100W resistor, how much is too hot?

Hey Guys,

Built this dummy load device from 100W alloy heatsink resistors from ebay.
Bolted them to an alloy heat sink with thermal grease and a 12v fan.

Anyway, the thing works great but at 28A for about 1 minute with 16pcs 100W 20 ohm resistors connected in parallel to a 36V pack the resistors heat up to about 75c
Is that a temperature range that these resistors can tolerate?

well it depends largely on the heat sinks that you using and how well they are thermally connected. That kind of temperature rise is fast. Are the heat sinks coming up to a similar temperature ? Also I assume that you want to load test the battery pack and a one minute load on time is not what you want.
Generally these type of resistors depend on the heat-sinks that they are mounted on to achieve their ratings. Without diving too much into thermal design work the use of a 20 ohm 100 watt resistor on a nominal 36 volt supply is going to load up the resistor with 65 Watts of dissipation. According to the spec on the resistor that (with the correct heat-sink) will give you a temperature rise of 65C over ambient (1.0C/watt) (I.E. if its 20C in the room the resistor will get to 85C surface temperature).
That is doable but I would of used a larger heat-sink than their minimum (that calls for a piece of aluminium with 995CM^2 surface area or 25cm X 20 cm X 3 mm thick if you use flat plate). so think about (~0.5 kg plus very roughly per resistor) and perhaps a fan to help it keep cool.
I would generally use an old rule of thumb for power resistor ratings of not exceeding half current rating or 1/4 power (same thing) so I would of connected two of your 20 ohm resistors in series (total of 40 ohms) and X4 up on the numbers. (that's a lot of resistors).
So I suspect that given the speed at which the resistors are rising in temperature you are a bit shy on the heat-sinking required and it would be a good idea to add fans. (a fan per heat sink if you have one heat-sink per resistor would be a good idea). A suitable fan can easily double the rating of a piece of heat-sink (properly designed with the correct baffles to shape the air flow, much more than that).
So bottom line you need to do better with cooling so that you don't get to much more in temperature (after 10 minutes say) than you are getting in 1 minute at the moment.

Bob

https://endless-sphere.com/forums/viewtopic.php?f=1&t=89229

There should be a specification for maximum allowable temp. A quick check of a similar looking resistor shows a max temp of 200C.
http://www.mouser.com/ds/2/303/acl_hs1-748789.pdf

Other models on the same datasheet only go to 100C, so hard to say for sure, but I wouldn't worry if it stayed below 100C.

in my discharger diy they heat up to 120°. still running fine. but it's a bit scary

Thanks alot guys, I guess that I will add more fans as that is the easiest way to go right now

rg12 said:

Thanks alot guys, I guess that I will add more fans as that is the easiest way to go right now

Click to expand...

here's my discharger: https://endless-sphere.com/forums/viewtopic.php?f=1&t=84372&p=1234642#p1234642

izeman said:

rg12 said:

Thanks alot guys, I guess that I will add more fans as that is the easiest way to go right now

Click to expand...

here's my discharger: https://endless-sphere.com/forums/viewtopic.php?f=1&t=84372&p=1234642#p1234642

Click to expand...

I know, I built mine according to your design :wink: