EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: SharpEars on September 29, 2015, 05:23:26 pm
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Well, the subject says it all. Let's assume that I have the equipment to measure a 0.1 ohm resistance fairly accurately. I am thinking of maybe rolling it to distribute the heat and the cutting a piece that gives me as close to 0.1 ohm as I can get.
Am I crazy or will this work as good as a wirewound?
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Yep, crazy.
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Yep, crazy.
Care to add on the "why am I crazy" front? It doesn't seem that unreasonable, except possibly for ppm/C issues, if any.
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It does not seem crazy, just not very stable over temp. Is it for DC or are there any high-frequency components? Is for a lab experiment or something more?
For low-cost SMPS design, it is not uncommon to see a wire or a PCB trace used in place of a resistor for current sense. It's a 'close enough' technique. Aluminum will dissipate heat quickly but I have no idea how the resistance will change with temp or if that change will impact your application.
Try it.
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It does not seem crazy, just not very stable over temp. Is it for DC or are there any high-frequency components? Is for a lab experiment or something more?
For low-cost SMPS design, it is not uncommon to see a wire or a PCB trace used in place of a resistor for current sense. It's a 'close enough' technique. Aluminum will dissipate heat quickly but I have no idea how the resistance will change with temp or if that change will impact your application.
Try it.
This is strictly for use as a short term DC current shunt. I am going to limit the length of time that current passes through it to five to ten seconds...
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Yes, I would agree with the others that your thermal stability will be poor. Also, are you really thinking about dumping 100 amps through this?
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3$ will get you a Dale 10W resistor:
http://www.ebay.com/itm/0-1-Ohm-1-10-Watt-Wire-Wound-Aluminum-Resistor-Dale-/251629436151?hash=item3a96487cf7 (http://www.ebay.com/itm/0-1-Ohm-1-10-Watt-Wire-Wound-Aluminum-Resistor-Dale-/251629436151?hash=item3a96487cf7)
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OK, I've relegated this into the bad idea pile...
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Well, the subject says it all. Let's assume that I have the equipment to measure a 0.1 ohm resistance fairly accurately. I am thinking of maybe rolling it to distribute the heat and the cutting a piece that gives me as close to 0.1 ohm as I can get.
Am I crazy or will this work as good as a wirewound?
And that is related to Test Equipment in what way?
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Steel would be better, but stainless even better still (not quite as good as nichrome, but similar). Steel is solderable, aluminum and stainless not so much.
Even assuming a good bond, conductive metals like aluminum and copper make very poor resistors. For most frequencies (over ~kHz, usually), and shapes of conductor, the inductive reactance is several times the resistance. If you're only ever measuring DC, that doesn't matter, but any little transient or ripple will dutifully be transmitted to your sensitive shunt measuring circuit.
Tim
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I have successfully used stainless steel for making high current shunts. I sometimes used M4 allthread and adjust the nuts to desired resistance or used 1mm SS plate cut to size for the desired resistance.
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I smell an audiophool.
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There is another problem with using aluminum. Corrosion, anytime You connect aluminum to certain types of metal, you can get a galvanic reaction, which will cause corrosion. Over time this corrosion will increase the resistance, making it unreliable. You can only connect to certain types of stainless steel, and a few other metal, Without having a galvanic reaction. It will be a very expensive experiment. You need to do some research on metal compatibility in electronic circuits.
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I smell an audiophool.
What? Why?
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I smell an audiophool.
What? Why?
(http://www.oneletterwords.com/weblog/assets/1430703381.065.8220.jpg)
Kinda reminds me of this:
http://studiozey.com/invisistor/index.html (http://studiozey.com/invisistor/index.html)
Combined with the fact that the guy's name is "SharpEars" ...
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Well, the subject says it all. Let's assume that I have the equipment to measure a 0.1 ohm resistance fairly accurately. I am thinking of maybe rolling it to distribute the heat and the cutting a piece that gives me as close to 0.1 ohm as I can get.
Am I crazy or will this work as good as a wirewound?
And that is related to Test Equipment in what way?
In the way that I can use the current shunt to calibrate test equipment...
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And that is related to Test Equipment in what way?
I use current shunts in my test rigs all the time. Seems rather valid to discuss here.
Combined with the fact that the guy's name is "SharpEars" ...
Throwing stones at people that you do not know based on the screen name is not a good way to create a community. Sharp Ears had a curiosity question that anyone on the forum could take a stab at or not. The desire to improvise a DC current shunt has nothing to do with audio specifically so why pull in some silly personal jab?
Participate or not....but personal attacks are weak.
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And that is related to Test Equipment in what way?
I use current shunts in my test rigs all the time. Seems rather valid to discuss here.
Combined with the fact that the guy's name is "SharpEars" ...
Throwing stones at people that you do not know based on the screen name is not a good way to create a community. Sharp Ears had a curiosity question that anyone on the forum could take a stab at or not. The desire to improvise a DC current shunt has nothing to do with audio specifically so why pull in some silly personal jab?
Participate or not....but personal attacks are weak.
He also made an obvious mistake assuming that any audiophool would use aluminum for anything, when it's all about pure silver, 99.9999999999999999999999% pure copper, silver plated copper, cryogenically frozen versions of all of the above, etc... At no point would any self respecting audiophool use aluminum, the black sheep of the conductive metals family!! :box:
Having said all of that, yes I am a card carrying member of the audiophool club. Fortunately, I know better than to ask questions related to audiophoolery on this forum - an electrical engineering forum.
Specifically, my question was related to creating a current shunt for calibrating the current reading of a power supply. If I told you guys what make and model of power supply, then you would have a genuine reason to laugh at me, so I won't.
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Kinda reminds me of this:
http://studiozey.com/invisistor/index.html (http://studiozey.com/invisistor/index.html)
...I want off of this planet. |O I heard they just found surface water on Mars?
99.9999999999999999999999% pure copper
That 0.0000000000000000000001% wouldn't be oxygen would it? I hope not, no audiophool would dream of anything but OFC.
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Throwing stones at people that you do not know based on the screen name is not a good way to create a community. Sharp Ears had a curiosity question that anyone on the forum could take a stab at or not. The desire to improvise a DC current shunt has nothing to do with audio specifically so why pull in some silly personal jab?
Participate or not....but personal attacks are weak.
Cheesus Crust, overreact much? Calm your tits.
Having said all of that, yes I am a card carrying member of the audiophool club. Fortunately, I know better than to ask questions related to audiophoolery on this forum - an electrical engineering forum.
(http://static.tvtropes.org/pmwiki/pub/images/Colbert_3741.jpg)
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Throwing stones at people that you do not know based on the screen name is not a good way to create a community. Sharp Ears had a curiosity question that anyone on the forum could take a stab at or not. The desire to improvise a DC current shunt has nothing to do with audio specifically so why pull in some silly personal jab?
Participate or not....but personal attacks are weak.
Cheesus Crust, overreact much? Calm your tits.
Having said all of that, yes I am a card carrying member of the audiophool club. Fortunately, I know better than to ask questions related to audiophoolery on this forum - an electrical engineering forum.
(http://static.tvtropes.org/pmwiki/pub/images/Colbert_3741.jpg)
Wait, you posted that picture with a response like: Cheesus Crust, overreact much? Calm your tits.
Talk about the pot calling the kettle black! You also quoted me completely out of context, since this thread's question has absolutely nothing to do with audiophoolery as has been already pointed out numerous times...
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Improvising a current shunt can be done easily with plain old wire from home depot.
http://www.rcgroups.com/forums/showthread.php?t=393591&page=7 (http://www.rcgroups.com/forums/showthread.php?t=393591&page=7)
I got a nice vishay 20 watt wire wound from mouser.1% .01ohm, for about 5 bucks it think. Im not sure if your looking for improvised or cheap, I understand the need for either, so there are two options :). I got the resistor for a PSU shunt too, its more compact than a wire shunt, and much cheaper than a a bulk metal 4 terminal or a proper high wattage shunt.
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Improvising a current shunt can be done easily with plain old wire from home depot.
http://www.rcgroups.com/forums/showthread.php?t=393591&page=7 (http://www.rcgroups.com/forums/showthread.php?t=393591&page=7)
I got a nice vishay 20 watt wire wound from mouser.1% .01ohm, for about 5 bucks it think. Im not sure if your looking for improvised or cheap, I understand the need for either, so there are two options :). I got the resistor for a PSU shunt too, its more compact than a wire shunt, and much cheaper than a a bulk metal 4 terminal or a proper high wattage shunt.
Do you know how much 18 gauge wire you would have to use to create a 0.1 ohm shunt? About 5 meters (16.4 feet). That is a lot of wire...
The point of this exercise was to save ordering time from DigiKey, if it would be something I could do at home in an hour or so...
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I just purchased a really cheap wirewound 50W 100 ohm wire wound with a slider tap on ebay to use for help in calibrating a cheap PSU. They also do other low ranges like 10 ohm.
But if time is of the essense then strip apart a toaster or hairdryer and use the (nichrome?) instead of bloody aluminium foil. I would use the foil to make capacitors though.
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Isn't one of the bigger problems consistency? Aluminium foil is a bugger to join to in a reliable manner both electrically and mechanically. You could make it, calibrate it but move /change anything and I would guess it has changed electrically significantly.
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I just purchased a really cheap wirewound 50W 100 ohm wire wound with a slider tap on ebay to use for help in calibrating a cheap PSU. They also do other low ranges like 10 ohm.
But if time is of the essense then strip apart a toaster or hairdryer and use the (nichrome?) instead of bloody aluminium foil. I would use the foil to make capacitors though.
I've gone with a variant on your idea with the toaster and ordered 25 feet of Kanthal A1 16 AWG (1.29 mm - 0.324 Ohms/ft Resistance - Heat Resistant to 1400 C). I can clip a piece of the length I need for the resistance I need and make all the shunts I want, reducing this entire problem to one of measurement only...
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Isn't one of the bigger problems consistency? Aluminium foil is a bugger to join to in a reliable manner both electrically and mechanically. You could make it, calibrate it but move /change anything and I would guess it has changed electrically significantly.
Thanks for pointing that out. Even if i would make an accurate piece of geometry, moving it without distorting it would be a pain in the rear.
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Yes I forgot to mention kanthal :) can be purchased locally at most vape/smoke shops. I use it to wrap heating coils for my vaporizer, local shop sells 22-30 guage for a buck a yard, and I live in a RURAL area
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Yes I forgot to mention kanthal :) can be purchased locally at most vape/smoke shops. I use it to wrap heating coils for my vaporizer, local shop sells 22-30 guage for a buck a yard, and I live in a RURAL area
FYI on my screen your user name is so long it runs into the title so I cannot read either. Now I can read the replies. :)
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Kinda reminds me of this:
http://studiozey.com/invisistor/index.html (http://studiozey.com/invisistor/index.html)
...I want off of this planet. |O I heard they just found surface water on Mars?
Nope it is official.
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Specifically, my question was related to creating a current shunt for calibrating the current reading of a power supply.
How on earth are you going to do a precision calibration using a wadded up piece of aluminum foil? If you take all the electric stove heating elements from your stove and wire them in parallel that should give you about 8 ohms. Good for 7,000 Watts, at a fraction of that power it should provide a stable load.
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He could also use the 10A/20A current shunt on his DMM, it's likely 0.1 ohm. Use the terminals for the 1V load and tap the sense wires inside the meter
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Hmmm well I actually typo ed my user name, left the g out! I would like to change it but i wasn't aware it was possible to edit user name?
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And that is related to Test Equipment in what way?
I use current shunts in my test rigs all the time. Seems rather valid to discuss here.
It isn't. The truth is that this discussion has very little to do with Test Equipment, and for projects like making your own shunts there already is a much more appropriate forum (https://www.eevblog.com/forum/projects/).
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aluminum foil is cheap, why not just try it? I think you will give up pretty quickly once you try to solder it...
I've tried making precision wire resistors from kanthol without much luck. If you look inside a decent decade box you will see flat wire wound resistors all over the place so it must be possible unless they are using other types of resistance wire. Anybody know what they use? I tried stretching it out and running a meter lead down the length until I hit the right resistance and kanthol, that which I had on hand, had too high an ohms per foot to be accurate. I tried various gauges. I guess you can get close then switch to a lesser gauge (thicker wire) for trimming.
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I smell an audiophool.
What? Why?
(http://www.oneletterwords.com/weblog/assets/1430703381.065.8220.jpg)
Kinda reminds me of this:
http://studiozey.com/invisistor/index.html (http://studiozey.com/invisistor/index.html)
Combined with the fact that the guy's name is "SharpEars" ...
I love the last line of text under Warranty and Disclaimer...
"Read the Precautions paragraph and above all - use common sense." ... to which should be added ... "and don't buy the kings clothes."
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Aluminum is a darn good conductor. One of the best, a pretty poor resistor material choice. aluminum foil is pretty thick, as thick as a 1/2 oz pcb copper pour. Unless you cut your foil really thin and make it really long, any appreciable resistance will likely be due to your contacts between the foil (which has a tendancy to develop a nice insulating sapphire layer) and whatever you connect to it.
That said, if you have a ohmmeter that can go that low, try it, then try whatever other metals you can find. Steel wire may be good, (pulled from steel wool? inside a twist-tie? section of cheap bike lock cable? you can probably find something around) . When you find something that seems to work, wiggle the contacts and make sure they are not contributing to the resistance, measure it again after it gets warm and see if it changes. ramp up the current slowly and keep an eye on the temperature rise to see if it will stand up to the current.
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Apart from all the other problems already mentioned, you also might want to consider the temperature coefficient of metals like aluminum, copper, iron/steel and the like, that is in the range of +4000 ~ +6000 ppm/K. Not exactly a good basis for measurements with reasonable accuracy, let alone calibration.
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As a side note....
I was in solder mode on my bench yesterday and thought I would try to solder to a small sheet of aluminum foil. 'Hard' is too small a word. I tried with just the iron and then with a hot air pre-heat on top of a heated steel block. Different fluxes as well. Nothing more than a few balls of solder rolling around.
I did not expect any more, but it did satisfy my curiosity. I learned to TIG weld aluminum years ago and was always surprised how much heat it takes. Anyway it was fun to try. I may put some solder paste on foil and put that in the re-flow oven to see if that works.
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IIRC there are special alloys to solder aluminum.
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I've tried making precision wire resistors from kanthol without much luck. If you look inside a decent decade box you will see flat wire wound resistors all over the place so it must be possible unless they are using other types of resistance wire. Anybody know what they use?
Constantan (55% Copper, 45% Nickel) in the early ones or Manganin (86% Copper 12% Manganese 2% Nickel) in later / better ones - better long term stability and lower (very low) TC.
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Apart from all the other problems already mentioned, you also might want to consider the temperature coefficient of metals like aluminum, copper, iron/steel and the like, that is in the range of +4000 ~ +6000 ppm/K. Not exactly a good basis for measurements with reasonable accuracy, let alone calibration.
So, is it realistic to make a temperature sensor from aluminum foil? ;)
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As a side note....
I was in solder mode on my bench yesterday and thought I would try to solder to a small sheet of aluminum foil. 'Hard' is too small a word. I tried with just the iron and then with a hot air pre-heat on top of a heated steel block. Different fluxes as well. Nothing more than a few balls of solder rolling around.
I did not expect any more, but it did satisfy my curiosity. I learned to TIG weld aluminum years ago and was always surprised how much heat it takes. Anyway it was fun to try. I may put some solder paste on foil and put that in the re-flow oven to see if that works.
I've seen people using candle wax to allow solder on aluminum. Worked. Good or not, I don't know, but they did.
Also, one time on a company I've worked before, they bought a little bottle of 'aluminum flux' that worked very well... Used once in a decade I think, but when you need...
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To 'cheese' it, you rub at the oxide while under a suitable cover (usually mineral oil or wax). Obviously this does a poor job, and the tin dissolves it (inconvenient for foil), while the lead does not (lead and aluminum are immiscible).
The slightly better way is to use a zinc or zinc-tin alloy that tends to flow along the metal surface, under the oxide layer. This also needs to be scratched in, but doesn't require a flux for modest results.
Real fluxes include ammonium bifluoride, which is nasty stuff. But it works on stainless, and I believe titanium as well.
Tim
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To 'cheese' it, you rub at the oxide while under a suitable cover (usually mineral oil or wax).
Ordinary cooking oil works too.
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Apart from all the other problems already mentioned, you also might want to consider the temperature coefficient of metals like aluminum, copper, iron/steel and the like, that is in the range of +4000 ~ +6000 ppm/K. Not exactly a good basis for measurements with reasonable accuracy, let alone calibration.
So, is it realistic to make a temperature sensor from aluminum foil? ;)
Yes! see this great signal path review of a very fine keithely source meter where he uses liquid nitrogen to show how resistance of materials changes with temperature. https://www.youtube.com/watch?v=fs9WsTKOMmo (https://www.youtube.com/watch?v=fs9WsTKOMmo)