I'm looking at parts to build a dummy load. I am considering using one of these 100W resistors: Acl Ap101 | Ohmite Mfg Co
I identified this heatsink: Sink F R | Ohmite Mfg Co
Based on a figure of 3.5W at 25C.
I have a hard time understanding how to match up parts and heatsinks, which is why I'm posting here. If I'm off track, could someone people explain the process for selecting a heatsink for a given part? When I google, I find mostly stuff about designing custom heatsinks.
I identified this heatsink: Sink F R | Ohmite Mfg Co
Based on a figure of 3.5W at 25C.
I have a hard time understanding how to match up parts and heatsinks, which is why I'm posting here. If I'm off track, could someone people explain the process for selecting a heatsink for a given part? When I google, I find mostly stuff about designing custom heatsinks.
A 3.5W heatsink rises 1 degree Celsius for every Watt you put into it (assuming the spec says something like "3.5K/W ").
So you need to figure out how many Watts your resistors will dissipate to know how hot the heatsink (and the resistors) will be.
Do you know the voltage and the resistor value? The power into the resistors is Voltage squared / R.
Jan
So you need to figure out how many Watts your resistors will dissipate to know how hot the heatsink (and the resistors) will be.
Do you know the voltage and the resistor value? The power into the resistors is Voltage squared / R.
Jan
The resistor is rated for 100W @ 25°C case temperature.
This is barely above ambient and an unrealistic operating temperature.
Seattle in June reached 42°C .
Go for a large power resistor.
Plenty of dummy load ideas on the web.
Dummy Speaker Load Box
Or water cooled - Advice for Building a Dummy Load for Amplifiers | Electronics Forum (Circuits, Projects and Microcontrollers)
This is barely above ambient and an unrealistic operating temperature.
Seattle in June reached 42°C .
Go for a large power resistor.
Plenty of dummy load ideas on the web.
Dummy Speaker Load Box
Or water cooled - Advice for Building a Dummy Load for Amplifiers | Electronics Forum (Circuits, Projects and Microcontrollers)
Hmm, fair point. But still, how would I calculate the required heatsink for whatever resistor I get? The datasheet says
How do I know what the designed heat sink is?
What spec do I need to reference to determine if that is correct? The resistor I'm looking at is rated for 100W. Does that mean it will output 100W of thermal energy into its heatsink?
Put power into your speakers: a small amount moves air in the room which ultimately warms the air. The remainder is wasted as heat in the speaker coil, which ultimately warms the air.
The amp I'm testing is rated for 255wpc, but bench tests show it going beyond 400wpc, so I was planning on running five of these resistors in parallel to dissipate 500W.
What's confusing me is the various terms used in the spec sheet:
Ok, but how big does the heatsink need to be?
What is the 3.5W figure referring to?
The derating curve shows rated power % falling off with case temperature "Made with ... component mounted on the designed heat sink". Again, what is the "designed heat sink"?
None of this tells me how large of a heatsink I need to fit to the resistor. It just says that it can handle 100% in a 25C environment when mounted to "some kind of heatsink"
The resistor is small.
It will handle 3.5W without heatsink at 25°C in free air.
That's it - 3.5W.
Mount it on a huge heatsink, on a cold day and it will handle 100W.
Your heatsink was rated at 3.5°C/W.
That means dissipating 100W it will increase in temperature 3.5x100 = 350°C.
+ 25°C ambient = 375°C.
A fridge would be more appropriate.
100W dissipation - think how hot a 100W lamp gets - you cannot touch it.
That's the amount of heat you need to lose.
I suggest you follow the links I posted and use a proven design.
It will handle 3.5W without heatsink at 25°C in free air.
That's it - 3.5W.
Mount it on a huge heatsink, on a cold day and it will handle 100W.
Your heatsink was rated at 3.5°C/W.
That means dissipating 100W it will increase in temperature 3.5x100 = 350°C.
+ 25°C ambient = 375°C.
A fridge would be more appropriate.
100W dissipation - think how hot a 100W lamp gets - you cannot touch it.
That's the amount of heat you need to lose.
I suggest you follow the links I posted and use a proven design.
The package for the resistor you chose definitely needs a properly rated heatsink. Its a pretty small area for 100W of power dissipation.
Using 4 of these to test a 400W amp is "on the edge" and thus not a good idea.
I´d choose resistors with (kind of) "built-in" heatsinks:
https://uk.farnell.com/vishay/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1/dp/1231719
These types of resistors already have a metal shell and can dissipate "quite a bit" short-term without any heatsink and also have a much bigger contact area for a heatsink than the one you´ve shown.
I use 4x4Ohm/50W to have a 4Ohm-200W resistor without heatsink.
For 8Ohms I connect a 2nd set in series.
Testing a class-d amp at 100W+ will get them piping hot of course and I have to watch out not to leave any burnt marks on the table but other than that they can take quite a beating. (in their datasheets quite often a max. temperature is mentioned which is often around 200°C!)
I´d maybe use at least 4x100W types if I were to test 400W amps on a regular basis:
https://uk.farnell.com/arcol/hs100-4r-f/resistor-wirewound-4r-1-solder/dp/2678739
They will need a good sized heatsink for tests lasting minutes.
If you want to test for hours you´ll need a powerful fan (or other tricks) too otherwise the needed heatsink would get real big and expensive.
Here is a nice example of how to make a resistor array/dummy load:
Building my own noninductive 8R 150W load using wire wound resistors
I´d ignore the non-inductive part though. I personally think a proper dummy load should have some inductance and even more so: some capacitance but that´s another topic.
Using 4 of these to test a 400W amp is "on the edge" and thus not a good idea.
I´d choose resistors with (kind of) "built-in" heatsinks:
https://uk.farnell.com/vishay/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1/dp/1231719
These types of resistors already have a metal shell and can dissipate "quite a bit" short-term without any heatsink and also have a much bigger contact area for a heatsink than the one you´ve shown.
I use 4x4Ohm/50W to have a 4Ohm-200W resistor without heatsink.
For 8Ohms I connect a 2nd set in series.
Testing a class-d amp at 100W+ will get them piping hot of course and I have to watch out not to leave any burnt marks on the table but other than that they can take quite a beating. (in their datasheets quite often a max. temperature is mentioned which is often around 200°C!)
I´d maybe use at least 4x100W types if I were to test 400W amps on a regular basis:
https://uk.farnell.com/arcol/hs100-4r-f/resistor-wirewound-4r-1-solder/dp/2678739
They will need a good sized heatsink for tests lasting minutes.
If you want to test for hours you´ll need a powerful fan (or other tricks) too otherwise the needed heatsink would get real big and expensive.
Here is a nice example of how to make a resistor array/dummy load:
Building my own noninductive 8R 150W load using wire wound resistors
I´d ignore the non-inductive part though. I personally think a proper dummy load should have some inductance and even more so: some capacitance but that´s another topic.
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The rating means that it is the max it can handle. It does NOT mean that it will always dissipate 100W. The dissipation is what YOU put into it.
Assuming it is an 8 ohms resistor, if you connect it to an amp and drive the amp to 50W output, then that's what the resistor gets. It is below 100W so it is safe for the resistor.
And if the resistor is put on your 3.5C heatsink, that heatsink will go to 50 * 3.5 = 175 degrees C above ambient. If your room is 25 deg, the sink will reach 200C. Not healthy.
If you want to dissipate up to 100W, get a 0.5deg heatsink. With 100W it will rise 50 deg so with 25 deg ambient, reach 75 deg. You will not be able to hold that, but nothing will break.
Jan
The heatsink will bankrupt you...
Why not use this type:
SURPLUS 2 Ohm 300 Watt POWER RESISTOR 8.5 inch X 1.25 inch OHMITE C300K2RO
Why not use this type:
SURPLUS 2 Ohm 300 Watt POWER RESISTOR 8.5 inch X 1.25 inch OHMITE C300K2RO
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Right, which is why I said I was going to use 5 of them, not 4 😉
How do you decide how much resistor power capacity you want for a given load? If you wanted to run that 100W amp without risking burn marks, would you add more resistors? Add more heatsink?
Because I was having trouble finding one in the values I need. I want to be able to run ~425W into 8 ohms. I figured 500W power capability would be enough. So I'm trying to find resistors that I can stack up to get to that value, since a 500W resistor is quite expensive.
Remember, you’re only going to get about 60 watts of dissipation at 75C and much less at higher temps. That means you’re going to need ten resistors minimum to be safe. Check with companies like Heatsinkonline.com to see what extrusion they recommend. It’s going t9 be big! Dale makes big aluminum cased 250w power resistors that are made for this kind of thing.
Search 225W or 300W on this page... They will be as hot as a space heater and easily cause a fire from curtains etc though... But for testing, they need no heatsink. Maybe a fan but that's free from the garbage.
40$ CAD for 900W power handling...
Gold Metal Resistor
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