I would like to test an amplifier (see my avatar) with a passive resistive load with voltage swings in the range of 40V peak. Since, using a speaker at home generates sounds which can damage hearing due to having walls reflecting sound wave at a short distance, I plan to use a resistive load using a network of filament lamps.
Is this possible without causing damage?
Is this possible without causing damage?
Filament lamps are terrible as a load, their resistance varies 15:1 from cold to hot.
If expensive/inconvenient to buy, make your own out of cheap kitchen heater nichrome wire.
used in this kind of heaters:
First you stretch it 1.5X or 2X to separate wire turns (otherwise it self shorts) , then measure and cut appropriate resistance sections.
Mount it to strips cut out of floor/bathroom tiles, drill holes near the ends and insert 1/8"-3 or 3.5mm screws, use nuts and washers to hold it in place.
Warning, first wire and then tile will get hot, put it on a brick/cement/tiled floor, never on wood, furniture or plastic.
Make them somewhat lower resistance than needed, say 7 ohm for a nominal 8 ohm one, resistance goes up with temperature.
If expensive/inconvenient to buy, make your own out of cheap kitchen heater nichrome wire.
used in this kind of heaters:
First you stretch it 1.5X or 2X to separate wire turns (otherwise it self shorts) , then measure and cut appropriate resistance sections.
Mount it to strips cut out of floor/bathroom tiles, drill holes near the ends and insert 1/8"-3 or 3.5mm screws, use nuts and washers to hold it in place.
Warning, first wire and then tile will get hot, put it on a brick/cement/tiled floor, never on wood, furniture or plastic.
Make them somewhat lower resistance than needed, say 7 ohm for a nominal 8 ohm one, resistance goes up with temperature.
Immersion-type water heater elements are 24 ohms (240V/2400W). Most water heaters use two for 4800W. Three in parallel make a very large 8 ohm load that can be water cooled. The 120 volt 2400 watt (for single stage, mobile home use) are 6 ohms, but will vary more with drive/loading because they are NOT usually the direct immersion type (exposed Nichrome wire). Heat strips from discarded all electric central heat “furnaces” are probably suitable too - they are in the right ohms range - just got to keep that blower motor installed if you want close to the 10 or 20 kW rating. If youre not testing a multi-KW PA amp you may not need the fan, and ohms will be a little lower.
What's wrong with a simple 100W resistor??
You guys are quite adept at turning the simplest of the simplest into a Manhattan Project 😎
Jan
You guys are quite adept at turning the simplest of the simplest into a Manhattan Project 😎
Jan
Depends on what exactly you are testing. A real speaker load has both resistive and reactive components, each producing different effects.
If the OP was considering using tungsten lamps than he was clearly worried about the duty cycle and overloading a conventional dummy load. I can still recall the carpet fire I had when I tried using a pair of 50 watt metal cased resistors on a heat sink to test a 600 watt amp for a short duration test. Should take it long enough for a powr/voltage reading, right? 5 seconds and FIRE shot from the end of the thing, landing on the carpet in my apartment and catching it. So much for trying to test an amplifier QUIETLY.
Who are you complaining to? You made an error of judgement, that's the things you learn from!
A 100W resistor for a 100W amp, not for a 600W amp!
Jan
A 100W resistor for a 100W amp, not for a 600W amp!
Jan
1) This is D-I-Y Audio.
2) OP is in Malta, halfway between Sicily and Tunisia, in the middle of the Mediterranean.
The $10 resistor requires $20 shipping + 10 days delay.
And that if it´´s the aluminum cased type, which requires bolting to a proper heatsink, which adds to cost.
And which on its own, amounts to a Manhattan Project, by your standards.
IF a classic "dynamite stick" ceramic body type, it costs U$D 25 + again U$D 20 and same delay.
It does not require a heatsink but don´t just place it on anything flammable or meltable.
3) I am in Argentina, guess very similar situation as Malta , last week I bought at my friendly hardware shop 3 blocks away a ceramic heater resistance as I showed in my post for one U$D
Being 1000W 220VAC total resistance is 48 ohm, I can cut it into 3 x 16 ohm sections (very useful) which allow me to dissipate 330W each if I let them run red hot (no big deal, they were designed for that) and of course can be used at less power any day of the week.
Manhattan project turned out to be drilling 6 * 1/8" holes in an old tile, passing 6 1/8" by 1" screws through them and attaching the 3 pieces of resistive wire.
Total cost 2 to 3 U$D, total time way less than 1 hour, delivery time was 10 minutes (walking 3 or 4 blocks), I would do that any day of the week, instead of ordering and waiting.
Hey, my load resistor would have even stood wg_ski´s 600W amplifier!!!!! 🙂
For prolonged high power tests you can either use a fan or drop it into a water bucket.
For my dummy load, I made a wooden frame and wound 1.5mm^2 ECW onto it, to give me 8ohms.
Power rating? No idea but it handled a 2400W amp on soak test all day with little temperature rise.
Power rating? No idea but it handled a 2400W amp on soak test all day with little temperature rise.
You're right, we forget all these simple things.
personally I'm a fan, thank you for bringing us back to earth
Would you care to recheck those numbers?
They do not exactly add up.
Unless you are trolling us, of course.
Or they were written tongue-in-cheek that is 🙂
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Your calculation is valid at 1000W when the wires are red glowing. Which resistance do you expect at 100W or at 10W?
TempCo of wire metal cannot be ignored totally.
Oh, if easily available and short of time, why not?
Americans and to a lesser degree Europeans "have it easy" , tons of product available locally, reasonable prices, not-that-cheap posting (lots of complaints about that),next day delivery, etc.
Anywhere else it gets complicated enough to turn DIYing not only "an" option but often "the" only option available.
Including ignorant but suspicious (or greedy) Customs personnel.
Or plain crazy Post/Courier fees.
Just last week PRR generously searched a book for me (History of RCA factory relocations along time, and eventually outside USA)
Now Books are "Printed matter/Cultural items" and pay NO Tariff (at least in the Civilized World), yet:
😱 😱 😱
For a book!!!
I guess that kind of shipping fees alone makes DIYing VERY acceptable 🙂
Notice most of people self building stuff or components or searching for creative substitutions come from, say, South Africa, Australia/New Zealand, Eastern Europe, India, Brazil, etc. , (lackof) market availability + long/expensive freight complicates things
In the 120V world, a 1,500W electric kettle makes a resistor about 10 ohms. Take a 6-outlet extension cord, plug in two kettles, and we have a 5-ohm load. Have some water in the kettles to keep from overheating. Plug in more kettles when lower resistance are needed.....just an idea.
You are right.
I am not ignoring it, at all, on the other side I was surprised that resistance variation was not THAT high and to boot it is lower than the typical "nominal vs DCR" assumption in loudspeakers in general, so it´s actually bonus point.
Extra detail: speakers at rated power have WAY higher DCR than when measured "cold" with a Multimeter, go figure.
Extra extra detail: I would love to know what actual DCR do commercial resistors (plain ceramic or aluminum clad) get to when dissipating rated power ,certainly higher than guaranteed/multimeter measured "8 ohms" or whatever.
If all made of Nichrome wire, both DIY and commercial will behave in a similar way.
You will be surprised (I was, big time), but Nichrome is WAY more stable than Copper!!!!!! 😱
Copper resistivity thermal constant: 0.00393 , you can safely round it to 0.004 per Degree Centigrade.
Nichrome?: amazing low 0.0004 per Degree Centigrade, 10 times less!!!
EDIT. If curious:
http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/rstiv.html
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Yeah, I just expected the metal ones to do as well as the white ceramic cased ones. I had it on a big amplifier heat sink. I’ve given the white ceramic 10 watters 10X overload for a minute or two before with no ill effects (the bipolar crossover caps, not so much). After that little incident I figured out the water heater element trick. Amps are a little bigger these days so it’s a necessity.
Nichrome is also in toasters, right? Buy an old toaster and trim the heating element to 8/4 ohm?