So... I recently got a clamp meter and found it odd that it did not have a user-replaceable cartridge fuse... turns out it uses two Polyfuses instead. That's the first DMM I've had that uses PPTCs rather than fuses for input protection.

(alongside a couple of MOVs)

Mind you, this is not a no-name product. It's a Klein Tools CL390, and it has an Intertek ETL Listed mark (which I've confirmed to be valid).

The meter is rated at CAT III 600V.

So why the Polyfuses?

The green round parts near the input terminals are the Polyfuses.

And from looking at the PCB traces... they're in series. I don't get why one would put two Polyfuses in series.

@bwDraco can you post a part number for said Polyfuses?

on a different note, should I expect AVRs to be around for the long haul? (i.e. the next few decades vs. going away with a big "poof" sound in the near future)

got a concept for a microcontroller-involving project, and the lead candidates for the uC are an AVR part and a LPC800 Cortex-M0+

@Shalvenay The markings are really hard to read. Give me a few moments...

...okay, it's WMZ12A-75HV.

That is a PTC thermistor, not a Polyfuse.

🤔

@bwDraco well, Polyfuses are a kind of PTC thermistor :P

Yeah. But I still find the choice of part odd.

@bwDraco that said, I figured out why there's no fuse. it's because it's a clampmeter and thus doesn't have a current shunt for a fuse to protect to begin with

(i.e. there's no current input jack feeding a shunt, all the current ranges work off the clamp save for perhaps a uA/low mA range)\

It does has a microamp range.

But even then, I'm used to seeing a small low-current cartridge fuse rather than a PTC as input protection on a DMM.

(and possibly a separate high-current fuse if there's a high-amperage range)

Also, why two PTCs in series?

@bwDraco voltage rating maybe? that's all I can think of

@Shalvenay AVR is already very old and outdated, from the 1990s. It's already been around for those decades and is long overdue for it's "poof". There's not really a single argument why you would ever use it professionally in 2025. There was no reasons to ever use it back in 2015 either. In 2005 it would have been considered.

I wouldn't automatically dismiss AVR as obsolete. 8-bit MCUs tend to be more tolerant of extreme temperatures and can operate at wider voltage ranges.

One way fuses have no advantage over polyfuses with similar voltage rating imo

(hot take possibly ;))

@tobalt One-way fuses can open faster than polyfuses. There are also different flavors of one-way fuses: fast- and slow-blow.

One way fuses [particularly in DMMs] have the secondary function as "You screwed up!" indicators.

@Shalvenay I wrote this blurb about MCU family selection. That was 12 years ago, and things have changed (mainly for the better). I still think that a lot of that holds.

I can ask you the proverbial "20 questions" about your project, which might help decide between AVR and LPC.

Are you already familiar with the firmware development environment for AVR or for LPC? Or are you equally new to both?

Is your project going to become a commercial product? Or a serious one-off instrument (for 3rd party use, not just for your own use)? Or is it a hobby project?

How many people will be on the project? Are you the one who's going to maintain it long term [if there's long term]?