How to Make a Shunt Current Sense Resistor

How to Make a Shunt Current Sense Resistor


Hi, I want to make a *beep* I said shunt! I want to make a *shunt* current sense resistor. Basically the way we measure current is by converting it into voltage passing it through a known resistor which we call a shunt and so the current is the measured voltage divided by the known resistance. To measure the current accurately you need a relatively accurate shunt resistor, for example here I have a 1V 5% resistor and I’m gonna run two amp through it and measure the voltage across it See now the voltage reading here is actually showing that there is 1.8 amp running through it. *beep*, it’s burning now *beep* *beep* *beep* Of course that means 4 watts of power that burns my resistor. You can buy higher power *shunts* that are made for this purpose, but why pay so much when you can make a relatively accurate *shunt* yourself you just need some length of wire. Every wire has a known resistance. If you search Wikipedia for AWG or American Wire Gauge, you will find a table showing information for every wire size such as its resistance per length and diameter and such and of course there are other standards too, like British standard wire gauge. So basically using a *shunt* resistor, you’re generating a voltage drop that’s not very desirable. Why? Because then you are dropping your supply voltage level by that much, which will affect the current running through the load. But if you keep that voltage drop small or compensate for it by increasing the supply voltage, then you are fine I like to keep my *shunt* maximum voltage small. To start, I like to have only 100 millivolt for a maximum of 1 amp, which means around hundred milliohms. Regular multimeters are no good to measure such low resistances, but they can measure voltage and current much more accurately. What I have here is a 22 gauge wire, which is around 53 milliohms per meter, and so I would need around 2 meters of this wire. Now I set my power supply to output 1 amp accurately, and I run the 1 amp of current through my 2 meter length of wire and I measure the voltage across it and I have to read hundred millivolts, but you see I’m reading a bit off. So I stripped some lengths of wire, and I’m gonna slide my probe across it and you see as I slide, the voltage starts dropping I’ll keep going until I get exactly 100 millivolts right there This means that to have exactly 100 milliohms I have to measure exactly between these two probe points so I marked them I’ll solder wires to the two spots I marked on my *shunt* wire And I measure the voltage between these two wires. The length of the measurement wire is not important because there is no current running through it. Now I’ll wind my wire like this so it takes less space. This is bad because I’m making an inductor. If I try to measure higher frequency AC spikes or transients, this will filter them. It’s easy to solve, though. First, I bend the wire at the center and then I wind it like this. This will remove the inductance. You think I’m joking? No! If I bend the wire like this and send the current in on the top wire it creates fields like this and when it returns on the bottom wire it creates fields the opposite way and the fields cancel each other, so no inductance is created. Elementary! So here’s the *shunt*. Now let’s do a comparison between readings of the supplies and meter and our *shunt*. *beep* Who set everything at maximum?! I guess I did. What can I say, I like extreme *beep* So here we are. I set the current to 100 milliamps, and you can see that the supply can’t show it accurately, but the shunt can. That 10 millivolt voltage times 10 is the current running through the shunt which is 100 milliamps. And if I change the current, our reading is still pretty accurate. I’d like to be able to measure higher currents like 10 amps, but using this, the voltage drop would be 1 volt, which is too high for lower supply voltages, so in order to get the same 100 millivolt drop, I have to change the resistance of my shunt to 10 milliohms. Simple, I can still use the same *shunt* and just inject the current and read it at 1/10 length of this wire. Let’s just see if it can handle 10 amps, or it will burn. I’ll run 10 amp through a short piece and see if it gets warm. Well, see ther mouth and the lip area is more sensitive to heat Slowly getting warm, but it’s not too bad, so you should be fine. And if you are wondering how I’m running 10 amps through my mouth without dying, there is no current running through my body because the voltage across my body is zero so the entire 10 amp is running through the wire. First, I strip this wire at one-tenth of the length with some margin for a calibration, which is around 182 millimeters in this case. Then I solder a wire here, and that’s where I’ll inject my 10 amp current. Now I’m running 10 amps through my wire and have to see exactly at which point I’ll read exactly hundred millivolts. Right around here, and I’ll mark it and I solder my measuring wire to that spots And this time to get rid of inductance, instead of winding it, I’ll just fold the wires side-by-side Something like this and now we can tape it together in a bundle and label the wire, so we know which one is for which and also don’t short the bare wires inside the shunt otherwise it will change the resistance and throws your reading off. Now let’s step up the game. I want to be able to measure up to a hundred amps, but my supply here can’t output more than 10 amps, so I’m gonna use my auto transformer that can output some high AC currents. I have a piece of 22 gauge wire here, which I’ll measure the voltage across. If I connect it to the output of my auto transformer, I… *beep* *beep* *beep* *beep* And burn my fingers again. The piece of wire simply shorts the AC output, so I start at 0 volt and slowly raise the voltage while I’m measuring the current with my clamp meter. Let’s see… *beep* *beep* I’m starting to think coming up with the idea of a light bulb didn’t require much genius. in fact the dumber you are the more likely you are to invent a light bulb. And remember, if your *shunt* gets too hot, its resistance rises so much that its current reading won’t be accurate anymore We definitely need thicker wire. I have this 18 gauge wire which is around 21 milliohms per meter, so 400 amps and 0.1 volts drop, I would need around 4.75 centimeters of this Okay, let’s see if we can do the hundred amp now Yeah Seventy-seven, starting to smoke Come on Eighty… *beep* Melted again. We need thickness! And fortunately, I’m in luck, I have some 8 gauge wire which is around 2.1 milliohm per meter so I would need around half a meter of this for one melliohm or 0.1. Volt per hundred amps. If this can’t do 100 amp, nothing can. There we go Let’s see There we go, 100 amps and it’s holding on pretty… *beep* *beep* My auto transformer can’t take that much power *beep* So I can’t calibrate at 100 amps because my auto transformer blows up. Anyways, I don’t really trust this current clamp multimeters because they’re fine, but they are not very precise. I’m thinking just to calibrate the length of this wire at 10 amps using my precision multimeter, which will give me enough resolution, and then I can use this up to 100 amps. There we go, I’m running 10 amps through the wire, and I’ll be looking for… 10 millivolts exactly. Right around there. Now let’s do a comparison between the clamp current meter and my shunt. Of course I trust my shunt a bit more. Just remember that whatever voltage you read there in millivolts times a thousand is the current. Okay, let’s do it. There, 50 amps… 47, 48… Wow, actually the clamp is not very bad either, which seems like the current is rising. *beep* Because the auto transformer is burning There, now we have a precise 100 amp shunt resistor. Now some of you might ask “Why don’t I just use a multimeter or a clamp current meter to measure current?” No, no, that’s fine, there are no stupid questions. Three reasons: one is that the current might be outside the range of your multimeter. For example, if I want to measure 10 microamps, I have to use a shunt of around 10 kiloohms and measure 100 millivolt across it. In that case I can measure accurately down to nanoamps. Secondly, clamp current meters are fine, especially since you don’t have to cut through your contacts, and you just clamp around them. But they are not very accurate at low currents and their precision can be as bad as five to ten percent, where a shunt can easily be as accurate as 1% Third and most importantly, the meters only measure DC and RMS, but with a shunt you can connect it to a scope and see all the current waveforms and transients. I have an example set up here: I have my hundred milliohm shunt resistor I’ve made in series with the primary of my microwave transformer, and there is no load on it, and I’m gonna measure the voltage and current of the AC line on my scope. Let’s see… You can clearly see that the yellow current line is leading the voltage green line as should happen in an inductor. What? Why the *beep* the current is not a sine wave There. It says the current should look like that if you are using an iron core because the iron core doesn’t have a linear magnetization The *beep* you see and learn using a *shunt* Thanks to the circuit specialists for providing the essential tools I need to make my videos. They will provide five of these awesome LCR meters and 5 of these USB oscilloscopes to my viewers and patrons. I mean their website is filled with this great low-cost tools that you can buy like this thing that you just plug your component into and it will tell you that yes, It’s a NPN transistor, and here are the parameters. As usual the patrons, are automatically in the draw, but for the viewers, if you need these tools, please leave a comment under the video like the usual

100 thoughts on “How to Make a Shunt Current Sense Resistor

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  2. Flexible cables as shunt for high current? lol. Use a copper busbar. Calibrate with a file. Give your transformer the rest:)

  3. I know I'm telling grandma how to suck eggs, but. Running a shit load of current through your Variac at the bottom of the winding will fuck up and cook the Variac winding, making it useless!
    It's good your doing it with a new Variac and not an old school one as it'll be sacrilege otherwise. Trying to get your hands on an old school Variac these days are very hard to come by.

  4. Can you explain why step down transformer melts seconder coil if you put the power suply on secondary in order to make step up transformer. N1/N2 =316/32; U=230V

  5. … They say you learn not to touch hot things, because it hurts, and it's a reflex….. then, there is: 0:37 – 0:45
    – Let's be real here: That's just impressive. XDXDXD

  6. I thought that the word SHUNT was only used for when trains SHUNT train cars. I guess I should either expand my horizons or learn to SHUNT up.

  7. I think Mehdi may have made a little mistake at 10:08. Correct me if I'm wrong, but what I think he should have said was that the yellow current line is LAGGING the green voltage line as should happen in an inductor.

  8. Hi mehidi saghdar I am a biotech chemistry zoology student but much interested in electronics I think you can send some facts in electronics to my email kindly please

  9. "In fact the dumber you are, the more likely you are to invent the lightbulb."

    This is possibly the greatest thing that anyone has ever said. It is deeply hilarious.

  10. I just searched all of the winning names here on youtube. Not one result….Are these giveaways for real?

  11. You make fun of yourself saying meter (mether) almost perfectly, but not for formula (formoulah) 😀
    Don't worry mehdi, your accent is bad but you teach people a shotload of stuff and know way more than people talking perfect english

  12. BEST WAY TO CREATE A 10 MILLI-OHM CURRENT SHUNT: Use a jumbo paper clip and calibrate with a couple of meters and Ohm's law.

  13. Say… There definitely are tables for wire gauges which gauge is required for which max current.
    Those want to keep inner resistance negligible (eg for house installations). If you don't want to buy resistor wire (which is offered), the max current still should give you a rule of thumb which wire gauge is acceptable for which current, and which gives you a melting fuse.

  14. A resistor wired across a current meter to bypass current is a (shunt resistor).

    A resistor wired across a volt meter to induce a small voltage in proportion to current passing through it, is a (series resistor).

    In this case, we have a series resistor but shunt.

  15. 6:04 I just noticed that you already had those burns in your fingers which made me really confused. What happened?
    I am so confused. 🙁

  16. At 4:50 you say that there is no current running through your body, which is wrong. (The ampmeter shows 0.6 V which seems to be off for a 10mOhm resistor at 10A, but maybe your leads add around 50mOhm to the circuit.) You have a 10mOhm resistor with 10A running trough it, that means the voltage across the resistor is 0.1V. Lets say your mouth's resistance is 100 kOhm so there should be ~1µA running through your mouth. I am pretty sure you know this, but as you said it, it sounds a lot like the current "looks" for the easiest way which is the 10mOhm resistor and "decides" to only go there.

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