121GW Multimeter Battery Mod

[Mr. Wizard]

Has the PCB layout for the 121GW been published anywhere? I am designing a PCB mod board for it to allow the use of lithium batteries, much as Dave demonstrated in this video, with a tiny voltage regulator and a few other components. I can measure the locations of everything myself with a caliper, but if the PCB layout is available it would save me some work.

Below is my rough, working schematic for a 121GW mod board to allow anything from NiMH to lithium batteries.

It is based around the ONSEMI MC78LC25 voltage regulator which I am planning to insert on a small mod board that will feed the ADR3412, instead of its voltage coming from VB+.

Please criticize and critique. All feedback is welcome!

[Mr. Wizard]

Here is a rough mockup (components not to scale) of how this mod board would fit onto the EEVblog 121GW PCB.

The capacitors may need to be bigger. If so, the mod board can be bigger and they can be below or to the left. C2 may be unnecessary.

Solder mask is not shown, but would be everywhere except where components are making contact.

The back side of the board would be a copper fill connected to AGND.

The three castellations in the upper right would connect electrically; the others would just be for physical support. The AGND castellation may work better over the via by the C18 label rather than on the corner of C18 itself.

Please critique and criticize. Be harsh.

[BU508A]

will you make those PCBs available at Oshpark (for example)?

[Mr. Wizard]

will you make those PCBs available at Oshpark (for example)?

Yes, that’s the plan. I’ll upload all of the files here, too (or put them on GitHub or somewhere similar) so anyone can use and/or modify the design.

[Mr. Wizard]

I am now considering if removing C18, then putting it back onto the mod board, would make the mod board much easier to install and less fragile. It would make the mod slightly more intrusive, but would give much larger areas to solder to on the main PCB. C18 provides 2 of the 3 electrical connection points necessary; only VB+ is missing, but that is right next to it at R94. I am not convinced that the PCB could be easily manufactured to fit in the tiny space between the RTC battery and C18 otherwise.

It might even make sense to remove R94, C18, and C62 and then use that whole area and the area to the right for the mod board (putting C28 and C62 back on top of it)—see below.

I think I probably need to do some actual measurements to see which of these options—if either—is possible.

[Mr. Wizard]

Just a quick update on this project.

I have been searching for a suitable replacement for the ONSEMI MC78LC25NTRG voltage regulator in my original design. Aside from Rochester, which is no good for hobbyists, it has only had availability from Digi-Key and in small quantities (with a total stock now of just 89 units), and nowhere else.

Of course I could just build this for myself, using one of those 89, but I'd prefer to release an open-source design that would be easy for other people to buy parts for.

Does anyone have any suggestions? The absolute requirements for a suitable voltage regulator are:

• Input voltage from between 2.6 V and 7.64 V
• Output voltage of between 2.3 V and 5.5 V

Realistically, since I don't want the complexity of a buck/boost converter, this likely means an output voltage of 2.3 V to 2.5 V from a 2.6 V input, and capable of handling an input up to 7.64 V.

The MC78LC25 2.5 V regulator has two great specs:

• I_Q < 3.6 μA
• V_DO < 60 mV

So it would have output 2.5 V ± 75 mV and drawn a negligible amount of current when the multimeter was in auto-power-off mode.

I've not been able to find anything commonly available that meets or exceeds this. Does anyone have any suggestions?

Thanks.

[kripton2035]

found these on octopart : analog devices parts ADP3335
but the dropout voltage is higher at 200mV

https://octopart.com/search?category_id=4320&maxinputvoltage=%287.7__%29&maxoutputvoltage=%282.6__%29&quiescentcurrent=%28__4µA%29&mininputvoltage=%28__2.6%29&minoutputvoltage=%28__2.3%29&dropoutvoltage=0.2

[Mr. Wizard]

found these on octopart : analog devices parts ADP3335
but the dropout voltage is higher at 200mV

Thanks. It looks like the 2.5 V version almost works, but perhaps not quite, in that we'd need 2.7 V of input voltage there and we may only have 2.6 V--that is assuming four partially-discharged NiMH cells at 1.1 V each, minus the 1.8V analog ground.

Requiring 1.125 V from each NiMH cell instead may be acceptable; I don't know enough about NiMH voltages over time to know if this would be a problem. It'd probably be nice to have looser tolerances if possible.

One other potential issue with that one is the package; looks like the 2.5 V version may only be widely available in a leadless package, which may be difficult to solder by hand.

Edit: It looks like the dropout voltage decreases with current; at 0.1 mA it is only 40 mV. I don't know how much current passes through that, but I'm guessing not much; it is definitely under 10 mA since that's the limit of the precision regulator that this feeds, so the dropout voltage would be under 110 mV.

[wizard69]

As long as this is an in production unit I really don't see the stock of 89 being a problem.  DigiKey will simply order more when the low limit is meant.   Beyond all of that how many do you really think will be installing this mod?   You could order 89 boards and have them in stock for some time if you went that way.    If you don't build the boards yourself even fewer will adopt the circuit.

To put it another way I wouldn't sweat it.   

[Mr. Wizard]

That does make sense. I do wish it was available from another vendor, such as Mouser or Newark or Arrow, but perhaps I’ll just go with that one and not worry about it.

[BrianG61UK]

Let me try that again without the stupid error:
A seller has popped up on Ebay selling MC78LC25NTRG
https://www.ebay.co.uk/itm/332286608596
I can't vouch for whether they are real though.

[Mr. Wizard]

I’ve been on a four-month walkabout (OK, actually, I’ve been swamped with “real” projects) but now will have a lot of free time going forward so am picking this up again. I think I’ll go with the OnSemi MC78LC25NTRG after all. I need to take some precise measurements of the board on the multimeter and then start designing. I am thinking that trying to get it onto a flatbed scanner might be the most accurate way to take precise measurements, but I’m not sure if that will be possible.

I don’t suppose there already exists a file of the 1.21GW PCB layout, somewhere?

[Mr. Wizard]

I'm trying to accurately measure the positions of things on this PCB. Does anyone have any suggestions on how to do it?

My plan was to do it from photographs since the PCB is so small.

I've taken about 80 high-resolution photos of that area of the PCB with a USB microscope. I figured I could then just stitch those together, use one known measurement as a reference, and derive the rest.

I am trying to stitch the photos together, but can't find software capable of doing it. I've tried Photoshop's "Align Layers" feature as well as a standalone open-source program called "Hugin" that merges photos into a panorama, with no luck. Even just trying one row of images at a time, rather than all of them, I get weirdness as shown below.

Every source image overlaps the previous by 50% so I'm not sure why I'm having such troubles. Photoshop is much worse. I thought this would have been the easy part... They aren't all perfectly aligned, so need to be rotated a bit, and doing it by hand is really hard.

If anyone has any suggestions, let me know. I'm going to investigate ImageJ next, which is for microscopy.

[Mr. Wizard]

I am trying to stitch the photos together, but can't find software capable of doing it.

I found a discontinued program called Image Composite Editor from Microsoft that did it perfectly in a few seconds. I had to download it from the Wayback Machine since Microsoft no longer offers the download on their site.

Here's the result, as a low-quality JPEG to fit under the 4MB size limit here. Next step will be to measure things!

[Mr. Wizard]

Here I’ve labeled the relevant signals, in red. I think the best place for a mod board is likely as shown with the blue outline, connecting to the pads circled in yellow. That prevents the need for any thin fingers on the mod board. (Remember, this whole thing is incredibly small!). The resistor marked with an X needs to be removed.

If anyone has any suggestions on a better design for position and connections of the mod board, let me know. Otherwise, I’ll measure the exact positions of the relevant pads and mod board footprint tomorrow and start work on the PCB design.

VB+ = Battery voltage
AGND = Analog Ground
Vin = Voltage input of original regulator (the modboard’s output)

[Mr. Wizard]

OK. I've carefully measured the locations of the relevant pads and of the overall area (attached below), using the width of the existing voltage regulator as measured with a Mitutoyo digital caliper as a reference. I decided that two digits of precision is enough.

This will now allow me to now design a PCB with castellations that match up with each of the relevant pads while staying inside the overall footprint of the unused area there.

[EEVblog]

I don’t suppose there already exists a file of the 1.21GW PCB layout, somewhere?

Sorry, no, there isn't.

[Mr. Wizard]

OK. I've designed a PCB. The PCB and schematic are below.

Recall that the goal is to take the voltage between AGND and VB+ and change it to 2.5V, to feed the existing regulator.

I included two capacitors; one 0402 and one 1206. Probably only the 0402 is necessary, with a 0.1 uF cap, but since there was enough space I added a second optional one.

I also included two 0-ohm SMD resistors. These could just be solder bridges. The idea is that this way the PCB can be easily disabled through those rather than by removing the entire thing. This is similar to R12 and R94 on the 121GW itself.

It's a little bit sloppy so I'll likely refine things a bit more before sending it off to OSH Park for a prototype, but if anyone sees any potential issues or has any feedback, please let me know.

[oliv3r]

I don’t suppose there already exists a file of the 1.21GW PCB layout, somewhere?

Sorry, no, there isn't.

That's so sad but maybe we can ask some of the guys that have blown up/ruined their multi-meters to sacrifice it for science
Like @vyruz for example here https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg3575120/#msg3575120 Descoldering all parts and scanning that PCB would get us a very nice starting point. If it's a multi-layer PCB; some sandpaper and more scanning, et-voila.

I'd be more then happy to do the work, if i'd receive a broken one :p (not gonna sacrifice my kickstarter one :p)

Sadly though it seems that vyruz is a 4 post 'read-only' user ... but I think there's more people with broken meters in that post. Maybe even Dave has a few busted ones he wouldn't mind sending out for science

[oliv3r]

OK. I've designed a PCB. The PCB and schematic are below.

Recall that the goal is to take the voltage between AGND and VB+ and change it to 2.5V, to feed the existing regulator.

I included two capacitors; one 0402 and one 1206. Probably only the 0402 is necessary, with a 0.1 uF cap, but since there was enough space I added a second optional one.

I also included two 0-ohm SMD resistors. These could just be solder bridges. The idea is that this way the PCB can be easily disabled through those rather than by removing the entire thing. This is similar to R12 and R94 on the 121GW itself.

It's a little bit sloppy so I'll likely refine things a bit more before sending it off to OSH Park for a prototype, but if anyone sees any potential issues or has any feedback, please let me know.

You sure you want to go the 0402 route? I have trouble doing 0603's already And I think you do have plenty of room to go up a couple of sizes for easier manual soldering ... the agnd plane can be moved a bit and made smaller; the silk screen is probably not needed to be exactly there, so I think you'd have plenty of space for something bigger ...; remember, designing stuff allows for super zoomed in stuff :p

I am curious in your mod and also probably really want this

BTW, what PCB thickness are you looking at and how many layers? I would guess 'the thinnest possible' and only top-layer, right?

Would be nice to see this in a next revision (any thoughts there dave?), dealing with lithium batteries is a good idea and I tend to have them here as well; in my case though; i use them everywhere when they are not good enough for my digital camera anymore so they tend to be slightly depleted already anyway. I wonder how long a full set of lithium batteries would last ... 1000 hours with backlight and bluetooth on?

[BrianG61UK]

Any news of progress?

[TheBay]

Would it be worth considering adding a castellation on the left hand of the mod PCB so it can be secured to one of the via's (depending on what that via is as you wouldn't want to introduce noise or capacitive coupling to the circuit) on the main PCB giving it a bit more mechanical strength?