I have two large current shunts a client gave me. One is marked 25 amps 50mV SE company The other is 20amps 50mV.
Am I to take it that they read 50mV at rated current? Or in other words the 25A one is .002 ohms and 20amp one is .0025 ohms? My VOM isn't very accurate down there.
As far as I know, you are correct. Every shunt I've used is marked for full scale current, and output voltage at that current.
If you want to measure that sort of resistance, you need a four- terminal (Kelvin) measuring system.
Upmarket multimeters offer this facility - the Thurlby-Thandar 1906 (Farnell order code 724-026) offers four terminal resistance measurement, but since the resoultion only goes down to 1 milliohm, it wouldn't do you much good.
Thurlby-Thandar do offer a micro- and milli-ohmeter - the BS407 (Farnell order code 381-2364) which is somewhat more expensive, but can resolve resistances up to 1.999 milliohm to one micro-ohm, and
19.99 milliohm to 10 uohm.Top of the line Hewlett-Packard (now Agilent) and Datron multimeters do better than the TTI 1906, but cost quite a bit more.
-- Bill Sloman, Nijmegen
But for the OP to simply check his understanding of the markings, he just needs to stick a few amps through it and measure the voltage developed across the terminals of the shunt. I usually use a power supply with an adjustable current limit.
-- John Devereux
Exactly. Most cheap DMMs have 100uV or better resolution at full accuracy, so at 3A you have 33 uohms resolution.
Be sure to apply the 3A to the outer (usually larger) terminals of the shunt and read the voltage from the inner (usually smaller ) set of terminals.
He'd better measure the voltage twice, reversing the direction of the current between readings - low level voltage measurements are bedevilled by thermocouple voltages developed in the junctions between dissimilar metals, and measuring with AC or at least reversing DC is the standard way of getting rid of these offsets (or at least of getting some idea how bad they are).
-- Bill Sloman, Nijmegen
Would you not need quite a big temperature differential (between the ends of the shunt), for that to be significant?
-- John Devereux
The last time I was using a really good DVM to measure low voltages, I found draft shields were absolutely essential to keep the voltage stable. Most measuring set-ups have different metals all over the place, and base metal thermocouples give you about 50uV/C.
You have to be very careful if you want to get anything useful out of the uV resolution of a good DVM, particularly when measuring resistances - which automatically involves dissipating some heat.
The fact that the manufacturers data sheets derate resistors linearly against ambient temperature doesn't means that the temperature rise of a resistor is a linear function of temperature - a low dissipations the Maclaurin number is below 500 and you don't get any significant convective cooling at all, so the the resistor is a lot warmer at low power dissipations than you'd expect from linear extrapolation.
-- Bill Sloman, Nijmegen
Several degrees C (more than 5 is my guess) at least just to tick the LSD. Forget about it. You'd see it anyway when the power supply is turned off because of the large thermal mass.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Right. And manganin has a low thermoelectric potential relative to copper, just a few uV per K.
John
So use a really bad DVM :)
-- John Devereux
The pairs for base metal thermocouples are chosen to give a relatively high voltage for a given temperature difference (among other things).
Typical connection material pairs are something like 5:1~10:1 better, and you'll very seldom see anything other than a "0.0" mV if you go around probing random bits of metal that have just been handled by
30°C fingers. A DC shunt will be made symmetrical in part so the substantial self-heating at rated current won't cause thermocouple voltages to affect the reading. Immediately when the current is shut off, the meter should go to 0. If it doesn't, then the reading can be corrected by that factor, but it will not be a problem with such a setup and a 100uV resolution meter.If you want to calibrate a really high current shunt (not just check it) at a current orders of magnitude less than the rated current, then such things would come into play (and perhaps you'd be using a meter with 100nV resolution rather than 100uV), but the OP just wants to assure himself that it's actually 50mV at rated current as marked.
We've supplied many, many, high current (up to 15,000A) DC measurement systems using such shunts, BTW. They're usually between 50mV and 150mV output at rated current.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
If you're down in low uV DC territory or below, for sure. But we can get 30 micro-ohm resolution out of a 100uV resolution measurement @3A, which requires no special care. That's a resolution of ~1.5% of the expected value, which is fine for the intended purpose.
Do you have a reference on techniques for nanovolt DC measurements? I'd be interested in that.
Heat in itself is not a problem. Nor even are thermal gradients. It has to be an asymmetrical thermal gradient with dissimilar metals.
Maclaurin number? Do you mean the Reynolds number? or maybe the Nusselt number?
Keep in mind that shunts typically dissipate a fair bit of heat at full rated current. The OP's wee 25A one will dissipate in excess of
1W in normal use. Larger ones are in the hundreds of watts. The lack of significant dissipation might affect the reading a bit.Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
ohms?
No. My impression is that any such reference would start off by recommending that you immersed the active part of the experiment in liquid helium and go on from there. Microvolt DC measurements are already tricky enough.
The English national standards laboratory at Teddington does offer "A guide to measuring resistance and impedance below 1MHz" ISBN 0 9044557
31.1I've got a copy, but can't recommend it - it doesn't say anything silly, but it doesn't help you understand what is going on.
You can't dissipate heat without creating a thermal gradient. Ad hoc connections are always asymmetric.
Oops. Raleigh number - the Reynolds number applies to flow, the Raleigh number applies to convection. Both show up in my Ph.D. thesis. Why I keep on thinking the Rayeigh number is called the Maclaurin number I'll never know. Check out
if you want a bit more detail.
Heat dissipation is proportional to the square of the current - 3A is going to generate about 1.44% of the heat dissipated at 25A. 3mV isn't too hard to measure, unless you expect the voltage to be accurate to a couple of uV, but reversing the current is a useful check.
-- Bill Sloman, Nijmegen
Never had any manganin voltage probes for any voltmeter I ever used. Most of them looked like chromium-plated steel.
-- Bill Sloman, Nijmegen
The shunt is probably manganin. The wires you put under the screws are probably plated copper, (AWG22 or something like that) so copper for T/C purposes, so the number is applicable.
BTW, the probes (if they are the probe type, and cheap) are most likely nickel-plated brass.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Go Sphero, i breifly googled and did not find any appropriate Mclaurin number application, Reynolds number, as i had learned in school, is pretty strictly related to aerodynamic drag of various shapes, but Nusselt number appears to be applicable. Well placed question, i learned something as a result.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
Probably not the most accurate thing, but may be helpful for smaller shunt measurements.
Thanks. We use the Reynolds number (and the Nusselt number) in mold cooling calculations- you want a Reynolds number high enough to assure turbulent flow and therefore efficient heat transfer from the metal to the cooling medium. A small number like 1,000 means you have laminar flow for sure, whereas a larger number such as 10,000 or larger means you have turbulent flow in the cooling passages. In between is transitional. I wish the powers that be had deemed a basic education in fluid mechanics to be of more importance to us EEs.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
We have a similar 1-amp home-made box. We used a good voltage reference, a 4-lead Vishay power resistor as the shunt, and a 10-turn pot to trim current. Once it warms up it is short-term stable to a few PPM. It has a load-short switch to keep it warm. We'll typically drive a precision 1-ohm oil-filled resistor in series with an unknown, measure the drops, and compute the unknown.
John
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