Resistive Load Box

[MathWizard]

I have a few low ohm power resistors, but nothing for any amount of power at mains 120/240V or 380V in SMPS. Ages ago I ordered a cheap bag of 3k, 2W Carbon composite resistors on ebay, they look like light blue cement or chaulk.

So I soldered up a perf board with 6rows * 8columns in parallel, and the legs match well by luck, and I plan to put it in a box with a fan, and some switches for different total load. For 2W each they can handle (in ebay land) 26mA @ 77V continuous (2945ohms avg) .

But when do they really start cooking/changing from factory spec's ? 

So if these are just your basic cheap Carbon composite resistors, what type of pulse power might be too much and damage them inside ? I should destructively test some, but not in the house.

At some voltage I'd have to worry about arcing, but I'm not using those voltages yet.

[jonovid]

incandescent bulbs make for good dummy loads. series or parallel depending on your voltage under test. 

[MathWizard]

Yup I forget about them too. What type of OV or OC does it take to fry a 60W light bulb ? They have different heating up times, and effects,  etc, so resistor's are still better for some stuff.

[Berni]

I keep some big 20W to 100W power resistors around for quick easy loads. The big aluminium body resistors take a while to heat up so you can run a 100W resistor at 500W for a few seconds just fine. Usually that is enough to determine if a power rail is capable of delivering the oomph by watching how much the voltage drops when connecting the load.

When it comes to more long term thermal torture tests on things then i do bring out an actual electronic load that can pull lots of juice for long periods.

Another easy way for high power loads is a bucket of water filled with resistance wire. The water takes a long time to heat up so you can put 10kW into one of these for some time. Even if you get the water to a boil it still won't go over 100°C so you can just keep going (Tho you probably don't want to do this indoors due to the massive amounts of humidity the boiling water produces)

But a big bank of 2W resistors cooled by a fan is a perfectly valid way too, just make sure you have a strong fan blowing trough it since the 2W rating per resistor is for natural convection open amibnet air. If your fan blows hard enough you can probably take those 2W resistors up to 10W no problem. Just stick a thermocouple to one of the resistors in the middle and see how far you can take it before it gets too toasty (They should run fine up to about 150°C)

[Gyro]

So if these are just your basic cheap Carbon composite resistors, what type of pulse power might be too much and damage them inside?

Carbon composition resistors can actually have very high pulse power ratings, but it sounds as if you are talking about something with longer time constant than a typical pulse (ie. long enough to get them up to high body temperature).

[wizard69]

How desperate are you for a high power load?   You could do something like repurpose a toaster or use some pallet binding steel strap (in a suitable container).   In the end all you need is the right resistance, in a device that can handle the applied power.