Hi welcome to the Eev blog an Electronics Engineering Video blog of interest to anyone involved in electronics design. I'm your host Dave Jones Hi it's a rather unusual one today. I've had a couple of people not just one, but actually two people ask me a very similar thing. uh, how can you? what's the simplest method or what's a real easy method to electronically trigger some sort of springload mechanism and they both gave an example of something like a mouse trap.

Even though that's what they weren't trying to do, they were trying to do something else. But you know, a basic spring-loaded mechanism like that? that's easiest way to electronically trigger it. And of course, one of the first things you'd think of for triggering something like that. It's some sort of solenoid base system, you know, apply a voltage to a coil and it pulls a plate back or something like that.

I don't know. Bit too complicated thought. got to be able to do it with only law surely and sure enough I reckon you can. Simple: A resistor is all you need.

a resistor and a voltage source. Here's your. this is how I would do it I would uh, this is your thing that you want to hold back with your spring mechanism. You just have some cord fishing line, nylon string, cotton, polyester, whatever.

I don't wrap it around the resistor, tie it off so it holds it back and then when you want to trigger the thing, pass some current through the resistor Heats it up and uh, it weakens the string or whatever it is. and Until It Breaks and then Bing gone simple. I'm going to try it. looks like fun and here we go.

I've got my mouse trap or it's actually a much larger rat trap. it's The Eradicator I Love it. Awesome sounding name. Um I've tied back the arming plate here because we're not going to hold down the arm here with the traditional arming plate.

we're going to hold it back with this. Uh, I've actually tied some fishing line on there and I'll show you that in a sec. Um, and I've just tied it back so this is actually armed at the moment. it's just tied back so it doesn't accidentally go off in my hands which would be, uh, rather nasty.

now. I've got this fishing line. It's tied off down here. it's probably hard to hard to see, but this fishing line.

I' I've wrapped one turn around the arm here and I've tied it off around the bottom. Uh, on on. sorry on the other side, but on the bottom here. I've got a 10 Ohm resistor and I've wrapped a uh one turn of the uh fishing line around the 10 Ohm resistor.

So when that heats up, it should eventually get to a point where it, uh, weakens where the fishing line weakens enough and it will break and bingo it'll lift the arm up and boom. it'll uh activate the Trap so let's give it a go. Uh, 10 ohm resistor isn't too bad. It means it should, uh, trigger on the order of volts.

We don't have to put much power through this at all. It's standard quarter watt resistor, so we should only have to put like a watt or something or maybe even two at most. Take the tape off here and arm this sucker. Fishing line should have more than enough strength to hold back that trap there cuz there's because we have a big lever arm here.

so there's uh, quite a large Leverage there. and uh, that should hold there nicely. And Bingo! it's ready to go. And let me get my blast! Shield Just in case.

I Don't want to damage my new camera here I Don't want the thing flicking towards the camera and I'll set up the highspeed camera on the side and we'll see if we can capture this and uh, I'm 100% sure this will work I'm going to switch my load voltage on I've got it to zero and I will wind up the wick and let's see if we can, uh, make this sucker do anything. Here we go! Woo! Awesome! And that sucker smoking too! I Love it! And there's the melted fishing line around our resistor. I Turn that up uh, fairly quickly because of my um. Highspeed camera.

I've got to, um, trigger this thing within 10 seconds. That's all it captures. So um, but you could, actually, uh, do that at a lower current. Uh, it would just, uh, take more time to actually, um, generate the heat in the resistor and uh, and melt the in in this case, fishing line.

But you can go and try um, uh, other things you know, nylon, polyester, all sorts of, uh, all sorts of stuff if you want I think they may have higher melting points than something like fishing line? I'm not sure, but great fun. Go and experiment with it. And really I don't think you can, uh, get a simpler trigger mechanism than HS law. And that, of course brings us to The Humble Quar Wat Resistor Let's take a look at it and see what happens when you pass, uh, close to the maximum rated power through one of these resistors.

Now we've got a data sheet here of a typical axial quarter wat resistor and the figure we're interested in down here is What's called the Thermal the thermal resistance r T here and as you can see, it's 140 K per watt or kelvin per watt or basically degrees Celsius per watt. So if you put one watt into the or if this, if this, if this resistor dissipates one watt, then its temperature is going to rise by 40 Kelvin or 140 C above the current ambient temperature. And the power rting of a resistor is something that beginners forget a lot about. Now here's two graphs which uh, show us, um, basically the rise in temperature here rise in temperature.

degrees Kelvin versus the power dissipated in watts and it exactly matches the that thermal resistance. We saw that figure we saw before. Look, if you put one4 watt, if if you dissipate a Qu Watt in that resistor, it will rise by 35 Kelvin or 35 C uh for a quarter watt and that means and if you multiply that by four for one watt as this thing wouldn't handle a watt, but if it did, then uh uh, you would get that 140 C per watt. Now a lot of people a lot of think oh, this Qu wat.

This quar wat resistor can handle a quar Watts Yeah it can. It's designed to do that continuously, but it will rise by 35 C above your ambient temperature and that's the key thing. It doesn't get to 35 gets to 35 plus your current. So if your Labs at 20 Celsius that's already 50.

that component that resistor is going to get to 55 cel. And if you're Labs you know if you're using the product outside in summertime and it's 30 C then you're going to be that resistor is going to be at 65 C and so on. And you can get higher temperature resistors which can actually uh H you can get uh, the not all quarter watt resistors are the same size. They can actually be physically different uh, sizes.

And yeah, they all handle a quar Watts but they might. The smaller ones might get, well, will get physically hotter with that quarter watt. They won't be the same as this particular one, so just something to watch out for when you're designing your boards. Don't If you are going to go to limit and dissipate a quarter watt in a quarter watt resistor, just make sure that your board can handle, uh, that your product can handle that temperature.

Make sure there's adequate air flow and things like that. If you put this resistor if you mounted on the board right next to an electrolytic capacitor, then well, that the temperature of the nearby electrolytic capacitor is going to rise, which may derate its life and things like that. I've explained that before on various episodes of the blog. Now if you're curious about this graph on the right hand side here, it's very similar to the one on the left.

Now, the one on the left we just looked at is what's called The Hot Spot temperature. And that's basically the temperature in the core of the resistor itself. Or you can take it essentially um, as the temperature of the case of the actual resistor itself. But this one over here, um, is uh, a temperature.

It's exactly the same Y axis temperature rise versus X-axis power dissipated. but it's the temperature rise at the end of the lead when it's solded into a board like this. When you've actually put it on there, you bend the leads and you've got x amount of lead length and you trim them off and they're solded onto the individual pads. Okay, now this is let's say you leave 5 millimeters lead length at each end of the resistor.

This here Y axis will be the temperature on the actual pad itself. Uh, and that's the key because you can actually use your PCB as as a heat sink itself. If you actually put it close enough to the leads, then the PCB can help dissipate the heat from the core of the resistor itself. but there are limits actually to that because the leads themselves have a certain thermal res resistance and it becomes a thermal resistance series equation and stuff like that.

I've done a previous Uh blog on that for um, heat sinks and things like that. but yeah, you can use your PCB as a heat sink to help dissipate the heat in your product. so that's something to think of if you are designing um a product which will which will dissipate close to the maximum power in that resistor itself. and as you can see, this is no longer a linear graph like this it it actually tap off like that depending on the lead length which you actually have now the uh, the greater the lead, the smaller the lead length, the more linear the line becomes and because of the thermal resistance in that actual lead itself.

So if you have uh, uh, a longer lead length of 15 mm as you can see, the temperature will drop off and and if you bent it right out here and had like 25 mm or 30 mm of length and bent it right there, you know, really extreme kind of stuff. It would taper off something like that and you would actually get very little Heat at the end cuz all the heat would be in the core plus a little bit dissipated um along the leads itself. So if you're going to do that, uh, try and trim them closer to the board so that you can actually get you don't lose as much heat in the legs and you can, uh, take the Uh heat out of the core of the resistor itself and this brings us onto a what's called a derating curve. Every resistor data sheet will have one of these.

Not all only few of them will actually have the actual Um thermal resistance graph like this, but they'll have what's called a derating curve and basically it shows It's uh, maximum power dissipated or 100% of its nominal rated value. Now, basic resistors like this are almost always specked at a wattage. In this case, a quarter watt resistor is specified at a temperature maximum temperature of 70 C and that's why it's showing on the graph here with this dotted line going up at 70 uh Cs on the X axis here and that's actually ambient. That's ambient temperature, not the temperature rise as in the resistor as we've actually looked at.

So if you if you're designing your product to work over a temperature range, say up to 70 C then the resistor can actually dissipate um 2 and2 it's nominal 2 1/2 watts and continue to function function correctly. And that's fine because the temperature rise plus the ambient temperature um will still be within the working limits of the resistor itself, but after 70 C it derates linearly like this until it gets to a point in this case for this particular resistor. even though it isn't this one, but this data sheet for this resistor is 55. So that will be the maximum absolute maximum temperature that you can use the resistor at, and then you won't be able to dissipate any power in it because you can't go beyond that temperature.

It just won't work anymore or it won't be reliable or something like that. What does all this have to do with our Mouse Tru Well, not much at all. Uh, really. but I Just wanted to show some things on resistors.

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