4R and 8R 100w resistors from Ali

[GuntherM]

Hi all,
I'm pretty much new here. I want to build myself a dummy load bench to test output stages from diverse amps.
I want to use 2 resistors from 4R and 2 from 8R, with underneath schematic.
The parts are partial in my possession. But not yet the big boys from 100watts.
I noticed on Ali they have several, cheap resistors for the job. The question is, are these reliable enough to use in a hobby environment?
Those they sell here costs about U$ 300 a piece.
Sorry for the bad English.

Gunther Mampaey

[PartialDischarge]

I really have not had experience with those resistors from Ali but the only caution that I would take is to derate the power, ie assume that a 100W resistor is in reality 75W at most and so on.
Other than that think of using fans to run air thru them

[GuntherM]

With 4 of these in one chassis, vents are needed (I have these from Pasp for this project), it's not said they will operate at 100% but I want a load on the outputs. But not with unneeded costs. That's why my question about the ali's.
Thanks

[Vovk_Z]

I've bought some small amounts of different passives from Ali/Ebay - they all were fine. I buy primarily small parts, not 100W ones.
I definitely won't buy semiconductors from those places, but I'm almost sure about passives.

[alm]

Keep in mind that 100W resistors are generally only 100W when bolted to an infinitely sized heat sink that keeps the resistor at 25°C. The resistor absolutely won't be able to handle 100W on its own.

This applies to any power resistor with mounting holes, which is most of them above 12W or so.

[Vovk_Z]

I'm a bit concerned about your 8R schematics - it won't have an 8 R impedance. It'll have a higher impedance than 8 R.
4 R IHF artificial reactive load I've seen has something like 2.7- 3.0 R active resistance plus an active and reactive resistance of an inductor.
The same for 8 R rated load - it has to have something like 5.6 .. 6.8 R active resistor.
We use active 4/8R resistor dummy loads typically without reactive elements. But if you want a reactive load according to Electronic Industries Association / Institute of High Fidelity (EIA/IHF) standards then the active part has to be smaller than full impedance accordingly.

[CaptDon]

If you are playing with audio amplifiers (I gather that from the load impedances) you really don't need those inductors and capacitors. An 8 ohm or 4 ohm (non inductive if available, usually marked N.I. on the ceramic body) resistor is fine. I have a switch setup with 8 ohm resistors for parallel (4 ohms at twice the power of a single resistor) or series (16 ohms at twice the power). It can also be done with banana plugs and jacks. Also, if you are careful to mind temperature rise you can put 100 watt ceramic resistors (the hollow type) vertically in oil (pick your favorite 10W30, 0W20, or even ATF) and push the resistors hard 100W can be pushed to at least 150W in oil. I have had horrible luck using the resistors that look like a flat pack transistor. They are rated I believe at 75 watts but that is ONLY with a nearly infinite heat sink and physically internally they are generating all that heat in a die of about 5mm X 5mm so they don't take abuse very well. We used them for locomotive power supply testing at GE and quickly went back to oil bath or air cooled ceramic tubular resistors!! If you go with oil bath watch the oil temperature as all oils are flammable!! I don't recommend leaving a kilowatt amplifier running for more than an hour into an oil bath dummy load!!

[GuntherM]

Yes, 2 8R's in parallel also gives 4R, stupid of me, well forget the schematic, these will be bolted in a case from about 40cm x 20cm, 2U and 2 Papst ventilators, I hope that will do the trick. Yes, it was suspicious they were that cheap for 100w resistors. It's like EDavid said, the price for a real 100watt is about €200 to €300 at least on this side of the globe.

Hey guys, thanks for the tips here. You helped me a lot.

GM

[Vovk_Z]

the price for a real 100watt is about €200 to €300 at least on this side of the globe.

Look at the attachment. They need a heatsink of cause, anr they have to be connected in parallel-series to obtain required rated power and reliability, but price still far from 200 eur.

[Jester]

This may not be relevant for your application, however one deficiency I noticed with these cheap resistors is there dielectric strength to the case varies a fair bit and is much lower than the name brand resistors that are often rated to 2-3 kV WRT the case. I tested a selection of the cheap ones and some failed at <500V

[SmallCog]

I imagine that e14's prices are similar in the EU to here

I'd just buy from them, you don't want to be doubting your test gear

https://au.element14.com/arcol/hs100-4r-f/resistor-wirewound-4r-1-solder/dp/2678739

[mariush]

Any particular reason you wouldn't just parallel a bunch of resistors to get your 100w rating?

Ex 30 x 240 ohm 5w resistors will get 8 ohm and 150w in theory... 40 cents each in a pack of 50 : https://www.digikey.com/en/products/detail/te-connectivity-passive-product/ROX5SSJ240R/9926211

Surface mount resistors will be closer to 1% tolerance but a bit more expensive at around 1-2$ each, for example : https://www.digikey.com/en/products/detail/te-connectivity-passive-product/3560240RFT/9927368

There's also to-220 or dpack packages rated for 25-35w each (with heatsinking/cooling)

For example attach 9 of these 75 ohm 30w to-220 resistors to a heatsink and you have a 8.33 ohm resistor for < 60$  which should be close enough for your needs : https://www.digikey.com/en/products/detail/caddock-electronics-inc/MP930-75-0-1/1284420

Note that resistors like the ones in SmallCog's post above and also the ones in my post are only rated at the advertised power at 25c or some other low temperature, they can only do the rated power with heatsinking / cooling.
For example, the TO-220 75 ohm 30w resistors are only rated for 30w at 25c - the datasheet has a graph on page 2 with the rating vs temperature : http://www.caddock.com/Online_catalog/Mrktg_Lit/MP9000_Series.pdf
 
For example, for those to-220 75 ohm resistors, they're rated at 40% at 100c (and 60% at 75c), so only rated for 0.4x30w = 12w at 100c... with 9 in parallel, you'd still get 100w+ you need at 100c, barely.


ps.  You may also want to be careful about wirewound resistors, as they may be inductive... the above to-220 advertise being non-inductive while the wirewound resistors... I guess it depends on how the winding is done.

[GuntherM]

Any particular reason you wouldn't just parallel a bunch of resistors to get your 100w rating?

Ex 30 x 240 ohm 5w resistors will get 8 ohm and 150w in theory... 40 cents each in a pack of 50 : https://www.digikey.com/en/products/detail/te-connectivity-passive-product/ROX5SSJ240R/9926211

Surface mount resistors will be closer to 1% tolerance but a bit more expensive at around 1-2$ each, for example : https://www.digikey.com/en/products/detail/te-connectivity-passive-product/3560240RFT/9927368

There's also to-220 or dpack packages rated for 25-35w each (with heatsinking/cooling)

For example attach 9 of these 75 ohm 30w to-220 resistors to a heatsink and you have a 8.33 ohm resistor for < 60$  which should be close enough for your needs : https://www.digikey.com/en/products/detail/caddock-electronics-inc/MP930-75-0-1/1284420

Note that resistors like the ones in SmallCog's post above and also the ones in my post are only rated at the advertised power at 25c or some other low temperature, they can only do the rated power with heatsinking / cooling.
For example, the TO-220 75 ohm 30w resistors are only rated for 30w at 25c - the datasheet has a graph on page 2 with the rating vs temperature : http://www.caddock.com/Online_catalog/Mrktg_Lit/MP9000_Series.pdf
 
For example, for those to-220 75 ohm resistors, they're rated at 40% at 100c (and 60% at 75c), so only rated for 0.4x30w = 12w at 100c... with 9 in parallel, you'd still get 100w+ you need at 100c, barely.


ps.  You may also want to be careful about wirewound resistors, as they may be inductive... the above to-220 advertise being non-inductive while the wirewound resistors... I guess it depends on how the winding is done.

Any trick that can do the job will be ok for me, I have a FM Kathrein dummy load from 100w and that heatsinck is huge, About 15cm in length and 6cm wide and high. These are pretty expensive themselves. And these have to be mount outside.
Maybe I can try your path, it's just a bunch of resistors, right? The case is big enough to station such arrays from resistors.
Thanks,

@edavid, I noticed the prices were much cheaper. But it still those heatsincks 85€ a piece isn't cheap. I was a little too enthusiastic in my reply.

Also thanks.

[magic]

Consider a dozen or two 5W/10W (whichever is more cost effective) cemented resistors, as they are cheap and don't need heatsinks. Buy some appropriate value (perhaps 1Ω or similar), fill the box with them in appropriate series-parallel configuration with free air between resistors, add a fan.

[Jeff eelcr]

One shop I worked at used a toaster as an 8 ohm load.
Good for a few hundred watts. It would glow with the larger amps.
I have a home heater coil I use now good for more than a few
watts. A thousand watt amp gets it hot but no glow.
Jeff 

[Mechatrommer]

just finished my passive load and use it to test my circuit... 5 x 20 ohm (100W), so i can configure say 1 ohm 125W, 2 ohm 250W up to 100 ohm 500W.. about $25 cost for resistors from aliexpress... fwiw..

[ledtester]

Here is a related thread:

https://www.eevblog.com/forum/beginners/dummy-load-for-bench-power-supply-resistors/

There's a pic of a dummy load built with those small-sized power resistors and active cooling:

https://www.eevblog.com/forum/beginners/dummy-load-for-bench-power-supply-resistors/msg3786482/#msg3786482

There's also a pic of a 120W 8R resistor made by Dale compared to the aliexpress "100W" resistor:

https://www.eevblog.com/forum/beginners/dummy-load-for-bench-power-supply-resistors/msg3786527/#msg3786527

[srb1954]

just finished my passive load and use it to test my circuit... 5 x 20 ohm (100W), so i can configure say 1 ohm 125W, 2 ohm 250W up to 100 ohm 500W.. about $25 cost for resistors from aliexpress... fwiw..

If you are hoping to dissipate up to 500W in that box you will need to drill lots of ventilation holes to allow plenty of air flow around those resistors.

It would also be a good idea to upgrade the wiring to a high temperature insulation like silicone or fibre-glass and maybe even use screw connections rather than solder - the solder on those resistor connections might melt if the temperature gets high enough.

[Mechatrommer]

only to test it on small circuit. like 10-20W.. the wiring only i want quick connection last night to make it work. and its fun to see that wire melting, and i need their higher resistance anyway. if i want it become 500W room heater, i know the drill, cheers.

[jonpaul]

Avoid Chinese knockoff junk.

We Use surplus Dale US made NH non-inductive R, from epay, ham radio fleas.

Here is an epay listing at $20

https://www.ebay.com/itm/193716203646

We accumulated  boxes of them...resistances 0.1 Ohm...1k Ohm.

extruded alum body, can be heatsunk, incredible overload capability.

Low inductance,  1..3 % accurate, used by all for audio amp testing.

We use only  the 4 or 8 ohm,  no other parts needed.

Enjoy!

Jon

[Psi]

Sometimes,  you need more resistors


It's my adjustable 230A 12V dummy load.  Various taps are paralleled to go from 20A to 230A.
I'm not sure if they're genuine 100W resistors or aliexpress copies as I got them in a box for $10 at a junk sale.
Mix of 0.1R and 0.05R.  They seem to work fine though.
In my application i only ever need to test for 60sec or so.
Even at full power it's only ~40W per resistor