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Calibrating vintage multimeter
(derp.foo)
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this post was submitted on 29 Sep 2023
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Their pricing for calibrating a device starts at around €400, which is rather more than I paid for this thing and way more expensive than building my own calibrator. So I guess I'll have to do that...
Thank you! Not having to invent everything from scratch is going to make this a lot easier.
400? I guess that's in the ballpark I expected it to be.
I haven't studied the BOM for the DIY solution, but something tells me that you'll only be able to keep the cost lower than 400, if you value your time at close to zero.
With all that negativity out of the way, I'd definitely want to build it myself too. Although my anxiety level is exponentially correlated to the working DC voltage and at 300VDC I'm definitely well in the thick rubbergloves territory. Be careful with the build!
Having been walloped by 230VAC, which is far more dangerous, I’m not too worried.
Looking at the schematic you linked above, the amperage is going to be low, which is reassuring. As a safety measure, I’m looking into running the thing off of a battery, so that if the worst comes to pass it simply won’t have the power to be dangerous (1A at 12V translates to a mere 40mA at 300V).
I’d be far more worried about using lead solder TBH (love my Sn100Ni+).
Well hello there Mr Edison, I didn't realize the calendar read the 1880s again, please refrain from elephant ownership 😀 First off if you've got 300V I don't care what form it's in, I ain't sticking my fingers anywhere near it! Secondly at least AC alternates, giving your muscles a break and possibly a chance to let go of the wire, DC isn't that forgiving. The reason 230V AC is so dangerous is because it's usually referenced to ground, meaning that if you touch a single wire, you'll be drawing a current from that point and to your feet.
But that is all theoretical, because while 300V is stored, and the current may become quite high, it's only backed by 100nF. Still, I'd want it discharged before poking about though.
The battery is a good precaution. Some of my co-students in university made an EKG apparatus. Our lecturer demanded that anything connected to the electrodes was to be powered by a single battery.
I may be a boomer in this regard, and I'm sorry for the rant I'm about to go on (that last part of the sentence was tagged on after writing the following). I prefer my Sn60Pb40 of which I got several kg from when it was outlawed, maybe throw in a little Ag in there for the 0.3mm SMD solder. TBH my love of lead solder may stem from the fact that I have more, in various diameters, than I'll ever use.
The first couple of lead free solder brands I got just didn't flow right. It didn't help that the solder stations it was to be used with at work, by students, was old school Weller stations that just delivers a constant 24V AC to the iron, and the tips set the temp. All the tips we had were at a temp comfortable for lead solder.
But I digress... if you just wash your hands after soldering (and maybe use a bit of ventilation) then your absorption of lead is harmless https://www.ehs.harvard.edu/sites/default/files/soldering_safety_guidelines.pdf
Interesting. Your comment made me read up on all of this. Note that, since V~rms~ = 1 ÷ √2 × V~peak~, 230VAC has a V~peak~ of 325V, so in that respect, it should be pretty much equivalent to 300VDC. I figured that the ability for AC to induce heart fibrillations was the most dangerous factor in all of this, but I hadn't figured in that DC induces tetanus and can also temporarily stop the heart.
It's not the volts that kill you though, it's the amps (the volts just make it easier). I found a table listing the effects of various amperages. It does present DC as generally more safe when compared against 60Hz AC, but I'm not sure how that generalises to the 50Hz AC we have here. I do conclude however that I should be limiting the output current to something something generally safe, like 20mA (which should be fine for a voltage reference). I'm thinking a PTC at the input and being conservative wrt capacitor sizing should do it.
TBH, when it comes to an EKG apparatus I'd also be worried about common mode across the chest, or the power supply having a disastrous failure mode.
Yeah, those were horrid. Even the supposedly excellent SAC305 gave me dull joints (tough it flowed adequately). Still, I can only recommend Sn100Ni+ (supposedly closely related to SN100C): flows well and gives me the shiny joints I crave. Having a good soldering iron (I'm using a Pinecil) helps with solderability.
I do however still have some rosin core Sn60Pb40 for reworking vintage electronics. And I do agree that it's just better when it comes to wetting ability. The peace of mind when working with lead-free solder really is worth it though, especially with pets or small children.