Hi I got an email from a Bm 235 customer who's had it for, uh, quite a few years. but it's um, started to give a problem where even if it was, uh, switched off like this, it would drain the batteries and uh, he linked me to an Ev blogger forum fred from 2018 where another customer had exactly the same issue. Now, I don't ever recall seeing that uh thread because there's like over 800 posts a day on the Ev blog forum. There's a lot.

Um, and I didn't reply to it, so I'm not sure if they contacted me direct or or what. But anyway, this is the first time I've heard about, uh, battery drain when it's off like this. of course. uh, it's got, uh, power saving mode after you know x amount of minutes.

Uh, if you leave it on and you do nothing with it, it'll auto switch off, auto power off, uh, function, and then of course the microprocessor and it will, uh, take some residual, uh, power and it's waiting for a button. uh, to, you know, wake up and stuff like that, but in the off position of course. um, it should have physically disconnected the switch. But um, I don't have a schematic for this because Bruhman refused to release schematics even to um, trusted dealers like myself.

So yeah, you know, it's all secret Squirrel and everything. So let's open this up and have a look inside because I don't remember um, the architecture of the switch and the battery and any other devices in there that may actually cause a drain. Um, so yeah, there must be something else in there, like a protection device before the switch or something else before the switch that is chewing. Um, some residual power.

So let's crack it open. All right. So first of all, let's actually measure um, the residual current of this in the off position. We can do that with the Bm786 here because it's got 10 nano amps resolution here.

Uh, we don't have to go get in like the microcurrent or something like that to get anything lower. because like a really low power uh device will take in the order of like micro amps. Like a standby thing. Like a really ultra low power device.

Uh, ultra low power micro might operate at like hundreds of nano amps or something like that. If you get down into like the 10 nano amp uh region, then you're like a really ultra ultra ultra ultra low power. So and have a squeeze. let's up.

get it the right way around and there we go. There we go. I saw something we're down in the least significant digit there, but I didn't see any jump up there. So obviously there's no capacitance on the input.

that's like charging up or anything. So yeah, there's just absolutely nothing there. And if you're wondering about the operational current in voltage mode, there you go. Two odd milliamps and I won't bother getting the uh standby, uh, consumption.

This thing. I have to wait till it turns off and then you can't break the thing. you've got to have it permanently wired in. And in any case, the report is, um, that it does it in the off position.

So let's crack it up. So my first and best guess to the customer was that, oh, maybe there's like a reverse bias diode protection in there. So uh, to basically, um, short out the batteries or actually, you know, limit them to 0.6 volts if you actually put the batteries in backwards. And if you've got a diode that does that, then of course diodes will have leakage.

And if it's using like a one in 401 something like that Absolute classic for that um, application, then they're not low leakage. Um, of course. so you know they might take microamps, but they certainly shouldn't take. uh, you know you to drain these batteries, uh, within weeks that he's talking about or something, then you're going to need like, in the order of, like, you know, hundreds of microamps or milliamps.

So here we go inside and we've got our spring contacts here for the batteries. and they contact these two points down here. Bingo. I think right off the bat as I suspected between those two contacts, Bingo.

A power diode that's likely like a one in 401 or something like that. And there you go. I was right on the money. It's a one in 4007.

the 407 is the highest voltage, uh part in that. range. The reason they're not using a four level one is because they probably use it for protection somewhere else. and there are, yeah, just reusing that bomb partner.

From a Bill of Materials point of view, it doesn't um, you know, make any sense to have you know high voltage parts somewhere else and then the 401 lower voltage one across the battery. You just, they're so, uh, cheap And yeah, you wouldn't bother having a different uh Bill of Materials item just to put across the battery of course. uh, the battery only needs a low voltage, so the 401 would work a tree. But anyway, now I had to go through a couple of dozen data sheets because Uh.

Murphy says it wouldn't be the first one I opened before I could find one with a reverse characteristic uh, graph like this that actually has the reverse uh current in microamps. uh versus the voltage the reverse voltage on here and um, yeah, I found this one. This is a fair child jobby. This is for the Uh through hole uh part, but it should be identical.

uh for the surface mount variety anyway, and they'll slightly vary between manufacturers. but basically all one in 401 407s are pretty much equivalent. So if you don't have one of these graphs, then the only thing you've got to go by is basically the uh, the spec table which is going to give you, uh, the worst case, reverse current at the maximum rated voltage. And of course, we're not operating at the maximum rated voltage and or the maximum rated temperature as well.

You can see they've got three different characteristic occurs at different temperatures. We're at ambient, uh, temperature here. and we're only operating this thing at basically, um, three volts here because we've got two double A batteries. Three volts tops.

So this is 10 volts and we're already down at 20 nano amps. Look, it just drops off a cliff here like it's it's 10 nano amps down here. at 5 volts. you know, roughly less than that.

This is why we weren't able to measure anything on our Bm786. Now, of course, this doesn't mean that this isn't the culprit. You could very well have a, you know, a faulty diode in there that has excess leakage. So if you've got this sort of problem, it can be in any multimeter.

This is a very common any product actually that has batteries you might typically have like a reverse bias diode like this. If you can afford the luxury of the voltage drop, then you put the diode in series with the batteries. But then you get your 0.6 volt drop and on a 3 volt battery. and that's pretty crippling.

So you have your reversal by stories. There's other ways to protect, but anyway, this is cheap and simple. and yeah, it could certainly have come the guts in that part. So if you have see this sort of thing in the product excess power consumption, I would potentially look at that.

and as I said, like, you should be able to measure that with an ordinary multimeter. you shouldn't need a microcurrent or any other fancy six and a half digit meter or something like that to try and measure the low current. If it's like in the order of microamps, you don't have to worry about it. If you've got significant battery drain, it's going to be in the order of like, you know, hundreds of microamps kind of thing or more.

Now of course, the other thing you might want to look at is any, uh, potential contamination around here. You might have, uh, you know, have moisture on the board? You might have. You might have spilled something, some liquid in through the battery compartment or something like that. So I'd give it a big good clean with um, some isopropyl alcohol and then you know, just give it a good dry out and uh and see if that makes a difference.

But um, yeah, I don't have the Cus. this is not the customer's unit, so I don't know. Now that Diode might not be the only thing that's in parallel with the battery. So let's have a close look at the Pcb here, shall we? I've got my original teardown photos which are included in the manual.

I claim it to be the only multimeter in manual. in the world with photos of the Pcb in it. So what I've got is the top and bottom of the Pcb here. And as you've seen in previous reverse engineering videos, I've actually taken these photos and I've skew corrected them.

I've rotated them so they're roughly the same. They're not absolutely perfect, but if you focus on, say, the center of this, pin, uh, down here. if you can see my, uh, cursor, then you know it does a reasonably good job of aligning these up. So I've rotated them, flipped them, and cropped them.

so they're pretty much, uh, line up good enough so that we can actually flip from top, side, to bottom side of the board and trace, uh, things like this. Now, unfortunately, I just realized, um, that these photos these would have come from my when I first, um, got this uh, meter So I don't know what the revision is. I think it might, is it chopped off? I don't know. Anyway, this would have been one of the original units and you'll notice up here.

where's the Diode? Where's wally? Um, it's not there where you saw it, It was actually wedged it. It was actually put. well, my, um, yeah, Earthen view skills aren't that good. But yeah, they've actually squeezed this on future models.

I don't know how many years this I can't remember. They might have mentioned this change way way back. I've been selling this meter for like six plus years. at least I think something like that.

So yeah. anyway. the new meters um in quote marks have the diode in there. but apart from that, I can't really see any other differences.

I am, you know, using the mark one eyeball at the moment, I'll have to get like some new uh photos of this and uh, put them up there on my Eevblog flickr account, which is where I put all my high-res photos for stuff. But yeah, it looks identical. Apart from adding the diode, everything up there else looks the same. So yeah, she'll be right.

So yeah, just imagine there's a diode in there. Okay, so this here is our positive terminal. Where else does it go? Well, there's this giant uh via here like a test hole. but it's you know it's actually connected.

So it goes down as well. and there's two little uh, piddly vias there and they go down to the bottom side and you can see there they are. Okay, so a trace buggers off this way like this goes up here. We'll follow the money there and these come down and this goes across here.

Please forgive my crude mouse skills and this goes off here. Let's just trace this one here first. Okay, so if we flip to the bottom layer, there it is. Follow the money.

Just follow the money and we're following. following, following there, and boom there it is. That's what I expected. It went through to the switch contact in there like that.

So the battery is like switched. So at the moment, the diode is still the only thing that's directly connected across the battery. But aha, what about this one up here? You're saying, well, let's follow the money there. Trust in Deep throat.

Follow the money. He knows what he's talking about. Anyway, that goes under there. I'm pretty sure there's nothing else under there that goes along here.

And Boom Goes to these two vias down here. Let's switch there. They are there and this goes around here. Interestingly, they do have the uh, solder mask removed from that.

Not entirely sure why. Anyway, it goes to a big hole over here and that pops up on this side and we might have to actually do some zoomy zoom on that. There you go. It doesn't connect to that, but bingo goes off to actually two capacitors c24 and C26 there.

So we've got a diode and two capacitors in parallel with the battery even though the switch is off. Aha. Okay, so where do these go? Well, this goes via an inductor here, which is basically just a short circuit and then it goes over to uh, the current shut here which then goes up to here and that would be that's our ground. I believe that's our ground terminal because our fuse comes in over here like this goes through our high rupture capacity.

Hrc fuse here goes through the 10 amp current jack like that. So this here would be ground and you can tell because they're basically doing some star splitting off here and then that buggers off over here. So this is, uh, ground over here. Then you've got star ground in over here as well so that it's a common point.

I mentioned star grounding technique in other ones and then and there's another inductor there and then once more got some star grounding going off here. They love this star grounding, don't they? And they're actually doing the same thing over here. So why do they have um, these capacitors across here like this across the battery? What's what's the point? Well, I don't actually. Um, no, I can only presume it's for some, uh, you know, Emi compliance thing.

But why would you care if it's switched off? I don't know why. I don't know if you've got any idea. leave it in the comments down below, but if the things switched off, I could understand having the reverse bias diet across there. it's to protect it when you plug them in, but even that you could have on the other side of the switch.

Really? So yeah, I I don't know the reason why they've got those um. caps in there. I can only presume some sort of Emi compliance thing that they had problems with. and they've got both of those capacitor there through this inductor to ground.

And they've also got this capacitor through um, L3 inductor to ground as well. So like why they've got two of them. Sure, this one branches off, uh, somewhere else. Okay, they're taking that, uh, reference point, that ground reference point off via that inductor there.

But these inductors are. these are just like little very low value, like Rfib kind of thing. So you know it might have some not only compliance issues, but they might, uh, be to help Rfi interference being picked up by the test leads going into the measurement circuitry and stuff like that. But still why you need them before the switch? That's what I'm asking.

I don't understand that at all. Anyway, if either of these capacitors here develops any sort of leakage, um, at all, then that leakage is directly across the battery from the ground, and I'm sure that this ground point is going to go back to the battery. Here's the ground point here. We'd have to, actually, uh, follow the money all the way.

And look, here's these. uh, star groundings. Again, look at this. They're a real Star grounding fanboy.

Aren't they just all these different paths, just, um, snaking off here. Absolutely incredible. They've got it up here again. Look at this like three different star ground.

They that might be not be ground, it might be another, uh, voltage potential. but yeah, these are just like and there's another three going off there and there's probably like a bunch more within the meter. So they're really, um, you know, star. Um, reference point, star grounding? uh, fanboys.

which is great design practice? Um, of course. So here I was just editing the video and I thought that I should measure that because it's not going to be connected. Yes, the actual uh, battery ground here is in a lot of multimeter designs. Most, um, yeah, it won't be connected to the actual uh, ground physical ground input here.

It's not uh, floating though. I actually measured it uh with the meter and it's 3k ohms in one direction and 33k in the other. So like there's active. you know, stuff in there.

But yeah, basically. um, if these caps. But once again, if these caps were actually uh, leaking then there. Yes, that would actually leak back to the negative terminal.

so all that stuff is still valid. But yeah. anyway, we've got a diode and two capacitors across there. like that, I can't see anything else that is across the battery.

So to the customer who's got a couple of year old faulty Bm235 yeah, I'd be looking at those three components. In fact, you could remove them and the meter would still work. Um, function and meet spec and everything else. It just won't be.

um, as maybe Rfie protected. Um, and also, uh, reverse battery. Uh, protection. You're essentially removing that.

I don't know what happened. if you're putting the batteries back to front. um, just don't do that. But anyway, you can put new ones in there.

so I would say it's most likely. Come on. your money has to be on these Mlcc multi-layer ceramic, uh, capacitors. They're you know, not the greatest things at the best of time.

And maybe you know, get some board flex in here. In fact, the meter actually did have a video a long time ago. I have to link it in. Actually, in this area, there were actually board flex issues and we'll get in.

What break-in in the inductors weren't we or something? The board was flex. I can't remember the exact thing I'll link in the video, but uh yeah, that was happening around there. That's not related to this battery issue because even if if these inductors, uh, break of course, then well, your grounding's completely ruined, all your measurements will be completely um, off. That's got nothing to do with the battery consumption.

So as I said, if you want to include why they uh, would put capacitors on the bat on the like the battery side instead of like after the switch. I like because it's not like it's gotta uh, perform or meet any Emi requirements when it's off. So I like, yeah, why wouldn't you have those after the switch Maybe it was just like a convenient routing something like that and uh, maybe you know this is not uncommon. By the way, the uh, Pcb layout? uh, asked me how I know the circuit designer designed the meter and then they threw it over.

You know, they threw it over the cubicle wall to the Pcb layout design engineer and they went, oh, look, I can't just like it's on the other side of the board. I can't put these things here and get it back here. And like all that, um, sort of jazz. like really, you want me to, can't I just put it before the switch and they asked the designers only as yeah, whatever.

it'll still do the same thing. You know, if that helps in your layout or something like that. Um, which you might have to do often. in actual design, you might have to actually compromise.

Um, your Pcb layer. I swear, pin swapping is another uh thing, right? If you've got your micro controller pins, your Fpga pins or something like that and your routing's not, you know you just can't you last trace it can't go in there. Well, you might swap some of your micro controller or Efpga pins or something like that. Um, just to ease your uh, routing, you know, basically it's it's got to come back to here so you know it's It's not a stretch to kind of get it back to there, but then you'd have to come in through the switch and stuff like that.

So yeah, yeah, no, I don't know. So there you go. I hope you liked that video and found it useful If you did give it a big a thumbs up. And if you've seen other multimeters that have stuff on the like battery side of the Um power switch, then please leave it in the comments down below.

I know everyone in the Ev blog: Test Equipment uh, Forum largest test equipment forum on the interwebs by the way. Um, yeah, they'll know so they always know. But yeah, that's interesting. I thought it would have been only a reverse biased diet in there.

That was my guess. But no, I I reckon there's good money on those as well. I think they're much more likely than the poor old deity up here because these you know, these one in four, Double a sevens. usually pretty grunty.

So anyway, there you go. Catch you next time you.