Hi last time we looked at this professional decade resistance box and I thought well why not check out my old one that I built when I was a kid. Just do it yourself. One could be interesting. Let's go.

haven't opened it for a long time now I've got no recollection of exactly when I made this I might have been you know I don't know 12 years old or something I was very young when I actually built this. it was one of my um, you know it was a it was a big thing I wanted to build back then and you know it wasn't uh cheap to buy these uh knobs and these uh uh well well the switches weren't that expensive, they were just standard uh PCB Mount ones I think from memory they might have been the lug uh terminal type. yeah, they probably the lug terminal type. Well I don't know I haven't opened this literally since I uh built the thing I don't think and it's gone a bit.

it's gone a bit Dicky now um, it's a six decade unit. uh it. it only goes from 10 ohms up to 10 Meg but you know that's a pretty darn usable range. and I didn't go to seven because well, seven was like an odd one out and you know it was just much nicer to.

you know, have the six decades in this one siiz uh Jiffy box which is an off-the-shelf Jiffy box you could get uh back then and you can still get now Australian made Jiffy box if you don't know about those, it's a very Australian thing and uh, you can probably see the the uh, the rust on the on the screws. it's just uh, it's terrible anyway. Let's uh, but before we crack it open I might do a quick uh check again. but as I said I it's been a bit uh Dicky lately and that's the problem with these things.

The contacts in the switches just go dodgy after a long period of time. Now as you can see, it might seem all nice and stable up here. I've got it set to uh 10K on my uh times 1K thing here, you know and it's not too bad at all. You dial it down 9k.

Okay, not a problem. seven six, hey, there we go. We're getting a bit dodgy. There's something something really going on with that switch and three two.

and yeah, look at that. it's all over the shop. and of course if I put it down to zero, that's supposed to be zero. Ohm I dialed zero into my box and well, as you can see, it's like all over the shop there.

so there's something at least wrong with that switch. Maybe a couple of the other switches as well, so you know it's it's only happened fairly. Uh, recently, it was working reasonably quite well. But yeah, like I said, this thing is probably uh, you know, 20 30 years old something like that, so you know it really is quite ancient.

It probably needs an overhaul, but now I've got this new uh I one I guess I don't need it, but uh, you know it' be nice to have a second resistance box. so I need to overhaul this sucker. but let's crack it open and uh, see what's inside cuz I completely forget what it looks like and I think it's quite appropriate that I use the exact same screwdriver I almost certainly used to assemble this thing. this is my Phillips that I've had since I was knee high to a grasshopper and uh, let's open this sucker up I've had this screwdriver since I was a kid and it's has I've lost it quite a few times over the years, but uh, it always seems to come back just like Indiana Jones's hat.

You know it just has something magical about this screwdriver and it seems to fit absolutely everything that I've ever done. and uh, it was my screwdriver I love to use as a kid and it was I almost certainly I don't actually recall putting it together with this, but I I pretty much guarantee this is probably the one I use. So let's jeez. crack this thing open if it.

Hey there we go. Oh there we go. Taada and there it is. Woo! After all that time, the magic smoke has just escaped I've opened it and of course there it is made in Australia you beauty and in this case it is actually uh, a 10 resistor box.

So it does actually go from as you can see Zero to what 10 So you can actually dial it around and you know 10 is going to be the same as the uh next one up but but you don't need to do that, you can just go from 0 to 9. Uh, which is fine because then if you want to go to 10 then you just go up to the next decade. Not a problem and uh, as you can see, I've got them going like this. uh, 10 ohms, 100 1K 10K and 100K and 1 Meg And of course they don't actually go in that sequence under the bottom or they don't have to.

It starts out at 10 and then it jumps to the 100 but then it jumps over to the 10K here and then it jumps over to the one Meg and back to the 100K and then over to the 1K and through. So it doesn't matter cuz they're in series. Makes no difference whatsoever. Now if you got a Keen Eye you'll notice that these two switches over here are different to these four.

These four here are the uh PCB type. They don't have the solder eyet uh bag like these ones here do. So I must have had like I maybe I had these four in my junk box or something or I had uh, these two, you know or these two and I had to go out and buy four more and maybe the local Dick Smith um didn't have a you know a particular type. maybe they only had four of these and I had to get two different types.

but yeah they are the cheap uh dicksmith uh switches which is no surprising. uh which would have been Asian sourc at the time. of course you know from the cheapest um supplier as they did and they would have changed suppliers every week. So you know the the quality of these switches is is quite poor.

so it's no wonder like you know 25 odd years later or something that it's just you know it's just gone to the dogs. uh really. so that's you know. let that be a lesson to you.

You use these cheap quality switches and it works perfect the first time you build it. but yep, down the track. sooner or later you're going to come a gutter now. of course, if you want to make one of these, it's not rocket science at all.

It's one of the most simplest implementations. A basic decade resistance box as you saw in my one here is just uh, M's are six decades so the first decade you've just got well I as I said I had, um, actually 0 to 10 but you don't actually need that. You only need a 10. so mine uses an an 11 position switch to get from 0 to 10.

but you can use a 10 position switch from 0 to 9 and that's good enough. And uh, you might start out at 10 ohms as I did and the next uh, decade you go up to 100 ohms and that's how you wire it on. Very, very simple and they're just wired in series in uh, in a Cascade configuration like that for as many decades. And there's your binding posts there and there.

Not a problem, that's just standard basic decade resistance box. Now of course, you can do that using either standard traditional uh rotary switches like this and and these knobs actually can be, uh, rather expensive. They could be the most expensive part of the whole thing. I Just make sure you use good quality uh rotary switches or you can use thumb wheel switches as IET have done in this box and a Uh Evev blog Forum user by the name of Uh Vti he actually made one using some cheap uh Chinese um, you know one hung low brand ones off eBay and this is what it looks like.

It's quite nice. it's quite compact, You, you know, make it into a little box. custom front, you know, a square front panel cutout for it. couple of uh binding person Bob's your uncle.

You've got a neat little decade resistance box. but if you once again, if you use these cheap switches, don't expect it to last for you know, 5 years or A decade or something like that, it's very likely to be quite bit Dicky down the track. Now, when you design in a decade resistance box like this, it's important to understand tolerances and temperature coefficient. Now, a lot of beginners make this mistake that if they think that they put, um, all these resistors in series like this, then the percentages.

Let's say you got a 100 Ohm resistor at plusus 1% They think that if you put them all in series like this, then the percentages will add up. So in this case, you got four 100 Ohm resistors in series and you'll end up with a 400 Ohm resistor, plusus 4% or some, uh, you know, some weird, uh, combination of that. Well, it's not the case you will end up. regardless of how many resistors you put in series, you will end up.

The final resistance value will still be plusus 1% And if you don't believe me, let's do some simple math and check it. now. a 100 Ohm resistor plusus uh, 1% means it can have a value from 101 ohms to 99 Ohms anywhere within that range. Technically, it can be any value there.

So let's take the case of that. They're all in this case. We got four resistors. They're all 101 Ohms.

What do you end up with in total? In that case, you get a uh total resistance of 400 and 4 Ohms And how much is our value out at 400? Uh ohms. Well, it's out by 4 Ohms. So if you do 4 / uh 400 because that's 400's the nominal value times 100, you get 1% Bingo. And if you do the same math with all the you know, with 100 resistors in series, it doesn't matter.

You're still going to end up with a total tolerance of plusus 1% So if you use plus- 1% resistors in your decade box, you can say your total uh decade. Uh, your total resistance box tolerance is plus - 1% Just like the Um I Labs one here, it uses. 1% resistors in it and they have no problems saying the accuracy is plusus 0.1% Now one of the things they don't tell you on this box is the temperature coefficient I Actually asked EIT and they said they use 25 PPM resistors as I mentioned. uh, right at the start.

but they don't actually mention that and that's um, if you're designing a Precision decade resistor box, it's important to take that into account. Well, what does 25 PPM means? Well mean. Well, it. It means that that 100 ohm resistor or whatever resistor in the Box will change 25 Parts in the million per change in De C And what's 25 parts per million in percentage? Well, you can simply convert those.

they're completely equivalent. It's .25% per degree C Now you might think, hey, that's that's nothing that's half a bees dick. It's tiny. Why do I need to worry about that? I'm only going to use this box in this Decay resistance box in the lab and the lab's going to be.

you know, 20 plus - 5? What's it? You know what's the big deal? Uh, you know at all. it's not going to change that much. And that's true, right? If it's only .25% per de celsus and you know your lab might change, you know, if it's air conditioned, it might only change a couple of degrees or something like that. And really, when your resistors are Point Uh, you know if you're using a.

1% resistor as you are in this case, or even a 1% well, what does it matter? Aha, What you're forgetting is that the resistors inside these boxes, they will heat up. Uh, they can actually heat up above the ambient temperature. depending on the current you're flowing through and that you've got flowing through it, you know? I S R Basic stuff. So let's have a look in the case of uh, this particular box here: I Don't know the exact resistor used in there cuz they're custom.

but we can get a generic resistor a half W resistor. It's pretty much going to have the same um, uh, thermal resistance uh for basically any uh value resist, any same size, same size, half W resistor. It's going to be very similar. So I'll take a generic V brand resistor here and this is the particular Uh model that is, um, half a percent, uh, half a watt? Sorry.

And you've got a thermal resistance graph here of uh, the power dissipated in the resistor and the Uh heat rise above ambient which is important, which uh, we'll look at later above ambient that the temperature will rise when you dissipate half a wattt. so you can't just magically, you know, make this thing, dis, put enough voltage in here and make it dissipate half a watt and not expect its temperature to go up because it will. Okay, And if you take a look at the graph down here, it is, uh, let's this is the line here for our particular size resistor. They do come in different body sizes, but this is the one that's rated for half a watt and you'll see if you go across here.

Well, there it is. Half a watt goes up there and it's around about 60 C So it's going to rise 60 C above ambient temperature when you dissipate its maximum power rating of half a watt. Crazy! So now we know that if we dissipate 0.5 wats into one of those resistors, then it's going to rise 60 C above ambient. Uh, but because we're uh, comparing it to the Uh, ambient tempure temperature will only take the Uh A Delta a delta T of 60 C But the temperature of the resistor actually gets to 80 uh Celsi if you've got a Uh 20 C ambient temperature.

So let's look at uh, what percentage change? remember it's not .25% per deg C which didn't sound like much. But if you take Uh 0.25 % and you multiply it by the 60 C change Bingo that gives you a value of 0.15% And if you're using 0.1% uh tolerance resistors nominal then Bingo you've already exceeded uh that uh value by another 05% Uh, you know it's not going to matter. maybe if you like use a 1% tolerance resistor or something like that. but if you using try and design a decade Precision resistor box like this one with 0.1% resistors in it, that can really matter.

And of course not to mention the stress you're going to put on the resistors and you can change their life uh, based on uh, if you operate them at a higher temperature. So and correspondingly, if you don't have to have Uh 0.5 Watts either, if you're dissipating 0.1 it's going to be you know 1/10th of that value Because as you saw on the graph here, it's a linear response. Completely linear so that even if you're dissipating a very small amount of power, it still can Rise by you know, uh, 5 or 10. Not a problem at all, so you got to take that into account.

And if we come back to our data sheet here, these high temperatures, you don't want to be running resistors continuously at high temperatures like that, because this is the one we're looking at here. Okay, to keep it up: plus-15% over the maximum typical variation in the resistance over the load life. The load uh, life Shift versus power rating uh table we've got here, we'll see that that resistor um, if you're talking if it's half watt dissipation which is what we which was what it's actually uh rated at over its life, it's going to be plus - 1% not plus -1% So just remember that and that's a power rating at + 70 C Now the other way you can make a decade resistance box is by using uh, four individual switches like this just uh, you know, single PSE single throw uh switches either slide or toggle or whatever. um, or dip switch or whatever per decade.

uh, instead of a rotary switch and you only need four resistors like this per decade in the 1, 2, 3, 4 sequence like this, no complicated switch Arrangement But instead of dialing the exact value onto the unit itself, you have to actually do a bit of math and add it up. So in this decade, Um, here, if you want to say have uh, 7 ohms for example or 70 ohms in this case cuz it's a 10 ohm uh uh, decade uh thing, then uh, all of the switches by default are closed. Okay, in this whole sequence here, they're all normally closed. and if you wanted to say do 70 ohms, you would open switch for and open switch three and it puts 40 Ohms and 30 Ohms in series.

And you can get any value in in this case from 10 ohms up to 100 ohms per decade. So you do actually get um, like the 11 waist which we actually looked at before On my particular box, you can actually D anywhere from um, you know, zero up to 10 on each decade and you put them in series. And of course 10 Ohms is A you know, 10 is a standard E12 value and Uh 20 and 30 are e, um 24 are values and um, of course 40 Ohms is you know, or 40 is not possible or 400 or 4K or whatever is not possible in any of the standard uh e uh ranges. but you've already got your Uh 20, 200 Etc so you just put two of those in series to use your 41.

or likewise, with a 300 you don't need a Um a 300 uh value. You could use two 150s in series if you wanted to using the E12 range. But there you go. That's just another method doing a um a decade resistance box except you know it's A.

It's a bit trickier in that you've just got to add up the Uh, add up the particular values and it's a bit confusing that the switches have to be normally closed. and one of the things with the normally closed switches one of the problems with this Uh configuration. Instead of having just one Um switch contact resistance per decade, you've actually got four. So if these are all normally closed, you've got those four contact resistances there.

So you know, if you're doing a box like this, you might start out with a uh, lowest, um, first decade of only 10 ohms As I've shown here, you might not want to go down to the 1 Ohm range and certainly not down to the Uh 0.1 uh ohm range, but you can if you want to if you're using high quality switches. Etc but just something something to consider. so that's another way to do it. But they all work the same.

They all put resistors in series for a decade. That's all it is. So go build yourself a handy decade resistance box and what a great, fun little project and you'll be surprised at the uses that you'll get out of the thing. It's amazing! Catch you next time.

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