| wintermute |
I've been using some very underpowered (2 X 5W|| wire wound) resistors as a dummy load when testing my amp, and have been looking to create a proper dummy load that won't burn the carpet, melt plastic it comes in contact with or imit strange smells ;) .......
I've searched and people gennerally recommend non-inductive types, but for 100W rated non-inductive resistors I'm looking at $40AUS each (and I'd want four... 2 X 15 Ohm wired in parallel). so that's $160!!! (luckily I have a heat sink).....
I can get ceramic wire wound (Obviously inductive) resistors for about $17Aus each which works out at about $68.........
Wouldn't the fact that these resistors have some inductance mean that they are a closer (marginally) approximation to the load presented by a speaker? Is it ok to save some bucks and get the cheap inductive ones, or should I just grin and bear it and fork out for the non-inductive ones?
Tony. |
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| tubetvr |
Connect multiple low power resistors in parallell, I use metal oxid type 5W and have connected totally 20 in series parallell to get 8 ohm capable of 100W continuous power.
9 39ohm + 1 47ohm resistors is connected in parallell giving very close to 4 ohm, 2 of these combinations are connected in series.
BTW, this is how dummy loads for RF was done long time ago by some manufacturers.
Regards Hans |
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| JOE DIRT® |
| Tony...I did post the resitors I use awile back with pics...Dale part# hl-225-07Z......225 watt and affordable |
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| wintermute |
Hmmmm that's two options Thanks guys.... I was trying to avoid paralleling up too many resistors, but I guess the advantage of using heaps of 5W is that a heat sink wouldn't be neccessary.
I did think about using 3 X 47R and 1 X 22R all in parallel for 7.6 Ohms at 100W but wasn't sure whether the power disipation would be even or whether the 22R would cop more of a beating since the whole thing isn't symetrical....... that'd work out at a tad under $70Aus.... I would like it to be able to handle about 150W though......
Those are some mutha resistors you've got there Joe :eek: I'll see if I can find some :) BTW why the change on the avatar?
edit: Ummm searched on dale, and dale electronics, but I'm not sure I have found the right site... Got a url?
Tony. |
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| Bill Fitzpatrick |
I have a few Dale resistors which are ideal for dummy loads.
See at http://www.kbacoustics.com/dale/
Do not reply to the email address on that page - it is obsolete. |
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| millwood |
how about using a heating element? You can take out the heating wires inside and use it as you wish.
I did that way back in the 1980s but haven't tried it lately. |
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| tubetvr |
| quote: | | I was trying to avoid paralleling up too many resistors |
Why?, because of the increased capacitance? Capacitance is not a big problem when building a dummy load for 8 ohm or similar impedance, the problem with using many resistors is when you connect them in series as it will increase the total inductance and that can have an effect even at audiofrequencies.
For a simple calculation using 20 resistors in parallell:
Capacitance of each resistor ~10pF, giving 100pF for 10 in parallell, 100pF + 100pF in series give a total of 50pF in parallell with 8 ohm. The reactance of 50pF at 20kHz is ~159000 ohm so the effect is neglible.
Regards Hans |
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| sayang001 |
| quote: | | 9 39ohm + 1 47ohm resistors is connected in parallell giving very close to 4 ohm, 2 of these combinations are connected in series. |
This is exactly how I build my dummyload, I used 20W wirewound resistors... but paid only $6 for the 20 resistors in Indonesia when I was there on holiday. :) |
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| wintermute |
Hi Hans,
No I wasn't worried about the capacitance, just the logistics of connecting them all up :) Ideally I just wanted to bolt a couple of resistors to a heat sink that I have and connect the speaker wires...... Then I saw the price!!!!
These resistors were the last thing I was going to add to an order I'm putting together for various bits and peices and it's allready up to about $140 without them....
Bill, the cheaper resistors I mentioned before look very simmilar to the ones you posted (although maybe your's are low-inductive ones.... I discovered you can get low inductance wire-wound).
Millwood, I think I'd be more likely to burn the carpet if I used heater elements :D But full marks for the diy element :)
Maybe I'll just get some of the cheaper 25W ones and mount them on the heat sink, will be better than the 10W power rating I have at the moment :). I can allways by some fancy non-inductive types later on, if I feel the need. After all my amp isn't exactly audiophile quality, I just need something that'll make it do some work while testing (which most of the time isn't at full power anyway.
BTW Joe, I found something very simmilar to your Dale resistors at RS Australia, but they were $65 each and 4.7 ohms :( (although that's not bad considering they are rated at 200W)
Regards,
Tony. |
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| Christer |
Tony,
Do you have any compponent surplus companys around? They seem
often to have a lot of power resistors in the 5 - 10W range, at least
here around. You will have to figure which of those values they have
to use and how to hook them up in series and parallel to get what
you want, but it can be a bargain and you won't need any heatsinks,
which save a lot of money. It'll probably end up a worthy candidate
for the ugly prototypes thread, but so what. :) |
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| AuroraB |
Just a word of caution.....
Most of the high power resistors you guys mention will have to be mounted on a fairly large heatsink in order to be anywhere near the listed dissipation figures.
If you parallell plain old fashion wirewounds, it will have to be a flat package,--- if they are made up to form block or a "roll". the innermost ones will have to take a lot more heat than the outer ones, and it will most certainly get very hot in a very short time, -- a minute or two only on full load.
The classical way to make a dummy load is to immerse a 20-50 W ( a parallell package will be just fine) resistor in 1 L of oil,- this will hold for 100W full load for several minutes. Even finegrain sand have been used,but doesn't distribute the heat as well as oil. |
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| tubetvr |
Hi,
Be careful when using wirewound resistors, normal types have enough self inductance to make a considerable influence even at audio-frequencies.
10uH is ~1.25 ohm at 20000 Hz, there are many wirewound resistors with that inductance and more.
Regards Hans
(I want to measure amplifier response as exact as possible up to >1MHz when I measure open loop response aso, therefore I consider it important to use a good dummy load.) |
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| wintermute |
:bawling: I think I need to stop thinking about components for a while, I'm going to go insane!!!!
I found some wirewound resistors at RS components that actually list their inductance, 5.4uH for 10R (100W) and 22uH at 100R .... shame the resistors at farnell don't have that info. I found what I thought would do me for the time being without breaking the bank too much (still expensive, but not as much as the first lot).
3 X 24R 50W in parallel at $6.38 each so that's a bit under $40 for two channels.
the heatsink I was planning to use is 15cm X 16cm with a base about 8mm thick and fins about 25mm high.
as mentioned by AuroraB the specs for the resistors call for pretty big heat sinks the above mentioned resistor specs a 30cm X 30cm X 1.5mm thick heat sink, now I'm guessing they are not talking finned, so I think I should be able to get away with my heat sink for a short while (few minutes maybe, considering I'll have 6!!!).
The heat sink is out of an old mainframe power-supply. was being used by 5 very chunky 2SD3036 transistors and 3 other power transistors of some sort. pic attached.
Tony. |
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| wintermute |
Peter,
I wondered about light bulbs but I thought the resistance would be too low..... Might be worth investigating.
Tony. |
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| AuroraB |
Basically, there is no need to worry about the inductance of the dummy load, unless you specifically want a purely resistive load. After all, loudspeakers presents a rather ractive load, mostly inductive.
The easiest load I think, is made of oldfashioned carbon comp resistors, 2 or better 5 W, 15 parallelles of 120 ohms, 2 or 5 W immersed in mineral oil, --- good for 200 W for several minutes.
I have a 50 ohms in a gallon box, - rated for 1kW, very apropriately named " The Cantenna".
Otherwise, - the mentioned Dale resistors of various values or combinations are very good, provided you have enough surface area to mount them on. |
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| Christer |
I was thinking of Caddocks myself, but they are quite expensive and
definitely needs a lot of heatsinking. Further, I think I read somewhere
on this forum that they are very sensitive to overcurrents. The have
a current limit you must adhere to, even if way below the rated
power dissipation. They might still be a good choice, though, if
taking these things into account. Otherwise, if we are talking
expensive solutions, Dale has low-inductance power resistors
with 4 and 8 ohm resistance. I don't quite remember now if they
are 25 or 50W.
Edit: Found the current limit stuff for Caddocks
http://www.diyaudio.com/forums/show...1223#post311223 |
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| wintermute |
Hmmmm, Just checked the max dimensions 5cm X 3cm should fit but it'll be crowded. Maybe I do need a bigger heat sink after all....
The immersed in oil sounds like a good way to go, but I think it is a bit hazardous to the carpet too :). Would be ok if I had a workshop.
I guess the other thing with the inductance is that if I'm paralleling 3 then the inductance will be 1/3 that of a single resistor and as you say a speaker has a significant inductance (as does the crossover if it is more than 1st order). I think I'll just get the 3 X 50W 24R stick them on the heatsink and see how it goes. It has to be better than the two 5W resistors I currently have paralleled (which got so hot they burned the carpet (and it's wool)!
Tony. |
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| CeramicMan |
| quote: | Originally posted by AuroraB
Basically, there is no need to worry about the inductance of the dummy load, unless you specifically want a purely resistive load. After all, loudspeakers presents a rather ractive load, mostly inductive.... |
Yeah, come on people! First you dish out on ultra-expensive super-dooper non-inductive resistors, and then you buy ultra-expensive super accurate OFC ribbon-wound hand-made inductors to precisely model the speaker's reactance! Has the world gone mad (rhetorical question)???
Don't mean to ruin the thread, but in New Zealand those 5W resistors that everyone seems to be avoiding cost less than a dollar. It takes something like a kilowatt-minute to bring a cup of water to the boil, so why not use water for heatsinking for those bargain resistors? Those retro-style glass coke bottles (or whatever other glass container) would be perfect for holding the water, and if you're worried about the water reducing the resistance slightly - just cover the exposed conductors with a thin layer of oil.
For the ultimate dummy speaker load you could even construct a notch filter network to better model the mechanical impedance peaks that speakers have. Or try to measure the inductance of that imperfect resistor and compensate for it by putting a capacitor (of the correct value) in parallel with it.
Maybe I'm getting cynical, but I can see it coming - a new thread entitled: "top-quality polypropylene or teflon capacitors required for dummy speaker load".... :bawling:
CM :eek: |
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| wintermute |
Jan & Christer,
Those caddoks look like some of the other resistors I can get at Farnell. Seems though any solution I try when it comes to the non-inductive types ends up being within about $10 or $20 of the cost of just getting the "big" ones.
These are the "big" ones I'm talking about :
http://www.farnell.com/datasheets/814.pdf
If I were to parallel two of the 15R ones It'd be good for 200W and about 7.5 Ohms. I worry about spending that much though because if the heat sink isn't adequate and I cook them then it's a lot of money up in smoke.... Seems like they would make a pretty good non-inductive resistive load though.....
When I drove my amp into pretty heay clipping it was delivering about 190W (poor little 5W resistors didn't know what hit them) which is Why I originally wanted to go for 200W rating.
They don't give specs on heatsink requirements, but compared to the others I've looked at the max power rating is at 60 deg C whereas everything else I looked at max power was at 25 deg C and you have to lower it for every deg above that.
Tony. |
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| pjacobi |
| quote: | Originally posted by wintermute
[...] I wondered about light bulbs but I thought the resistance would be too low[...] |
One cold 230V 60W bulb has just been measured to 80 Ohm.
Ten of them paralled will give you
- about 8 Ohm cold
- about 600 W max dissipation
- sort of overcurrent protection by increasing its R to about 800 Ohm at working temperature.
Apart from unexplored HF behaviour, the most problematic aspect would be to balance power and number of bulbs, so that they stay "cold" during the test.
This wouldn't put precision resistor manufacturers out of business, I assume.
Regards,
Peter Jacobi |
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| Christer |
Correction,
the low inductance power resistors I mentioned earlier are not
manufactured by Dale, as I thought, but by Arcol. The tech specs
are a bit brief, but it seems the low-inductance versions have
about 1/10th the inductance of the standard series, but also
more restrictions on voltage etc.
http://www.arcol.co.uk/pages/produc...-Resistors.html |
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| wintermute |
Hi Christer,
RS Australia have the Arcol ones, but only the Standard ones, not the low inductance ones, that's where I got the figures of 5.4 uH for 10 Ohms from...... It seems to be the way of things here. I was looking for wima FKP caps I could get one of the values I wanted from Farnell but no others, they don't even have MKP at all, only MKS..... Also when I was looking for panasonic FC series, they have quite a few, but some values they just don't stock, I was having a hard time getting the caps for the mic preamp I'm making because its a tight board, and I needed 16V to keep the size down, but everything was 63V or 100V even when there were 16V versions in the data sheets.
I spent a large part of the last three days doing the layout for the IC experimenter board I have, and then trying to find all of the components that would fit :( (moral find out the size of the caps before doing the layout, doh)..... got there in the end with some revisions to the layout. Also finally got the list together to replace all the caps in my amp.... I think I'll put the oder in tomorrow, could be a busy weekend next weekend :)
This thread started because I suddenly thought, hang on if I fix the amp I'll need to test it, and I really should have a better dummy load, and if I'm going to do that I should get the resistors with everything else. So the orders been put off for another day :)
sometimes I think it would be easier If I was still blissfully ignorant, and a resistor was a resistor and a capacitor was a capacitor and the only reason there were different types (of caps) was because ceramics covered the pF range, greencaps the nF range and electros the uF range ;)
Tony. |
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| tubetvr |
| quote: | | Yeah, come on people! First you dish out on ultra-expensive super-dooper non-inductive resistors, and then you buy ultra-expensive super accurate OFC ribbon-wound hand-made inductors to precisely model the speaker's reactance! Has the world gone mad (rhetorical question)??? |
Of course it is correct that a speaker is not a pure resistive load but that is not the point. In my mind there are actually at least 2 points:
1 It is adviceable to use a load with well known performance when measuring an amplifier otherwise it is not possible to compare the result of different measurement setups, as an example if I say that my amplifier have a -1dB upper frequency limit of 100kHz, is that because it really is so, or is it because the load I am using has so much inductivity that the load impedance is 20 ohm at that frequency?
2 The second reason is that as I wrote earlier I want to measure open loop response up to > 1MHz, how should I trust the result and be able to use that for calculation of optimal feedback loops if I dont know what I measure.
It is very easy, (and very cheap) to make a load with neglible capacitance and inductance to be useful for audio measurements by using multiple non wirewound resistors so why dont do that instead of using wirewound resistors that will effect the measurement result?
Why dont use a little professionalism when it doesn't take that much effort?
Regards Hans |
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| wintermute |
| quote: | Originally posted by pjacobi
One cold 230V 60W bulb has just been measured to 80 Ohm.
Ten of them paralled will give you
- about 8 Ohm cold
- about 600 W max dissipation
- sort of overcurrent protection by increasing its R to about 800 Ohm at working temperature.
Apart from unexplored HF behaviour, the most problematic aspect would be to balance power and number of bulbs, so that they stay "cold" during the test.
This wouldn't put precision resistor manufacturers out of business, I assume.
Regards,
Peter Jacobi |
I just realised that my misconception about the resistance of light bulbs comes from when I measured the resistance of a 100W halogen 12V globe, which pretty much registered less than 1 ohms (from memory) which I guess it would have to in order to pass enough current at 12V to get 100W.....
I just measured a 60W 240V bulb and it measured 66 ohms which now that I think about it seems awfully low! As you say that would work out at much higher than the rating of the bulb, so as it heats, the resistance must go up substantially.
I wonder whether I can find some 48V high wattage bulbs might be the way to go :) (at least for doing non critical testing, don't think it would be much use for testing power output!)
Tony. |
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| wintermute |
Hi Hans,
I found some metal oxide resistors but they are $2 each so $80 for 40 required for 2 channels at 100W..... Seems that no matter which way I go it ends up being expensive. That's the same cost as if I get two of the $40 non-inductive 100W resistors, and it would be a lot more work..... I don't think there is any cheap way out of it for me except to get the inductive resistors.
BTW the config you described right at the begining, I would have thought was only 50W power handling. If you have 10 5W in parallel that gives you 50W, putting another group of the same in series is still only 50W........ you only get increased power handling when in parallel right? or am I missing something?
Tony. |
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| UrSv |
| quote: | Originally posted by wintermute
Hi Hans,
I found some metal oxide resistors but they are $2 each so $80 for 40 required for 2 channels at 100W..... Seems that no matter which way I go it ends up being expensive. That's the same cost as if I get two of the $40 non-inductive 100W resistors, and it would be a lot more work..... I don't think there is any cheap way out of it for me except to get the inductive resistors.
BTW the config you described right at the begining, I would have thought was only 50W power handling. If you have 10 5W in parallel that gives you 50W, putting another group of the same in series is still only 50W........ you only get increased power handling when in parallel right? or am I missing something?
Tony. |
I am sure you can get cheaper metall film/oxide resistors. Here the 3 W metall film are USD 0.25 each at qty 100 so USD 25 for 2 * 150 W max dissipation. And if you make sure that you parallell and series connect everything symmetrically then the power gets distributed symmetrically so for you example that would indeed be 100 W. |
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| joensd |
Hi Tony,
have a look here for more suggestions.
I personally use 50W resistors mounted on a heatsink.
So far I got away with two of them.
If I´m doing permanent test near 50W I just use a fan in addition and indeed they work wonders.
Those 50W resistors you see cost about 3.26€ new. I got them much cheaper through a diyaudio-member and they are also regularly sold at Ebay.
The advantage of those resistors IMO is that they´re slightly heatsinked already. You could even install them on a metal plate, put some in series and build a little tunnel where you can blow some fresh air through. No need for heavy heatsinking.
(unless you wanna test some 1kW-amps of course)
Cheers
Jens |
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| Jax |
| I have Arcol NHS50 8 ohm myself ;) |
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| Tensop |
| how about 30 5 watt jaycar resistors? cheap, dirt cheap :) |
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| iCebReakEr409 |
What about
Isachrom (5,65Ohm/m) or Konstantan WIRE ? It's pretty cheap.
 |
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| peranders |
| It's probably OK if you wind bifilar (was is spelled right?), non-inductance winding. |
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| tubetvr |
| quote: | Arcol Type NHS 300
8 ohms low inductance 300 Watts! |
That sounds good, any information about capacitance and inductance?
Regards Hans |
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| Christer |
| quote: | Originally posted by tubetvr
That sounds good, any information about capacitance and inductance?
Regards Hans |
Check the link I posted earlier:
http://www.arcol.co.uk/pages/produc...-Resistors.html
There seems to be no info on capacitance, but inductance figures
are for example around 200 nH for 1 Ohm and 2 uH for 10 Ohms |
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| peranders |
Have you checked the datasheet? Don't know myself.
I have used those resistors attached on a heatsink (rather big with long flanges) from a swedish locomotive (RC, ABB) and I put it in a bucket of water. I tested to run 500 W continuously for a while. |
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| Christer |
| quote: | Originally posted by peranders
I have used those resistors attached on a heatsink (rather big with long flanges) from a swedish locomotive (RC, ABB) and I put it in a bucket of water. I tested to run 500 W continuously for a while. |
You mean a heatsink from an RC!!!? How big is that? Or maybe
it wasn't a heatsink for the thyristors? |
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| EchoWars |
Perhaps not an option for those in Europe or Oz (don't know their shipping policies), but a pair of these work pretty well for me. Have 4 of 'em mounted on a board for testing...400W 8-ohms for two channels, 200W 4-ohms for 4 channels.
Pic doesn't show it well, but they are spaced above the board by about 4-5cm to allow air to circulate. During high-power testing I have a fan trained on them...they do get toasty above 150W. |
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| tubetvr |
| quote: | | There seems to be no info on capacitance, but inductance figures |
2uH is very good for a wirewound resistor but will still have a effect on very high frequencies.
Using multiple non wirewound resistors it is possible to make something that is much better but the problem is of course how to measure the performance. When I built my dummy load about 10 years ago, I used Resista metal oxide resistors and measured the performance using a network analyser at my work, the dummy load behaves like an almost perfect 8 ohm load up to at least 10 MHz.
I have now built a similar load using resistors I found here in Tokyo, but I have no way of refering the exact performance but I think the performance is similar.
Regards Hans |
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| Christer |
| quote: | Originally posted by tubetvr
Using multiple non wirewound resistors it is possible to make something that is much better but the problem is of course how to measure the performance. When I built my dummy load about 10 years ago, I used Resista metal oxide resistors and measured the performance using a network analyser at my work, the dummy load behaves like an almost perfect 8 ohm load up to at least 10 MHz.
|
I certainly see your point about the usefullness of a low-reactance
load. I considered buying the NHS resistors, but decided to go
for a large number of cheap surplus wire-wounds instead. That
saved me money both on the resistors and on not needing
heatsinks. Maybe, it was a good decision after all, since
I didn't waste money on
something that's better but maybe not good enough. :)
However, then immediately arises the question about the
inductance of metal-oxide resistors. I don't think any metal oxide
resistors are easily available locally and I haven't been able to
dig up any info at all about those metal-film that are easily
available here.
Considering that these are often spiral-cut AFAIK it seems not
obvious they should have a low inductance, although they seem
to usually exhibit a low capacitance. Do you have any info on
typical values for L and C for "standard" metal film resistors and for
metal-oxide ones?
| quote: |
I have now built a similar load using resistors I found here in Tokyo, but I have no way of refering the exact performance but I think the performance is similar.
|
That is always a problem, how to assess the quality of lab
equipment unless you have even better lab equipment to measure
it with? Of coure, if using a precision resistor with know or
guaranteed low C and L in series, with the dummy load it should
be possible to measure
the phase angle and impedance at various frequencies.
Edit: I temporarily forgot the Caddock film resistors which are a
bit expensive and needs heatsinking, but they do spec inductance
in the datasheets: 20nH for the 100W type and 10nH for the
lower rated ones. |
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| wintermute |
Hmmm lots of activity while I was asleep :) thanks for all the suggestions guys..... the 30 jaycar 5W wirewound resistors (60 for two channels) is indeed a cheap option (about $11) but I don't know how I'd mount them, 30 in parallel would certainly make the inductance negligible as well.
I've had another look at the Arcol range, and one of the things I like about them is that they say the 75W ones can handle 45W with no heatsink, and the 100W ones can handle 50W with no heatsink....... I suspect they are pretty robust.
Tony. |
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| Christer |
| quote: | Originally posted by wintermute
I've had another look at the Arcol range, and one of the things I like about them is that they say the 75W ones can handle 45W with no heatsink, and the 100W ones can handle 50W with no heatsink....... I suspect they are pretty robust.
|
Well, if you look at them they already look like heatsinks. :)
Add a cheap computer fan or something, and you can probably
go quite a bit higher. |
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| tubetvr |
| quote: | | Do you have any info on typical values for L and C for "standard" metal film resistors and for metal oxide ones? |
Stored somewhere in Sweden I have extensive data on Resista metal oxide and metal film resistors I would imagine that similar size resistors will have similar values of capacitance and inductance, but I can not remember any exact values.
One old rule of tumb I learned is that a 1/4 W standard resistor have a capacitance of ~2pF almost regardless of type, (carbon composite, carbon film or metal film) the capacitance is dependant mostly on the size.
The capacitance is also almost independant on resistor value but as expected the inductance is dependant on how the resistance film is spiralised.
I did some quick calculations on the expected inductance of the 5W metal oxide resistors I bought here in Tokyo. The resistors are 8mm in diameter and 25mm long and have totally 6 turns of resistance film, (this is for 39 ohm resistors). The calculated inductance for one resistor is then ~0.08uH. As I parallell connect 10 resistors and then connect 2 of these packages in series the total inductance is 2*(0.08/10) = 0.016uH. This low inductance is for the resistors themselves and eventual inductance of connecting wires need to be added to this value but it should not be a problem to keep the total inductance low.
BTW, the reason why I prefer metal oxide instead of metal film is because the metal oxide types can withstand higher power.
Regards Hans |
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| tcpip |
Anyone tried ordinary incandescent lamps meant for auto use, rated for 12V? Unlike mains-rated lamps, these will run at proper rating, when driven by an amp or its transformer (I like to load-test my power transformers to see load regulation), which means they will glow and get hot, which means they'll give you the right rated impedance. (There's a huge difference in impedance of incandescent lamps when cold and hot. All ratings are for when they are hot.)
So, if you want to pump in 36V rails into this load, put three lamps in series. They are inexpensive, need no heat-sinking, and are widely available. The only headache is some ingenuity needed to mount them in a stable way, in case you want a reusable permanent setup. And each lamp is rated for 50W quite often. Make sure they are ordinary incandescent lamps, not some new hi-tech gas-filled alternative... those may have different inductance characteristics. However, they'll all behave well at rated currents. The only doubt I have is how they'll behave with an AC supply, since they get DC in cars.
I was also wondering about using the El Cheapo 5W wirewound resistors in parallel, all fixed using Araldite to a heatsink. Shouldn't they be good enough, unless you're specifically looking for high-frequency measurements?
Tarun |
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| tubetvr |
| quote: | | Anyone tried ordinary incandescent lamps meant for auto use, rated for 12V? |
I have an old book describing simple transmitters and receivers built with valves, the circuits are very simple with one valve transmitters and regenerative receivers. In that book it is described to use incandescent lamps as dummy loads when testing transmitters as they are apparently relatively free of inductance and capacitance. One problem I see is that the resistance of a incandescent lamp is changing with applied voltage which is not a good thing when measuring audio amplifiers.
BTW, incandescent lamps can be used as simple current regulators as current trough the lamp is more or less constant, (if current would go up the resistance goes up which counteracts the increased current)
Regards Hans |
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| peranders |
| quote: | Originally posted by Christer
You mean a heatsink from an RC!!!? How big is that? Or maybe
it wasn't a heatsink for the thyristors? |
Not for thyristors, something else, dont know what. The weight was a couple of kg's. Not that huge but very handy. |
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| wintermute |
Hi Tarun,
That's not a bad Idea, I might go to the local auto shop and see what I can get, non halogen globes should be pretty cheap too.
I suppose what all this really comes down to is whether or not the idea is to do accurate measurements of freq response, power output etc, or simply a way of loading the amp to check for things like oscillation, PS ripple, clipping behaviour etc. For me (at least at the moment) it's more the later.
Tony. |
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| AuroraB |
Something doesn't stick here.....
First- there is massive discussions on the nonlinearities of power resistors, inductance and capacitance included....
Then the suggestion is to use light bulbs,--- a highly nonlinear resistor in response to temperature. The light bulbs may have very little reactive elements, but releives heat badly. Thus a filament temp of 200 deg. is hardly noticable as a light glow, but changes the resistance almost 100%.Remember resistance is directly proportional to delta-T ( in Kelvin). You would have to use a lot of lamps to avoid the heating effect.
Good old fashioned carbon composition resistors, or metal comp, is probably the cheapest way to get something reliable. Immersion in oil is really not a problem either. Use a paint can with a good lid,- just avoid topling it!
Or- buy the expensive bits.................. |
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| tcpip |
| quote: | Originally posted by wintermute
That's not a bad Idea, I might go to the local auto shop and see what I can get, non halogen globes should be pretty cheap too. |
Precisely my thoughts. While you're at it, also look for holders or mounts of some sort, so that you can set all them bulbs up in some way.
| quote: | | I suppose what all this really comes down to is whether or not the idea is to do accurate measurements of freq response, power output etc, or simply a way of loading the amp to check for things like oscillation, PS ripple, clipping behaviour etc. For me (at least at the moment) it's more the later. |
Precisely. I too wanted to build dummy loads basically to test (i) whether my power amp PSU (specially the transfi) heats up after an hour or two of full-load use, (ii) whether my power amp behaves, (iii) how hot my power amp heatsink gets. I think that this sort of test is a must if you want to trust the reliability of the poweramp for many years to come. In such situations, I don't want to vary the signal frequency much (it'll either be 50-100Hz or something "standard" like 1KHz), and I don't want to take any but the grossest of measurements like output current, rail voltage, temperature, etc. So a bit of inductance or capacitance doesn't bother me, as long as the power amp doesn't get into oscillation.
For precise bandwidth measurements, I might not even want to take things to full load, and if I do, I'll think about pure-resistive loads then. Presumably, I'll be rich enough by then to buy a couple of hundred linear resistances with very low tempco and low L and C. :)
Tarun |
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| tcpip |
| quote: | Originally posted by AuroraB
Something doesn't stick here.....
First- there is massive discussions on the nonlinearities of power resistors, inductance and capacitance included....
Then the suggestion is to use light bulbs,--- a highly nonlinear resistor in response to temperature. |
I too was confused. :D
However, even light bulbs can be treated as quite stable impedances, if you choose bulbs which will be driven to "full load" by the amp. Hence my choice of auto bulbs, which are designed to glow brightly and reach a stable thermodynamic condition with just 12V. A few such bulbs in series can be good test loads for amps at full power and low frequency, provided you choose the number of bulbs in series to match the voltage output by your amp at full load.
Trying to use incandescent lamps, and then fussing with them to ensure they remain cold, is too unstable an approach, I feel. You must drive them so that they glow at full brightness.
One assumption here is that you'll be pumping a steady signal of whatever frequency but at unchanging amplitude into your amp. Varying the output power rapidly will allow the bulbs to cool and re-heat, thus sharply changing their impedances. It all depends on what kind of tests you want to do.
And to find out the impedance of the bulb, it's of no use measuring it with an Ohmmeter when cold. The best approach, it seems to me, is to take the rated wattage and voltage of the lamp, and calculate --- not measure --- the impedance, and then realise that this impedance is valid only if the rated voltage is applied on the lamp from a low-impedance high-current source, i.e. a power amp (not an Ohmmeter). :)
This auto-headlamp approach was suggested to me by The Great Audio Guru Angshu. :)
Tarun |
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| AuroraB |
I agree in principle to using lamps as loads, - this has been done. However- during my army days in the early 70s, we used som special bulbs as RF loads, but I cannot recall what it was......
Personally I think there is only two easy choices- wire wounds or paralled compositions. Otherwise- no way around the expensive stuff...
Actually, -some years ago, I needed to test a 24V / 30 A PSU in a hurry,...
I soldered a handful of "Dales" at hand to two wires and hung it from my desk.....after a while there was some strange thumps...looking closer, I found the resistors on the floor.....:clown:
Heatsinks are absolutely recommended....!! |
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| wintermute |
| quote: | Originally posted by AuroraB
I soldered a handful of "Dales" at hand to two wires and hung it from my desk.....after a while there was some strange thumps...looking closer, I found the resistors on the floor.....:clown:
Heatsinks are absolutely recommended....!! |
:D
Originally I asked the question about inductance, simply because I didn't know if there was some nasty thing that it could do, which would make my amp look like it had problems when it didn't.
I've wondered a few times whether some of the nasty things I've seen were in fact due to the cheap jaycar 5W resistors I currently have (only two in parallel so 10W, although good for 100W for about 20 seconds before they start smelling!)
Seems like the only real concern is whether or not you want absolute perfection in the measurements, which at this stage I don't. It's certainly been an interesting discussion though!!!!
Tony. |
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| JOE DIRT® |
| this is the resistor I was referring to in my previous post |
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