nature, but curiously, Ohmite makes them smaller still:
Length:89.7 mm
Width:72.5 mm
Height:41.8 mm
And $60 a piece, vs. $8 from eBay.
Starting to wonder if a change in materials is causing them to be smaller.
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Well no, there's another way, allowing the resistor core to run at higher temperatures means the thermal conductivity can stay the same and the dissipation rating be higher, so long as the heatsink is rated for that power coming in. That does mean the shell of the resistor will be a lot hotter where its not in contact with the heatsink, affecting nearby components. This wouldn't make for a precision resistor value (!)
" ..but curiously, Ohmite makes them smaller still" 🙄
Well, obviously you are an expert on this subject and don't need any "expert" advice.
Good luck!
Well, obviously you are an expert on this subject and don't need any "expert" advice.
Good luck!