Building a Shunt From Nichrome Wire, W-3205

Here is an excellent website detailing temperature coefficients for various elements and alloys, including nichrome. If you can't find the resistance of the wire at room temperature, you can easily measure it yourself by connecting a ohmmeter across a one-meter length (resistance per meter is usually specified at 20C).

This is a bit of a shot in the dark as I haven't looked at heater wire for a verrry long time, but I believe Omega used to publish data on nichrome. You might try www.omega.com

Tom

Thank yous.

sometimes one can miss the obvious... testing with a multimeter, now why didn't I think of that?

I'll check out those sites also.

As to your question about the resistance changing as the current increases, I must point out that the resistance changes with temperature, not current. For example, you could run 20 amps through the wire in still air, and the wire will heat to such-and-such a temperature and its resistance will increase to such-and-such ohms. But run 20 amps through the same wire submerged in oil and the wire will heat to a different temperature and will have a different resistance. Same current, same wire, different results. Consequently there are no clear-cut predictors of temperature vs current because the wire's specific thermal environment radically affects the rate at which the wire loses heat energy. Your best bet, I think, is to build a simple experimental rig and take measurements.

I'm not sure if you're agreeing with me or not? it seems that you arrived at the conclusion that current-increases forced down a shunt will naturally generate/meet-with greater resistance through the shunt increasing the heat as a "consequence". thus heat is a "consequence" of resistance which naturally acts to hinder current as the heat increases. Well, that's my understanding of resistance, heat and current.

Thanks for the suggestion about the rig, I think you're probably right.

I need to know what its resistence value will be at room temperature, and how I should expect that resistence value to increase, as amps are increased.

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The resistance value will increase as a function of increasing temperature. The relationship between resistance and current is a function of how much the wire temperature changes for a given current. This change in temperature, in turn, and hence the resistance, depends heavily on the wire's specific thermal environment which is not specified and cannot be expressed by a general, 'one size fits all' formula. Tell me the wire temperature, and I can tell you the resistance at that temperature regardless of current. Tell me the current, and I can't tell you anything about the resistance because I have no idea what the wire temperature is. For all I know the wire could be submerged in liquid nitrogen or it could be wadded up in a tight ball and stuffed into a chunk of R-100 glass insulation. But I can tell you that the wire resistance in each situation will be very different for any given current.

Ah, well, you should also be able to calculate the resistance using current I=E/R. therefore given I= 3 amps and E = 12, R must equal 4, but what that would mean in heat is still unknown to me.

basically we're talking about the two sides of the same coin... seems to me.

Here's one thing you can do: build a test shunt comparable in construction to the one your design calls for. Connect a constant-voltage DC source (a good, well-regulated power supply will do so long as it can handle the current) across the shunt in series with an ammeter. Measure the voltage across the shunt (only. As the ammeter also contains a resistance, the total power supply voltage will divide between it and the shunt). Note the current on the ammeter.

With current and voltage in hand, you can now compute the shunt resistance (R = E/I ) at temperature, and you can compute the power being dissipated by the shunt (P = E x I). If you're really ambitious and you know the exact length of wire used to build your shunt, you can estimate the operating temperature of the wire using the tempco formula in the link I provided earlier. This experiment will give you a good feel for your shunt's behavior at temperature, and you can then use this information to build exactly the shunt you want.

thanks

You're welcome (and thanks for your patience)!

Just curious, but what is this shunt for, Mr. Guest?

I don't wish to appear rude or ungreatful, but I've read a lot of the posts in this forum, and judging from the debates that issue forth, I think I'd be better off saying nothing. except, the basics, basically to limit current, and alter Electro motive force.

RUN AWAY! RUN AWAY! He's going to use it to measure the current output of his free energy device! EMF is a dead give away.

EMF is about volts.

as for what I'm using it for, only a moron would hazard a guess, and spark a thread of baseless nonsense... is that what you're trying to do?

Blink --- Nicely stated, my sentiment to the word. A problem or project that is not described, with all of the parameters stated, is very difficult to respond to and involves guessing from the most professional and informed folks. What else can a person do when all of the necessary information is not provided.

My guess is also "Free Energy" --- maybe this guest also might have a theory on "Perpetual motion".

The only thing of value in that entire post is "that it's 28 gauge nichrome 80". Thanks

the rest is very strong testiment proving the case as stated by the thread that initiated the response.

Shunt up Fry. You're making him nervous...

Ooops, sorry Ken. Blame my Inner Moron.

On a more serious note, I found Guest's responses to be a bit disconcerting. Kinda like helping some guy push his dead car off the highway and being dismissed with "Meh."

Oh, well.

(PS: Can I borrow your sig? Again? I've been told that mine's too cryptic. )

(PS: Can I borrow your sig? Again? I've been told that mine's too cryptic. )

Access Denied. Sharing Violation.

Actually, I think you should keep it a while longer. I honestly, really, laugh out loud (not just that virtual LOL) when I read* it. It's absolutely perfect, I think. Stuff goes in**, nothing comes out. At least EOF is near.

*(pronounced: reed, not red -- it still works)

** We can only guess: by his own admission, he doesn't read. Can he hear? Maybe the stuff never gets in there?

look

I am and was greatfull; beyond that, I hope you can show some understanding. seems only too easy for people to dart off into longwinded garble that really has nothing to do with engineering -the very thing I warned against initiating.

Sorry we couldn't be of much help.

Good luck, at any rate.

Its too less information to tell you how to build your shunt best.

You must know:
1. Whats the current range?
2. Whats my requirement on resolution/precision?
3. Is it a DC / lo frequency-measurement or do I need higher speed for current transients like in automotive applications?

ad 1) Your current range determines the size of wire (or, commonly spoken, the dimensions and shape of your shunt). If you need to measure bigger currents then you`ll possibly better a busbar-shaped shunt. At bigger currents and nothing else usable than your wire you can build a shunt with parallel windings to increase the current without increasing the temperature losses (leading to errors). As a rule of thumb: take your wire dimension, go into a list of AWG/current, read the current and take half of that for your measurement range - so you get a lower temperature increase.

ad 2) If its for hobby or home you need mostly low precision, that means that the influence of temperature rise at higher currents may be neglectible.
If high precision is needed then you should not take NiChrome but something like Zeranin or Manganin.
Nichrome has a tempco of about 100...400 ppm/°C, that means that a shunts temperature drift of 25°C between I=0 and I=max makes an error of up to 1% with NiChrome; Zeranin has 3ppm which is only 0.0075% at maximum current.

ad 3) The recommendations of 1) are only for DC or lo frequency (<1kHz) measurements, if you have higher frequency components in your signal as with rectangular shapes you need other types of shunt. Parallel windings have to be wound bifilar in a special way to reduce the inductive components.
Regards Uwe

Let's not forget that the diameter of the wire is going to play a big part in the temperature rise vs. the current through the wire, which is going to change the resistance...

You're going to have to pick some of the variables and force them to be constants if you want to avoid the partial differentials in the solution. There should be curves available that give the resistance of a certain diameter nicrome wire versus the temperature, but that's going to depend on ambient temperature as well. You've got a real can of worms using nichrome as a shunt. Guess I really don't know why you're using nichrome, but don't burn yourself.

thanks, some very good information there. Thanks for the awareness of needs also, and I was already aware of them.

Not sure why I'm replying to this after the others, but...

A good shunt is made from material that is made to have a small temperature coefficient at room temperature. The resistance will rise with temperature, then fall at higher temps, having a parabolic curve, with the peak slightly above ambient.

You can get a decent ready-made shunt for about $20, or an expensive one for about $2000. Learn to use Google. You can't make a good one from Nichrome wire. It can't be soldered, so you will have to use bolts, and adjusting it's value will be tedious. How much time is it worth?

Manganin has a temperature coefficient very close to zero. This property of the alloy is desirable because of the heavy currents that often flow through shunts, producing heat. An ideal zero-tempco material is not affected by this heat; its resistance remains constant regardless of temperature. Most other materials (nichrome, for example) increase their resistance as they are heated. Even if a shunt made of another material did not self-heat to any degree, perhaps because it is operated at a very low current, the temperature of the shunt's surroundings very well could change - say, if the shunt were located inside a power amplifier cabinet, for instance - and these changes will affect the shunt's resistance value. If Guest's application requires a stable, temperature-independent shunt resistance, nichrome is not the way to go. And you're right, shunts of all stripes are commercially available.

I was going to use a dfar simpiler method than soldering (clamp). And I will be using it. Thanks, some more usedful ideas and considerations in your post.