 The Great Search, brought to you by Digikey, and I thank you Digikey for supporting The Great Search. Every single week, Lady Aida uses a power of engineering to find you all the things on digikey.com. Lady Aida, what is The Great Search this week? Okay, so this week I am working on the ST Spin Breakout Board. It's an ST stepper motor driver with current limiting, and it uses resistors to make sure that the current limiting stays within the desired limit. And to do that, we need to have this breakout board that we're designing, and it's a fixed size. We need to have these kind of large current sensor resistors, so I want to show how I found the current sensor resistors of my dreams. Let's go to the computer, I'll show you the design. So this is the datasheet for the ST Spin 220 to see the stepper motor, and then there is two resistors, one for each coil. This resistor actually has the current pass through it, and it's quite a high power resistor. If we search for, you can determine what sensor resistor you want. The sensor resistor sets the current limit. So I'm going to follow the datasheet recommendation values, but of course depending on the stepper motor you want to drive, whether you want to drive at a full 1.3 amps, or a little bit higher, maybe 1.5 or 1, whatever, you know, do the math. But in this case it's .33 ohm, so it's 330 milliohms, because it's current sensing. You want to measure the voltage going over it, and the current is quite high. So in this case we're dealing with a .33 ohm resistor times, you know, maybe 1.25 amps times 1.25 amps. So it's over half a watt, because voltage times current is power, but we don't have the voltage necessarily in this case, so it's just current times current times resistance, which is like the same thing. And it's over half a watt. And usually when people are just buying resistors, you're buying a .402 or a .603, you don't care about the wattage, because it's a pull-up resistor, or a signal resistor, or a voltage divider. The amount of current going through is like under a milliamp, and so that, you know, you're never going to get over a quarter watt, or a tenth of a watt, or a sixteenth of a watt. So you can use these little itsy-bitsy resistors. But in this case, these current sensor resistors, because there is a full amp plus going through them, the wattage is quite high, which means physically the package has to be larger in order to dissipate that much current. Which is why you see on like our INA, 219, or other current sensing or power sensing breakout boards, there's this big ass, like, you know, .1 ohm resistor. That's what the current is passing through, it has to be really large in order to dissipate that heat. One thing, you know, early in your engineering design, you'll probably spec the wrong size resistor for current sensor resistors. It's easy to be like, oh, I'll just put an 0805 or a 603. And then it'll immediately overheat and melt off, and it'll be super cool, because this will just go right off your board, pop. So let's go to Digi-Key to find a current sensor resistor. So again, I'm going to follow that datasheet recommendation. They say 330 milli ohms, one watt, why one watt? Because you don't want to have a half a watt if it's, you know, like, give yourself some head room, right? You never know. That could be ambient temperature rises. It could be plus or minus 10%. So they recommend one watt, and we'll go with that. So in this case, we can look for this resistor. I know it's like, you know, you don't think you search for current sensor. But even though current sensor resistor is what I would consider a subclass of resistor, it's not sold separately than plain resistors. There are chassis mount resistors and such, but, you know, and adjustable power resistors which are kind of specialized. In this case, it really is just, it's just a resistor with a very high wattage. So first off, we're only going to look at stuff that's in stock and active because I'm, I want to purchase them and I also want to spec them. You might want to purchase them. I want to purchase them. Make sure you can purchase the things that you want. Well, there's always like thousands of, you know, the resistors, there's literally a million different resistors. And so I'm like, look, I just want to get the ones that I can buy. For resistance, you know, you can put it, I'll actually use these limit boxes. So let's go from, you know, 250 to 350 milli ohms. So that's kind of nice. It will select for me the, the, the area, I guess, if you can't select both. And then of course, the most important thing is that wattage. So under power watts, go down here and select one watt. Now there's 0.75 watts and I don't know, you can also select that, but I'm going to stick to one. You know, I think as I, as I really, you know, cause I'm just laying out the design to test it. Um, as I do more testing, I can actually verify like how much wattage am I burning off of this resistor. And then if I can get away with 0.75, you know, cause it's still more headroom than necessary, but I have to, um, want to, you know, verify that. Um, tolerance 1%, 5%, it doesn't matter. I mean, current sensor resistors, you're not going to get high precision because they're, they're just differently made. Um, they're meant to dissipate a lot of heat. And in fact, if you look, um, there's some cool weird, um, resistors out here. First off, um, they can come in, you know, uh, diode mouth because physically they're just larger, they can just be more current. Um, see, there was a really cool one. I don't know if it's, there's one where you can actually see there was like three elements in it. Let me, uh, let me go with stock. This one. No. There was just, oh yeah. Here it is. So this was kind of cool. I saw this, I was like, oh, that's kind of interesting. So you can literally see there are three resistors in parallel so that each one can dissipate and it has a little bit of this, you know, space in between them. So it's, you know, it's the old hack of like, oh, you need a, like half watt resistor. You only have a quarter watt. You pick twice the resistance and double them up and you get twice the wattage and half the resistance. Um, okay. Same thing, but done for you in SMT format. So one thing, um, I do want to note is, you know, which I've never seen before and you do have to watch out for is, it's like, it's an O805, but it's wide style, right? It's the wrong way. It's, it's like couch style or whatever. It's, um, you, you don't connect across the pads. And that's important because, um, you know, usually for this resistance you, you get 2512, right? Which is, which is kind of massive, um, for a resistor. I mean, we can, um, I think, uh, this is the resistor, right? Yeah. So this is 1206. Here's a 1206 on my board. Um, and if you change this package to 2512, it's like half the board is this resistor. I couldn't fit two of these. And so in this design, normally I go with price and I don't care about the size because who cares, but like I am fixed. I have to have it in this package so that this break up board can fit into like the standard, you know, 3d printing, uh, stepper motor controller. So the size of it is important. Um, in which case I'm going to do what I normally don't do, which is, uh, make sure that I don't have these massive packages. So I'm going to say, you know, 1210, 2010, I don't even want 2010 to be honest. I kind of want the smallest possible. I think kind of 12, 1218 or 1206 is the largest. And that'll really, um, reduce the number down. And, um, let me see if I can see what this was the 0805 wide, but this is a weird package. Oh, okay. So what happened is because it's a weird package, it actually doesn't appear here anymore. Uh-huh. So let me go back. So under supplier device, I think I need to click this little tick. Yeah. So what I ended up doing is that the one I found is, oh, interesting. It did not bid in one moment. It was, let me get rid of this package case. Oh, okay. So yeah, it's sometimes called 0805 or 0508. Um, so I did find this one, um, which I really liked because it has, uh, 48,000 stock, which I really like. Um, however, like I said, it's, it's weird. It's, it's like sideways style. So I actually have to, in Eagle CAD, make a new package because there actually is no default resistor package that I've already used that is this like wide style. Um, but what's nice is that it's a really good price. It's like, you know, under 20 cents per resistor and it's 0805, which will like fit quite nicely. It's just like, it's just what, you know, it's just, it's just backwards. Um, so, you know, one thing I learned is, is that as you saw, when you're searching for the package is just be careful because it's like, you call this 0805, but it's actually not, it's 0508. And, and I see this whenever you're dealing with stuff like, um, resistor networks and other like kind of non standard, like not jelly bean, um, resistance capacitors, if they come in a slightly unusual package. Um, sometimes you have to be a little bit flexible on how you use the search terms to determine it, because this would be categorized under 0805 even though like technically it's not. And then of course, uh, don't forget you have to create a new, um, package layout in ChiCad or Eagle if it doesn't exist yet. Cause this is an unusual package. But that said, um, this is, you know, a very small resistor for this size and it'll fit very nicely in my design. So I'd haven't done that, um, the layout for that, uh, 0508, but it'll be about this size, just, you know, a slightly different, you know, layout. And this, this will give me plenty of space. You know, I can have two of these and not worry about, um, taking up too much space on my tiny little board. So, um, current sensor resistors, very interesting, uh, you know, I've never had to pick one based on size, but as long as you're kind of flexible with your, your package options, um, you can get something quite small and there's a lot of it in stock. And that's a great church.