Hi, welcome to Tear Down! Tuesday Now I was going to do something slightly different today as opposed to the usual test gear tear Downs I seem to be doing lately, but I couldn't resist I got this really nice bit of Kit in the FedEx box the other day. You can't stop me now. I've done a couple of power supply uh tear Downs recently and I've got another few coming up. Um, so if you love power supplies, hang out for those.

But I've got a very useful, incredibly useful companion device to power supplies. and I haven't done a tear down or review of one of these before. I have done a video of how to make your own. What Is it? It's the BK Precision 8500, 300 W DC Electronic Load and this looks like a really awesome bit of Kit Oh, smells good too I Love it! So uh, what this is is.

um, it's the BK Precision 8500 but uh, it's it's actually um, designed and manufactured by a company called Uh Itch and it's sold. Um, it is also sold under the Itch brand as well. But it's not just rebadged by BK Precision because um, Itch is apparently a BK Precision company. It's all part of the one big uh, you know, Global Corporation or something like that.

So I'm expecting good things out of these because, uh, itch, This is all they do. They do. You know, power supplies, electronic loads, system LS you know things like that. So I'm expecting this to be really well engineered, just not some one hung low slap together cheapy.

And check out these big beefy binding posts. Look at that. How can you not love that? You know what we say here on the Eev blog? Don't turn it on. Take it apart now! I've got to say thanks to Greg at BK Precision for getting me one of these puppies because it's going to be really handy for my uh power supply.

uh Tes in videos my power supply design videos because you've seen my Uh video with the uh electronic um load where I built my own. you know, very, uh, simple, crude. as crude as you can possibly get. Well, you can't beat having a proper um, you know Precision DC electronic load and this one really is a Precision bit of Kit and it goes up to 300 watts as well.

It's awesome! so thanks Greg So you can expect to see this thing. uh, get quite a bit use in the Eev blog lab here as I designed various uh, power supplies and things like that. very handy bit of Kit and probably one of the most overlooked uh bits of Kit in any lab. And if you're designing uh Power Supplies you really should have any sort of power supply switch mode, linear, whatever, small or large, you should have a Precision DC electronic load of some sort.

And of course, I'll do a proper review of this thing later cuz it comes with uh PC Control software and all sorts of stuff like that. But look at these huge big binding posts on the front. Absolutely awesome. Lovely knob here for setting stuff.

current, constant current, constant voltage, constant resistance, constant power modes, everything. more stuff than you can poke a stick out I Love it and it's a really nice big Clunan power switch on here and it's a really nice solid unit and check out the base of it down in here. Huge metal base with tons of screws. I Love it! So let's crack this thing open and see what's inside.

I Think just a maybe a couple of screws on the back. we get the back panel off and maybe the front panel as well and then maybe those four on there and uh, this might slide off. Let's find out. Yeah, and that came off pretty much uh, exactly as I thought.

The rubber uh surround on the front just slid off nicely. No screws holding that in. I Really like that uh design, so it's really easy, so let's this should just slide right open. Tada And there's no major surprises here at all.

It looks off the bat, looks very well built, very well designed as you'd expect from a specialist U designer and manufacturer of, uh, these sort of electronic loads and uh, no surprises in the uh layout and um, the amount and type of circuitry in here. We've got a Main's transformer for Paing the thing. We've got some uh, big current shunts over here. We've got uh, some current shunt power resistors down in here.

We've got some mosfets hooked onto some large heat sinks with with some uh, pretty decent fans here on each one. I Rather like this actually this. Jewel uh Arrangement here with the fan inside sucking the air from these vent holes here through the heat sink and boom out the back. like that separately on both of these and Mains input over here.

and uh, some control circuitry. we'll take a look at that. we've got a number rubbed off there. Don't like that.

We got a couple of relays which I love of course I Love products that have relays in there bit of miscellaneous power supply to power the control circuitry and the input stuff. so this looks really neat. Let's take a look at the individual. Parts in more detail now.

I Find this, uh, rather strange. Here's the IC input connector. It's uh, heat shrunk. Of course they've got some uh, Ferite beads there for a bit of RFI going into decent connectors onto the board there and the voltage selection switch on the back panel 240 Vol 110 is there as well and that goes down to a second connector uh, down in the bottom down in there.

but the Transformer is all the way over here so it's it's. right down in here so those main tracks have to run all the way around here down the board. probably maybe on the top side or the underside there, down into the Transformer down in here. and uh, why they didn't just run some cable in down there? you know, neatly lay it around the side or something.

Something like that cable tight. I Don't know, but they decided to do it all through the tracks. I Guess it's neater, um, that way. but they've got the 240 volt tracers.

Main's tracers looks like, uh, running under the heat sinks. It could be on the bottom side of the board. It can't just run down the center here. CU There's all that, um, all that current sense circuitry down there.

so it looks like the traces go off here somewhere under the heat sink here. and they pop out down here. and you can actually see the silk screen on top of the mains tracers there. So presumably they have actually got them running on the top of the board.

And here's the big clunk in power switch down in here. I Love that. And uh, here's the Um output connector, which the Main's output connector 240 Vols which goes into the primary of the Transformer So I don't know it's there's nothing inherently wrong with that. I'm sure they've done all their design, uh, calculations, and clearance.

uh, things like that, so it's not a problem. Um, one thing it is certainly is neat, and if you have a careful look down there, you can see a couple of signal tracers here. these are the 240 volt Mains tracers and there's another power supply. um, you know, a low voltage, um secondary side power supply Trace down there, it looks like there only maybe five or six mm uh clearance tops in there.

Ah, there, I See it? It does actually drop down to the bottom layer there, so it looks like it may not actually run under the heat sink and you'll notice they've gone to a fair bit of trouble to actually celastic down that IDC uh ribbon cable there on the header. they celastic down. uh, these connectors down in here, the mains connector and the ribbon cable down on that side. And they've done that in quite a few places, so they've certainly uh taken vibration into account here and we'll see that again.

Over on the Um input connectors over here. And by the way, the Transformers Uh held down very nicely with Uh Shake Proof washers. They've done that really well in the Uh Earth connection down there has some Uh locktite on it as well so it can't come loose. And here's the input connectors.

Aren't they beautiful? Check that out. Huge, big solid threaded bolt coming from the input connector cuz remember, this is a 30 amp Um input, 30 amp uh capable and high voltage as well. We're talking about a 300 W DC electronic load here. and then that comes up into a a custom Uh right angle bracket which is soldered um directly onto the board there.

That's a really nice implementation and of course you can see the red Loctite around there as well to stop them shaking loose I Love it! Very well engineered and the other input circuitry around here. We've got a couple of big uh 1 ohm power resistors here once again celastic uh down so they don't vibrate loose. We've got a couple of uh, high voltage caps, we've got some Ms and uh, we've got some high voltage um isolation SL cut through the board down in there so they've paid a bit of attention to detail to there I like it and I'm going to presume that these two large shunts here are the main input current shunts and they would have possibly been uh tweaked to Uh value or their Um or their tweaked in software of course to actually null that out because this is a real high Precision unit by the way. uh, we're talking uh 0.05 5% class voltage measurement and at least 0.1% class uh current measurement as well.

Very precise bit of Kit So um I expect it to uh, that would be a very low uh Tempco uh metal to uh use as the Uh current shut and we should have um, some um, some Precision voltage reference and uh ADC circuitry elsewhere. but yeah, um, it looks like there's room for two more there. maybe uh, different model units cuz there are various models in this entire range and they have different capabilities. And we've got a date on this sucker.

uh, 10th of the 906 so it looks like it is quite an old design. it's been around for quite some time, you know. And these DC electronic, uh loads. They don't change much, they don't need to be upgraded so that uh doesn't surprise me at all.

And there it is. It's the It 851, uh load and presumably this baseboard can be used for the other models as well. and in the logic control power supply circuitry here. they've got some Lelon brand uh, electrolytic caps which I believe are reasonably good quality there all 105 so they haven't uh, skimped on there and that looks quite reasonable.

but um, you'll always note that I'm not a huge fan of these freestanding to 220s and they've done that a couple of times. and there's the other two freestanding to 220s and you can give those a little bit of a wiggle. I Would have liked to have seen those Uh mounted flat or mounted on a small heat signal somehow. uh, rigidly retained cuz they've done, um, a really good job on all the other locti in the nuts and the connectors.

elasti in those down, and things like that. so I'm always a little bit weary of that, but it's a very minor. Point We've got our main processor here. It's a small one, tiny little microcontroller of some sort.

and boohoo, they've rubbed the number off. Who? Why come on people? Really? Is it that important that you got to rub the number off that chip? And that chip Only give me a break. Anyway, there's the main oscillator. Crystal We got another 32 khz watch crystal over here.

Once again, that one's elastic. uh, down instead of the more usual uh soldering uh, down. And here I am trying out my new uh Times 10 Oper uh macro lens I've got for my Canon Hfg10 camera and this is working really well and you can see the gouges taken out of the main processor there. They've really ground that out, they haven't.

You know? they've really got in there with like a a Dremel or something and really dug that out. And there's a 24 LC 64 E prom that would hold the uh calibration values one would presume. and there's a whole bunch of Op 07 Precision uh. Op amps around this thing thing.

They're all over the shop here. these Oppo 7s. there's like 78 nine of them. so uh, no surprises there.

They're the industry standard um Precision Op amp and there's an ad uh 708 and that's a 16bit Delta Sigma ADC No surprises at all. Uh, built-in programmable uh gain amp. and the reason you need a 16bit uh converter in this thing is because it's got a 1 molt resolution in an 18 Vol range. That's one uh part in 18,000 and uh Uh to do that.

Um, you can probably get away with a 15-bit converter, but they don't make those as well. So they use a 16bit converter which is one part in 65,000 So it's ideally matched. And down here we have a bur Brown Love bur Brown Parts which are now TI of course, but that's a daak uh, 76, 31 and that's a matching um, pretty much a matching 16bit voltage output Dack And there's a voltage reference for the ADC and the Dack to be shared between them. It's the ADR 421.

That, and that's like a better than 0.005% class voltage reference with 3 PPM Tempco Very nice, and that's something a lot of people forget. With Uh Precision designs like these, you don't necessarily need absolute uh Precision components, you just need uh, low temperature coefficient or low um, you know, highly temperature stable components and then, uh, you can take care of the rest with uh software calibration. So it's um, you know, you can get a really precise instrumentation by using a 1% voltage reference for example. but as long as it has a very low Tempco then you can, uh, take care of that.

the software calibration now. I Love how in this section down here, they're Tl074 quad uh opamps and they're in a old style dip package and there's Lm324 and it's in an So package. so that's just really. It's just quite strange how they've actually got those in A dip package.

and look at those passives surrounding that, they're quite spread out with the tracers going a long way. I Just wonder why they've done that? And as you can see, also, we've got a whole bunch of these um Rx21 series uh, silicon resin coated, uh, 8 wat power resistors in there and they're available with 1% uh tolerance values and about 250 PPM I believe. And the big mosfet we've got on here is an Irfp 250n. It's quite, uh, hard to get the camera in there, but there you go.

There's a couple of there's um, quite a few of these devices scattered around the heat sinks to spread the load. and they 30 amp 200 volt mosfets and you can see, uh, a couple over here. You can see another couple up there as well. So that's four.

But if you look down the side of the heat sinks, they got another one and two there. So they've got a total of four uh, mosfets per side here. So a total of eight. Um, of those huge power mosfets.

That's to, um, share. That's where all of most of your load gets, uh, dissipated into these mosfets and through to these large heat sinks. Here there's a little bit in your current shunts and things like that, but uh, most of your power. Most of that 300 W capability is going to be delivered into these large heat sinks and the fans are going to suck the air in from the side here, suck it through the fins and take it out the back.

Now as you can see, there's a whole bunch of factory tests it past and each uh stage through the production. uh testing. it gets a little sticker on it. Cal Once it's done all the calibration and the trim pots and they've tweaked it.

Um, because this as I said is a Precision instrument very important process to calibrate it, then they would have done the high voltage test. It would have um, some sort of uh burning test I don't know. They might run it at full load for 24 hours or something like that. may possibly in a thermal chamber to see if it, um, you know, at an elevated uh ambient temperature to see if it Keels over or something like that.

um PR PRT test I'm not sure exactly what that is. uh, that's PRT is usually a production Readiness uh test. but I think it could be like a um, a highpot uh uh, Main's uh, compliance test or something like that and ICT is probably um in circuit uh test as well. So ultimately, what that means is that when you buy this thing, you can be pretty darn sure that uh, it's going to uh, function correctly out of the box and continue to function correctly because it's had that burning and that's what you're paying for for these uh quality bits of uh instrument from a uh proper manufacturer that uh specifically develops equipment like this and there'll be a few people who are just Keen to see what's on the back.

I Input connector fused Uh 220 1110 Vol selection We've got a Serial Uh control interface and my unit uh came with Um isolated by the way, completely isolated um Rs232 and USB cables. very nice and it comes with remote sense uh, spring terminals as well. You can just get in there and push your wire in and uh trigger input as well. Very nice and there's not much uh doing on the front uh display L board up here with the uh uh, soft button keys and the vacuum fluorescent display.

Apart from the two vacuum fluorescent display driver um chips they're they're a PT 6315 and that's a Serial input uh vacuum fluorescent display driver and there's two of those and it is a very nice display and they've got that of course. um uh interfacing with the ribbon cable over here going down to the microcontroller and there's a Teensy bit of circuitry there for the very nice rotary encoder knob and a buzzer to make some sound at you beep and I Forgot to mention a couple of other moths are scattered around the place. and uh, they've got another couple of devices connected to the heat sink here as well, a 3 pin To220 and a bridge rectifier uh down in here as well, connected to this side of the heat sink. So as you can see, there's not a huge amount inside these electronic loads.

There's a microprocessor to control it all. There's a Dack and ADC to uh not only uh, set the value required, but read it back and there's a Um mosfet or a bunch of mosfets with some load resistors and that's you know, pretty much the basic operation of any DC electronic load. except this one is uh, really nice cuz it's high precision. and I really do like how it's nice one bit.

Big solid bit of folded sheet steel which goes right over the top. like that, it really works. Oh the bottom. sorry, this is the bottom of the unit.

It really is quite nice. Everything uses Shake proof, uh, washes on there. and by the way, in case you're wondering, there is no circuitry on the bottom so there's no advantage to uh taking that apart anymore. But doesn't it look really? Schmick and I couldn't really uh, fault the design or build quality in this thing.

it? uh, it. It's certainly uh, first class and I would certainly give it a big thumbs up. So there you have it, that's inside the BK Precision 88500 100 Precision DC Electronic Load If you're into power supply design, um I Highly recommend you pick one up. This isn't a review, but uh, people want to know the price.

Street PR Um, sorry. recommended Price is about 1,00 bucks for this particular model. Street price is around about 900 bucks or lower. or you know, even substantially uh, lower than that depending on where you get it from.

and uh, this is actually the Uh It 8512 is equivalent to the BK Precision 85 500, but there's like a dozen different models. This is the 120 Volt 30 Amp 300 W version, but they're available in different voltages and currents and power ratings. so thanks to BK Precision for this, it'll be a very handy bit of Kit and you'll no doubt, um, see it coming up when I do some more videos on the Uh power Supply design stuff cuz this is fantastic for testing any sorts of power supply switch mode linear. you can do pulse loads as well.

you can set it up. It can do battery discharge uh testing as well under PC control. Fantastic! Very useful bit of Kit And remember if you like H tear down Tuesday Please give it a thumbs up cuz that helps a lot. And if you want to discuss this, jump on over to the Eev blog.

Forum Catch you next time.