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.
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.
wintermute said:
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
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/products/Aluminium-Housed-Power-Wirewound-Resistors.html
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/products/Aluminium-Housed-Power-Wirewound-Resistors.html
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.
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.
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
pjacobi said:
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.
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 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.
wintermute said:
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.
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
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
Arcol Type NHS 300
http://www.elfa.se/pdf/60/06063002.pdf
Art nr 60-699-59
http://www.elfa.se/elfa/produkter/se/2012534.htm
8 ohms low inductance 300 Watts!
http://www.elfa.se/pdf/60/06063002.pdf
Art nr 60-699-59
http://www.elfa.se/elfa/produkter/se/2012534.htm
8 ohms low inductance 300 Watts!
What about
Isachrom (5,65Ohm/m) or Konstantan WIRE ? It's pretty cheap.
Isachrom (5,65Ohm/m) or Konstantan WIRE ? It's pretty cheap.
An externally hosted image should be here but it was not working when we last tested it.
tubetvr said:
Check the link I posted earlier:
http://www.arcol.co.uk/pages/products/Aluminium-Housed-Power-Wirewound-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
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.
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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