A caution with power resistors are loads. - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 05:35:40 pm
content: So I had 2x 50W 0.5Ohm current shunts for high power stuff.I decided to put them in parallel and use them as a DC load. Varying the voltage through them I was putting about 60W through the pair. I was thinking, well, that's fine, they are 50W resistors aren't they.After about 5 minutes I noticed the smell. Put my hand near the resistors and felt the heat. Lowered the power down to about 40W and thought not much of it.Eventually when the smell persisted I tapped one of the resistors with a finger and "OUCH!" so.... I pointed an IR camera at them.235*C!Wow. Hot enough to melt solder and not that far off softening the aliminium casing!So, I gather that the 50W is only available if you can bolt them to a bit enough heatsink and the little anodized heatsink they have is only good from about 5W.Note. There was no damage to anything and they were still functioning find and still do. The smell I expect is most likely glue, plastic sealants, paint and the plastic caps on the ends baking.

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2023, 05:40:32 pm
content: If those are the popular metal-cased bolt-down wirewound resistors, then the "50 W" rating is like unto the top-line power rating for power transistors with mounting holes: it assumes a specific case temperature (a function of the heat sink) to determine the power dissipation that will keep the internal temperature within safe limits.Old-fashioned wire-wound resistors (with vitreous enamel or silicone coating and no mounting holes) were specified for a given ambient (air) temperature.Even then, a wire-wound resistor spaced away from the board run at full rated power will get very, very hot, since the resistor materials can tolerate much higher temperature than can silicon.(Guess if I learned that the hard way, by reading the datasheet, or the easy way, by touching one.)

author: jonpaul
author_rank: Super Contributor
timestamp: March 10, 2023, 05:49:02 pm
content: see original Dale type RH,and, NH resistor spec sheets.Used these for decades 10w..500w, on specified heatsink, châssis or fan cooledNo problems.easy to predict temp with some thermodynamics....Jon

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 06:15:53 pm
content: When I was about 15 I had an RC car power by 2 11V NiCads. They topped out at something like 20A. The speed control for the car was a rotary arm, on one side was a rotary switch with contacts:STOP-> Disonnected.MID -> Connected via wire resistorFULL -> Directly against the batteryDuring the mid point arc the other end of the arm moved across a wire coil. Battery voltage went in along the arm the output powered the motor.So if you drove around at half "throttle" for a while the wire resistor, wound around a ceramic core and shielded by a grill would glow cherry red.As a 15 yo I evaluated this to mean there was no point driving it anywhere at 1/2 throttle. Zero or FLAT OUT.

author: Gyro
author_rank: Super Contributor
timestamp: March 10, 2023, 06:32:59 pm
content: Quote from: paulca on March 10, 2023, 05:35:40 pmYes, those are casings, not heatsinks when it comes to using any significant proportion of their rating.QuoteBeware that the ends can get blown out with significant force, there's nowhere else for the contents to go once the internal epoxy insulation reaches its flashpoint.P.S. You also need proper precautions when fitting to a heatsink - remove any case burrs, nice flat surfaces and heatsink compound.

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2023, 07:11:34 pm
content: Check leakage resistance to the housing. Although the insulation of the resistance winding is usually high temperature rated and non-combustible, its possible the insides of the end seals have carbonised.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 10, 2023, 07:22:43 pm
content: I have a 50W (100W?) Ohmite wire wound resistor as a load. I don't use it often. But my way of getting near its power rating is to put it in a plastic/glass bucket of tap water. Works well enough for what I needed. Don't worry about the electrical resistance of the water. It will always be high enough not to affect the resistor.

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 07:23:11 pm
content: Instead I bought a bunch of adapters and DC extensions and start running actual stuff off the 12V rail to drain the battery. It's far more productive, produces the same amount of heating (for the room) and is much safer.I should have known better. I already made a 5A load about 3 years ago and I know it took a seriously powerful modern gaming PC cooler to keep the mosfet under 60*C at 50W. Cooler rated for 100W @ 105*c.25W through that heatsink.... I'm not surprised it was 235*C. I just made a lovely little heating element.... not designed to be one.On breaking them and causing leakage... they will never be used for actual measurements or high voltage.

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 07:25:04 pm
content: On heating elements.... that is going to be my ultimate end of line power dump for the off grid system. A set of 100W heating elements dumping heat into a butt of water which I can use to heat something...

author: Gyro
author_rank: Super Contributor
timestamp: March 10, 2023, 07:38:31 pm
content: Old school rheostats (often in both senses), the linear ones, not the rotary ones make excellent variable load resistors. Much more rugged than electronic loads. I have a 3R3 @10A one - 330W at 33V continous, in my set.