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Old 12th Aug 2018, 4:16 pm   #1
G6Tanuki
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Default Quick-and-dirty testgear: mains voltage monitor.

A few decades back I built a 3-digit mains-voltage monitor; it used a "General Instrument Microelectronics" chip [the part-number started AY but I forget the rest of the number] and three of the rather-nice Monsanto MAN-series LED displays, the type that rather than being '7-segment' had lots of smaller LEDs so digits-with-curves could be rendered properly.

It cost around 100 in 1970s-money, and though the 3 digits were only about 1/3 inch high the electronics-box was about the size of a brick.

Today, LED mains-voltage monitors cost 2.50 shipped from China. Red, Green and Blue versions! These are also available at radio-rallies. I've recently acquired three, they'll be great as quick-and-dirty voltage monitors, avoiding tying-up a DVM unnecessarily. OK, they are only allegedly 1% accuracy - indeed the least-significant-digit counts up from zero in even-numbers only - but that still gives 1% accuracy at 250V and the ones I've got are close-enough to my DVM to be believable, whether running on the output of a 230-115V isolating transformer or connected direct to the mains.

No noticeable RFI from it between 1.6 and 30MHz, either!

The question comes - how to house one of these little displays in a 'case' that doesn't end up costing rather more than the display itself? I'm pondering a way to repurpose an offcut length of square black rainwater-downpipe and some old PCB material....
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Old 12th Aug 2018, 4:30 pm   #2
barrymagrec
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Would it fit in an old 35mm film cannister?

Or just a piece of adhesive heatshrink?
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Old 12th Aug 2018, 5:06 pm   #3
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Alas since giving-up on chemical photography nearly two decades back I don't have any 35mm canisters to hand.

I'd prefer a case-with-sides if only so I can sit it on a shelf and not have the stress of the mains-lead cause the thing to take-up an aesthetically-unpleasing angle. Hence my thoughts about using up some of the offcuts of square drainpipe I have lying about.
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Old 12th Aug 2018, 5:47 pm   #4
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Could you modify the top cover of a 13 amp plug?
You can get pill bottles the same size as film cans but not in metal.
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Old 12th Aug 2018, 6:11 pm   #5
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

I really want something to sit on a shelf. And not rotate.

Building it into a 13A plug would mean it stuck out a good couple of inches, and in my 'office' (where I intend it to live) means it would be concealed under dark recesses of the bench where the power-outlets live.

If I wanted a circular encasement I'd use a section of round plastic drainpipe: at least I could then use a couple of easily available and same-coloured wall-mounting brackets to keep it stable.

Another possibility: an offcut of steel scaffold-pole. Heavy, so better able to resist the weight of the cable. But it would still require engineering something to stop it rotating on its long-axis.
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Old 12th Aug 2018, 7:13 pm   #6
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

A small plastic "sandwich box" container? Might even be translucent enough to see the display through.
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Old 12th Aug 2018, 7:34 pm   #7
Mr Moose
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Hello,
You could use a chunky piece of wood. Drill a large hole at one end for the display and a small one at the other for the cable. A piece of say 3"x2" about 4"-6" long should have enough weight to hold it in place. Chamfer or round off the edges and paint or polish to your taste.
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Old 12th Aug 2018, 7:41 pm   #8
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Interesting suggestions... I still think a square drainpipe is the best option. I don't really do wood - if I made a housing out of wood I'd spray it either WWII-style black-crackle, piano-gloss black or shiny-aluminium. To me, wood just doesn't say 'precision instrument'.

OTOH I know someone who has a mill and could machine me an 80x80x80mm cube from 4130 steel. But it'd still cost me significantly more than the display it housed...
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Old 12th Aug 2018, 9:59 pm   #9
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

How about using a standard domestic surface box and blanking plate?
Even better would be the similar ABS boxes intended for network sockets etc.
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Old 13th Aug 2018, 11:24 am   #10
OscarFoxtrot
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Yu don't give size, but would it fit inside a Kee Klamp bracket - no. 63 or 64 might make a desk stand for it. (There is also plastic plug no. 77)
https://simplifiedbuilding.co.uk/tub...ings/kee-klamp
On second thoughts, rather more expensive than 2.50! Unless you can find an equivalent product also shipped from China!
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Old 13th Aug 2018, 3:46 pm   #11
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

A dead PP9 can be gutted, leaving behind a black plastic box that can be used for all sorts of things. Very handy, and effectively FOC to those of us who run our old radios with them

I've been tempted to pick up a couple of these myself, if only out of curiosity. The first port of call would be to check the safety, based on previous experience of that sort of stuff. The capacitor used to drop the mains often isn't the correct class X2 type, for example.
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Old 13th Aug 2018, 6:06 pm   #12
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Voltage dropping capacitors do not need to be Class X rated, since there is a series resistance. In fact, the self-healing property required for Class X is undesirable in this application.
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Old 13th Aug 2018, 7:02 pm   #13
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

I bought one identical to this at the Kempton radio rally earlier this year, the intention is to fit it into a standard blank single plate on the wall in my workshop.

A few years ago I was working on a DJ console which suddenly devoloped a nasty hum after repairing it. I was looking everywhere for a fault, couldn't find anything obvious, saw what I thought was a dry joint and fired up the trusty soldering iron, couldn't melt the solder on the tip, iron must be faulty I thought. Off to the kitchen to make a cup of tea as it seemed to be one of those days. 10 minutes later the kettle still hadn't boiled, suddenly it dawned on me, got the DMM out and the mains was down to 160V! Must have been a local substation fault, a device like this would have saved a lot of time mounted above the bench!
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Old 13th Aug 2018, 7:36 pm   #14
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Back in the old days, a voltage reduction to 160 volts would have been obvious due to the much reduced brilliance of incandescent lamps, and fluorescents would have been either very dim, or failed to light at all.

These days however, many LED lamps are multi voltage and give full light output down to 90 volts or so. As do a minority of fluorescents.
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Old 13th Aug 2018, 9:09 pm   #15
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by julie_m View Post
Voltage dropping capacitors do not need to be Class X rated, since there is a series resistance. In fact, the self-healing property required for Class X is undesirable in this application.
The jury is still out on this one. It's quite complicated when you start peeling back the layers...

Yes, some types of X2 caps can slowly lose capacitance with time - that issues crops up on the forum from time to time. But not all do. Good quality plastic film types that are properly encapsulated have been 100% reliable in my experience. But of course, those aren't what are used at this end of the market.

So if you decide to not use a self-healing X2 type, you've got quite a tricky design problem on your hands. Surges are inevitable, and there are 2 places where these end up.

The series resistance (low 100s of ohms) is the obvious one. But if you use a film resistor here - like these cheap products always do - it's guaranteed to fail in normal use (after only a year or two IME). So you need to use a carbon-comp or wire-wound, and a fuse (which should be there anyway, but often isn't).

The other recipient of the surge current is the circuit itself. If it's a good quality type, the electrolytic smoothing capacitor should cope well enough (when new), but it's worth making the zener (or whatever shunt device you're using) a relatively chunky sort. Again, not commonly seen on this sort of cheap tat.

In short, cheap capacitive droppers are guaranteed to fail, and not necessarily safely - and the circuitry being powered is likely to become "collateral damage" when this happens. At the very least a fuse should be added so that you're not relying on a film resistor to interrupt what could be a serious amount of energy.

Personally, with these sorts of cheap and cheerful Chinese things I like to - at the very least - replace the cheap dropper cap with a decent X2 type and replace the series resistor with a carbon-comp type. Did I also mention adding a fuse

I've got projects here with capacitive droppers that have been working for 20+ years continuously - for those, I used BC (now Vishay) MKP338-4 types, and would happily carry on using up my stocks for my own use. Today, for a new design I'd use Vishay F1772 types - these are still X2 types, but are sold as "high stability" with series connections in mind - so they are, by design, immune to the falling-capacitance problem (although the MKP338-4 seems to be fine in my experience), and it makes the issue of dealing with surges much easier.
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Old 14th Aug 2018, 9:04 am   #16
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by G6Tanuki View Post
I really want something to sit on a shelf. And not rotate.
A Colmans Mustard Tin, painted with hamerite would do the job.

John
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Old 14th Aug 2018, 8:17 pm   #17
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

But that would mean I had to spend money actually *buying* mustard, a substance I find utterly without merit and entirely representing the terrestrial manifestation of Satan's evil bodily-secretions!

I'm going with the offcut of square drainpipe, along with a couple of wall-mounting brackets to provide a nice 'stand' for it.

As to whatever capacitors [class-X or otherwise] are present inside the module, TBH I just couldn't care: whatever they are, they seem to work - and if the module fails after a year I'll happily shrug, hurl it into the skip and buy another one. That's for me the 'path of least-astonishment' which I've spent decades following in business.

"Don't sweat the petty things" is one of my innate principles [equally, don't pet the sweaty things].
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Old 14th Aug 2018, 9:23 pm   #18
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

bigclive did a feature video on these 3 digit mains voltmeters a while back.

If I was to use one I would either incorporate it into the incoming fuseboard/consumer unit board, or alternatively reverse engineer an LED GLS size lamp (assuming I could still find one with a clear envelope)and fit it in there. At least then it would be portable.
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Old 14th Aug 2018, 9:23 pm   #19
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by G6Tanuki View Post
As to whatever capacitors [class-X or otherwise] are present inside the module, TBH I just couldn't care: whatever they are, they seem to work - and if the module fails after a year I'll happily shrug, hurl it into the skip and buy another one. That's for me the 'path of least-astonishment' which I've spent decades following in business.
You might not care about the long-term reliability of these modules - and at these prices, I completely get that - but to shrug off the potential safety issues is another matter. These sort so of cheap things can and do fail in fairly exciting ways - I've seen it first-hand. Your call, obviously.
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Old 18th Aug 2018, 10:48 am   #20
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

If connecting one of these cheap volt meters to the mains, I would suggest protecting it with a small mains rated fuse.
This fuse will only blow in the event of a failure that requires replacement of the meter module, there is therefore no need for a fuse holder.

I would solder the fuse in place in series with the module and cover with two layers of heat shrink. Treat the whole assembly of module, fuse, and supply flex as disposable.

A 1 amp fuse will make any failure much less dramatic.
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