HOW TO ADD IN A 10A HRC FUSE INTO A POC DMM

[splitzzzzz]

Hi all,

Newbie here with a recent interest in electronics, knowledge level very low!

I have a Micron (Altronics) Q1130A DMM which I bought before I knew better.

On watching some EEVBlog videos I decided to take it apart to see how it was fused. To my horror it wasn't fused at all on the 10A and had a crappy M205 glass fuse on the 200mAh (which I have now replaced with the equivalent in a Siba HRC Ceramic fuse).

I now have 2 better DMMs so as a learning project I'd like to add in a 10A HRC fuse. The PCB has a footprint for one with the outer holes 20mm apart. Looks maybe like designed for AG3 but possibly M205.
I'm confident I can solder a surface mount fuse holder but I'm unsure about how to inroduce it to the circuit? I mean the 10A circuit works so it must be closed so if I add in a 10A fuse I'm unsure it will function as intended? I assume I need to open the 10A circuit and introduce the fuse? The footprint has 4 mount through holes plus two SMD spots. I have continuity tested and they are all closed to each other?

As I said it's a POC DMM so I don't mind if I brick it in the process but obviously safety is a big issue so if it's too risky and difficult I'll give it a miss!

A side issue is the front cover says fused 200mA max and the fuse was F200mA however the PCB says 0.5A/250V. Can I put a 0.5A HRC fuse in there instead?

I will try add some photos but this is my 1st post here so unsure how?

Cheers + thanks in advance.

EDIT: BTW the closeup showing the 10A Fuse footprint shows what appears to be side cutter marks in the 10A shunt! Unsure if this was dodgy, intended or a faux pas?

[ebclr]

Buy this probe https://www.ebay.com/itm/Metrix-Electronics-fused-probe-MX67-meter-Fluke-meters/371784563947?epid=0&hash=item5690163ceb:g:9AsAAOSwgmJXzBzS

and maintain your meter as is

[splitzzzzz]

ebclr
Thanks I was aware of those and as I said have two better DMMs, just looking at this as more of a project
Cheers

[Brumby]

Firstly, there is the question as to whether this is actually worth doing.  Changing the fuse arrangement to HRC is just one part of the safety equation.  The rest of the circuitry and layout is part of the deal as well.

Having said that .... if you really want to try...


The 10A circuit actually does look like it is fused .... by the thin PCB track joining the two square pads in the middle of the 20mm section you have marked.

The two holes each end would be for fuse holder clips like these:


You could solder these in, break the "PCB fuse" and put in your M205 HRC fuse in the clips.  The clips would need to be pretty decent if you would be putting some heavy current through.

A side issue is the front cover says fused 200mA max and the fuse was F200mA however the PCB says 0.5A/250V. Can I put a 0.5A HRC fuse in there instead?

I would check the fuse that's already there and get the HRC version.

EDIT: BTW the closeup showing the 10A Fuse footprint shows what appears to be side cutter marks in the 10A shunt! Unsure if this was dodgy, intended or a faux pas?

Quite possibly that might be a "trimming" action to get the shunt to within the accuracy necessary.

[Nusa]

Re the 200mA fuse:
a) If you change the value, then the exterior labeling will be incorrect. As it's molded in, it'll be a real pain to change.

b) There's probably an actual reason they chose 200mA instead of 500mA. It's not like the higher value glass fuse costs more.

[Paul Moir]

To add a possible example to Nusa's b) point, likely this meter overranges at 200mA but the current shunt (possibly with additional protection) can handle 500mA.  The overrange makes sense since this is a 2000 count meter.

[IanMacdonald]

A British 13A plug top fuse may fit. These are only rated for 250v but it would be a big safety improvement over no fuse.

[Fungus]

so if I add in a 10A fuse I'm unsure it will function as intended?

Even if you put in a $100 fuse you still haven't fixed any problems to do with gaps, track clearances, MOVs, PTCs, basic circuit design (does all the current go through the selector switch?), etc.

Best advice: Don't bother. Accept that it will never be "safe" and use it on low energy stuff only. Glass fuses are fine for this.

You want a "safe" meter? Buy something that's certified. Safe meters start at around $42, delivered, (Fluke 101) so price isn't really an excuse

[splitzzzzz]

Brumby
Thanks for the info. Apologies I haven't worked out this forum's quoting yet so won't until I do!
Yeah I assumed that the rest of the circuitry is already capable of carrying 10A and the intruduction of an HRC fuse wouldn't change that?
With the two square plates in between the 4 PCB mounted holes, I assume by breaking the circuit there you mean, desoldering using wick?
As per my post, I have already replaced the original POC fuse with the equivalent in a Siba HRC F200mA/250V. Mind you I cracked one open and found it wasn't sand filled as I thought HRC fuses are supposed to be.....essentially just a ceramic bodied fuse so how the HRC and 1500A breaking capacity is there I don't know?

[splitzzzzz]

Re the 200mA fuse:
a) If you change the value, then the exterior labeling will be incorrect. As it's molded in, it'll be a real pain to change.

b) There's probably an actual reason they chose 200mA instead of 500mA. It's not like the higher value glass fuse costs more.

Thanks Nusa
(a) The case labelling isn't an issue as I'd never pass on to some-one who wasn't aware. (b) Yeah just struck me as strange, I assume the same PCB is in other DMMs and specced at 500mA and this is down specced in other components!
Cheers

[splitzzzzz]

To add a possible example to Nusa's b) point, likely this meter overranges at 200mA but the current shunt (possibly with additional protection) can handle 500mA.  The overrange makes sense since this is a 2000 count meter.

Thanks Paul, yep makes sense, will leave it at 200mA

[splitzzzzz]

A British 13A plug top fuse may fit. These are only rated for 250v but it would be a big safety improvement over no fuse.

Unsure about them, they seem to just be a fast/medium 13A Ceramnic HRC fuse. I assumed going 10A would be advisable? Cheers

[splitzzzzz]

Yes Fungus the PCB is sadly lacking in input protection.

Yeah as I said, looking at this as more of a project.

I already have two better (ie still in the cheap range but fused satisfactorily) DMMs.

[Fungus]

Yes Fungus the PCB is sadly lacking in input protection.

Yeah as I said, looking at this as more of a project.

If you're aware that this is just soldering practice (and nothing more) then fair enough.

As per my post, I have already replaced the original POC fuse with the equivalent in a Siba HRC F200mA/250V. Mind you I cracked one open and found it wasn't sand filled as I thought HRC fuses are supposed to be.....

It depends on their rating. Sand isn't really needed until there's no circuit breakers between you and the local substation.

I already have two better (ie still in the cheap range but fused satisfactorily) DMMs.

The fuse is only one small part of the equation. eg. The fuses probably aren't even part of the circuit when you're not measuring amps.

[Brumby]

Yeah I assumed that the rest of the circuitry is already capable of carrying 10A and the intruduction of an HRC fuse wouldn't change that?

No it wouldn't - but that is not the point!  When everything is functioning normally there is little to worry about.  The problem is when something goes wrong - and how wrong things go. A mild overload from a lab power supply will simply melt a fuse - and any fuse will do the same job.  However a high energy source such as mains can deliver substantially more destructive power - and that destruction potentially extends to every corner of your meter not just the fuse!!!  Tracks and components can fail, explode or even vapourise.  Arcs can jump around inside unless special care is taken to minimise these risks.

The point is - safety is MUCH MORE than just the fuse!

With the two square plates in between the 4 PCB mounted holes, I assume by breaking the circuit there you mean, desoldering using wick?

The "fuse" is a short section of copper, etched as part of the PCB as I have highlighted here:

It's just a piece of tinned PCB - with a little bit of solder mask running across it.  There is nothing to "desolder" - you would have to cut or grind through the track.

[macboy]

I wouldn't bother with HRC fuse unless you plan to use this on CATIII or higher circuits. You don't intend to do that do you? (please don't). Low voltage, low energy circuits like those commonly found on the electronics bench can be opened with an ordinary glass fuse. Get a cheap ceramic one if it makes you feel better, it will add some protection from explosion. No need to spend $20 or so per HRC fuse when the circuit you are breaking doesn't have the energy to sustain an arc anyway. I have several bench multimeters that originally cost >$5000. Guess what kind of fuse they use: glass or HRC?

Go ahead and add a fuse holder, or even PCB mounted fuse . I'd remove not only the small fuse track pointed out by Brumby, but also those SMD fuse pads. Hot soldering iron + scraper or knife will make short work of that.

The pinch mark on the current shunt is the "calibration" done at the factory. It was reading a little low on the 10A range, so they pinched the shunt with a side cutter just a little increase its resistance.

[splitzzzzz]

Thanks Fungus

[splitzzzzz]

Thanks Brumby. Yep best to drill/cut away...cheers

[splitzzzzz]

I wouldn't bother with HRC fuse unless you plan to use this on CATIII or higher circuits. You don't intend to do that do you? (please don't)......................<snip>............... I have several bench multimeters that originally cost >$5000. Guess what kind of fuse they use: glass or HRC?

Go ahead and add a fuse holder, or even PCB mounted fuse . I'd remove not only the small fuse track pointed out by Brumby, but also those SMD fuse pads. Hot soldering iron + scraper or knife will make short work of that.

The pinch mark on the current shunt is the "calibration" done at the factory. It was reading a little low on the 10A range, so they pinched the shunt with a side cutter just a little increase its resistance.

Thanks macboy. Nup won't be using this for anything but DC voltage now I've seen inside it! I'm guessing glass for the $5K bench DMM? Yeah the shunt calibration looks dodgy...cheers