UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 18th Aug 2018, 11:07 am   #21
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,403
Default Re: Quick-and-dirty testgear: mains voltage monitor.

That'll be easy to do - following a job recently[1] I've got a batch of the small wire-ended fuses that are usually concelaed within the windings of 'wall-wart' transformers. Not sure what their rating is but it can't be more than an amp.

[1] Once postage was included it was not-really-that-much-more-expensive to buy 100 of the things than ten of them...
G6Tanuki is offline   Reply With Quote
Old 18th Aug 2018, 3:13 pm   #22
broadgage
Octode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 1,128
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Before re-use of the fuses that you have, do confirm that they are in fact fuses that protect against overcurrent.
If intended to be concealed within the windings of a small transformer they may well be thermal fuses that open at a certain temperature.

Such thermal fuses WONT open on overcurrent unless so gross that the plug fuse or circuit MCB has probably operated first.
broadgage is offline   Reply With Quote
Old 18th Aug 2018, 4:41 pm   #23
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,403
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Checking the spec for the things I bought, they're actually "polyswitches" so they deal with both excess current *and* excess-temperature events in one convenient package.
G6Tanuki is offline   Reply With Quote
Old 20th Aug 2018, 7:07 am   #24
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 624
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Itís quite possible that your Polyswitches will be unsuitable for 230 Volts AC.


Most of these have maximum voltage ratings less than 100 Volts. I would not use these until I could confirm their spec.

SimonT.
__________________
The honesty of imperfection..........
thermionic is offline   Reply With Quote
Old 20th Aug 2018, 2:53 pm   #25
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,334
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I must admit that the notion of burying a Polyswitch in the windings of a transformer is a new one on me. Is this actually done?

The only thermal fuses that I've seen embedded in transformer windings are the non-resettable types. On the face of it they seem like a bad idea, but every time I've met one that's gone open-circuit, it did so for good reason.

Littlefuse give data about derating for ambient temperature, but as their fundamental MO is self-heating, they don't appear to give any data about thermal resistance, which you'd need to know if you're hoping to conduct heat into them.

Ultimately, I can't work out if this is an incredibly smart bit of lateral thinking, or an awful bodge that is likely not supported by whoever makes the Polyswitch. Part of me wants to like it, but I feel that I'd need more information before I can make my mind up.
mhennessy is offline   Reply With Quote
Old 20th Aug 2018, 4:53 pm   #26
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 3,514
Default Re: Quick-and-dirty testgear: mains voltage monitor.

If you place one against the core of a transformer it will go high at a lower current.
If it were in the windings it would to some extent monitor the temperature of them.
Refugee is online now   Reply With Quote
Old 24th Aug 2018, 9:56 am   #27
TrevorG3VLF
Octode
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,095
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I had some of these voltmeters delivered yesterday. One seems to work as described in #1. It works from 30V up using my Variac.

I want to add one to my Variac output which gives sine wave but how would it react to a dimmer reduced supply? Would the funny waveform overdrive the works?
TrevorG3VLF is offline   Reply With Quote
Old 24th Aug 2018, 1:06 pm   #28
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,403
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I don't know! Never having opened one of the displays up, I don't know what technology it uses as a dropper to prvide power for the LED display.

I'm thinking though - if it uses a capacitor dropper (like a lot of LED bulbs) then it could have issues in the same way as a non-dimmable LED bulb does when fed from a dimmer.

Also, the waveform from a typical dimmer is non-sinusoidal which could again compromise the display's voltage-accuracy: remember that a typical thyristor-dimmer is essentially a power-control not a peak-voltage control like a Variac - the peak voltage across a dimmer-connected load is still the full supply-voltage.
G6Tanuki is offline   Reply With Quote
Old 24th Aug 2018, 3:17 pm   #29
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,334
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Yes it uses a capacitive dropper: https://www.youtube.com/watch?v=LmO1y1ISuak

Quite apart from the accuracy of the voltage readings, you also have to consider the design of the capacitive dropper.

The cap will have been chosen to pass the right current on the assumption of 50Hz mains. But at different frequencies, the capacitor will have a lower reactance, so will pass higher currents. Whatever shunt mechanism they are using will need to absorb this higher current without overheating or similarly misbehaving...

So when powered from a dimmer, which chops up the sine wave and produces lots of harmonics, you have to ask how much extra energy passed through the capacitor. It's an interesting experiment to try.

It's been a while since I experimented with dimmers and capacitor droppers, and IIRC my findings were that my capacitor dropper power supplies didn't draw enough current for the triac-based dimmer to work correctly, so I had to use an incandescent light bulb as a load for the dimmer before I could think about the power supply in isolation. But suffice to say, it's one of the reasons why I fit larger than required zeners to these sorts of power supplies.

I've had similar thoughts about these sorts of meters before. There's a variant that measures current in addition to voltage (http://www.markhennessy.co.uk/mdu/) that is a natural addition to a variac, for example, but for that, I'd separate out the PSU and the voltage sensing so the meter reads right down to zero volts. Very easy with that meter, and I'm sure if I reverse-engineered the meter we're talking about in this thread it would be similarly straightforward.
mhennessy is offline   Reply With Quote
Old 24th Aug 2018, 4:28 pm   #30
dseymo1
Dekatron
 
Join Date: Dec 2007
Location: Shropshire, UK.
Posts: 3,050
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Similar dual I & V meters are available with independent power supplies. I used a pair in my bench supply - the meters are powered by an additional winding on one of the main transformers.
dseymo1 is offline   Reply With Quote
Old 24th Aug 2018, 5:12 pm   #31
Biggles
Octode
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 1,994
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I just have an analogue panel meter permanently connected across the mains in the workshop, wall mounted and with an in line fuseholder and fuse at about 100mA. The meters are available for direct reading 0-300VAC and as long as you aren't wanting a reading to three decimal points they are fine. They would be ideal for your variac too. On my mains supply, the meter shifts about quite a bit, depending on the time of day. I got mine from ESR Components. I do use one of those little 12V digital modules for monitoring my 12V "ring main" in the workshop. I was initially a bit concerned about RFI from it, but there doesn't appear to be a problem.
Alan.
Biggles is offline   Reply With Quote
Old 26th Aug 2018, 3:46 am   #32
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 6,848
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I had a dimmer switch that was controlling a bedside lamp with a 40W tungsten filament lamp; but it blew up the triac when the light bulb failed. The replacement dimmer I bought doesn't like dimming a 40W lamp; it comes on suddenly at about 1/3 brightness and flickers alarmingly until turned up full.

The dead triac is a BT138, but the only triacs in my parts drawer were BT137s; which I believe have a less-sensitive gate and so probably would need a higher minimum load if they worked at all.

I think the point of these meters is a quick "sanity check", indicating whether or not it might be worth investigating a possible oddness on the mains supply with a better instrument.
__________________
Julie {formerly known AJS_Derby}
julie_m is offline   Reply With Quote
Old 26th Aug 2018, 4:00 pm   #33
TrevorG3VLF
Octode
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,095
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I made a power supply for my neighbour to heat a wire to cut expanded polysyrene for model aircraft wings.

I used a lamp dimmer feeding a 100W bulb in series with the transformer primary. The only indication of output level is the brightness of the bulb. I thought that one of these little giggers would help to set the output. If the spikey waveform would overdrive the works, then it could not be used.

P.S. The wings should be smaller than the door.
TrevorG3VLF is offline   Reply With Quote
Old 27th Aug 2018, 5:15 am   #34
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 6,848
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I don't think the dimmer switch would actually be letting any more energy through than if it were passing the full sine wave; the energy in the harmonics has to come from somewhere; whether it's from the "filtrate" that passed through the dimmer or the "precipitate" that would have been blocked.

Accuracy on a phase-chopped waveform would depend what detection method (peak, mean or RMS -- I know, but a gal's got to dream, right?) the meter module was using.

Still, these modules are cheap enough for it to be worth finding out the hard way .....
julie_m is offline   Reply With Quote
Old 27th Aug 2018, 1:08 pm   #35
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,334
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by julie_m View Post
I don't think the dimmer switch would actually be letting any more energy through than if it were passing the full sine wave; the energy in the harmonics has to come from somewhere; whether it's from the "filtrate" that passed through the dimmer or the "precipitate" that would have been blocked.
It's not about the energy that the dimmer passes through to the load; rather, it's about the energy that the dropper capacitor passes through to the meter circuitry.

It's a fascinating experiment to try - you just need a dimmer (plus suitable load to ensure the dimmer is behaving and producing realistic output waveforms), a suitable dropper capacitor, a bridge, a zener, and a couple of resistors. A smoothing cap across the zener is optional. The whole thing can be knocked up in 5 minutes... Obviously, as this is a public forum, I must point out the electrical safety hazards with such an experiment.

If you have an isolation transformer, you can observe the waveforms on a 'scope to get an insight into what's happening. Otherwise, you can simply measure voltages across resistors - but make sure you have a DMM with a decent AC bandwidth, as we're looking at very spiky waveforms that are rich in high-frequency harmonics.

Which gets me back to the original point. The dropper capacitor is chosen to have the required reactance at 50Hz. But its reactance at higher frequencies will obviously be much less.

So if you monitor the current in the capacitor (by observing the AC voltage across the typical 100 ohm anti-surge resistor in series with the dropper cap), you'll find that it reaches a peak value when the voltage across the lamp is in the 150V region. Above that, the current drawn actually falls as the waveform starts to look more like a sine wave.

It also follows that this increased current makes its way through the bridge rectifier and ultimately the shunt element has to absorb it. By putting a 1 ohm resistor with the zener you can observe the current in that - it also peaks at around 150V.

So if the design and components used are in any way marginal, then yes, powering the unit via a triac-based dimmer is a bad idea - as counter-intuitive as that might seem
mhennessy is offline   Reply With Quote
Old 8th Sep 2018, 12:19 am   #36
TrevorG3VLF
Octode
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,095
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I have done some measurements on the little mains indicators.
The dimmer was driving a small bulb.
The DM25 was set to a specific value, either with a Variac or a dimmer and the indicator noted.

DM25 meter (RS) Variac Indicator Dimmer Indicator
50 62 40
100 100 110
150 152 156
200 202 210
236 238 236

It looks as if the meters are measuring in the same way, presumably mean voltage.
TrevorG3VLF is offline   Reply With Quote
Old 10th Sep 2018, 7:43 pm   #37
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,334
Default Re: Quick-and-dirty testgear: mains voltage monitor.

Curiosity got the better of me. Mine arrived today...

When used with a variac, it gives good results.

Code:
Fed from a variac:

Applied voltage    Indicated voltage    Current drawn
50V                 50V                  7.2mA
100V                99V                 16.6mA
150V               149V                 21.8mA
200V               200V                 29.1mA
240V               240V                 35.1mA

However, the same can not be said of the dimmer:

Code:
Fed from a dimmer:

Applied voltage    Indicated voltage    Current drawn
54V                 24V                 25.8mA
100V                57V                 47.4mA
150V               102V                 56.2mA
200V               163V                 50.0mA
226V               206V                 32.7mA
The seriously incorrect voltage readings are bad enough, but look at the current taken by the capacitor dropper PSU. As I pointed out earlier in the thread, capacitor droppers really don't take kindly to the chopped up mains from a triac dimmer.

I haven't yet taken it apart to see what components have been used, but looking through the vents, I can see a fairly substantial (by usual standards) series resistor. This resistor was cooking during these tests; I had to switch off and allow a cooling down period in-between measurements - the smell of hot electronics and melting plastic was all too obvious. I bet that if left for a few minutes, it would fail. Perhaps I'll test that in the future, although I'd set it up outside for obvious safety reasons - at the very least, there will be smoke

All measurements were taken with true-RMS Fluke meters with good AC bandwidths, and the dimmer in question was a standard type from MK. I used a 60W lamp to keep the dimmer happy.

It's interesting that Trevor's measurements are better via his dimmer - I wonder what sort it is?

It looks like a teardown on these might be destructive. I might have a go later. I don't intend to keep and use it - if nothing else, I don't like the unreasonably slow multiplex rate of the LEDs (as usual for these cheap things).

Cheers,

Mark


EDIT: Typos in the numbers - not sure how that happened! Double-checked - all correct now, and conclusions remain the same

Last edited by mhennessy; 10th Sep 2018 at 8:00 pm.
mhennessy is offline   Reply With Quote
Old 10th Sep 2018, 11:55 pm   #38
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,334
Default Re: Quick-and-dirty testgear: mains voltage monitor.

It turns out you can get these apart without cutting things (like Big Clive did in the video I linked to in post #29). You can de-solder the wires from the screw terminals.

With it all out in the open, the 56 ohm resistor in series with the capacitor dropper runs at about 140 degrees when fed from the MK dimmer. It's quite cool down here in the workshop, so it'll be a lot hotter than that when back together and installed in a product.

But the AC voltage across the resistor is much lower than expected for the temperature of the resistor - which shows the difficult nature of these sorts of measurements. As a reality check, placing a DC voltage that is the same as the measured AC voltage results in far less heating, which shows that the AC measurement is a gross underestimate - even though the Fluke I'm using has an AC bandwidth of 100kHz.

With the addition of a mains isolation transformer, I'm able to observe the voltage waveform across the resistor on the 'scope, and see just how very spiky it is (and those spikes are very narrow indeed). A measurement with my Fluke 8920A (which is earth-referenced, uses a thermal RMS converter and has a bandwidth of 20MHz) finally gives a voltage measurement that correlates with the heat this resistor is actually making.

Let's consider the current. While the trend of the earlier measurements fits my expectations based on previous experience many moons ago with these sorts of circuits (highest current in the 150V region), I was aware that the currents measured were likely to be underestimates because of the very high crest factor of the current waveform (and the "hot" smell from inside the unit). But as we now have a reasonably accurate measure of the voltage across that 56 ohm resistor, we can determine the current much more accurately - it's about 110mA, believe it or not! Roughly double the earlier reading.

The final thing to try was using my Rigol 1054Z, as this has a much higher bandwidth than the 8920A. It gave an RMS voltage that correlated to a current of 135mA.

So anyway, I hope this helps to demonstrate that there's more to this subject than meets the eye. I remember being fascinated by all this when meeting it for the first time, and I hope it's of interest to some.

Just to reiterate, this is when the unit is being fed from a triac-based dimmer which, for my particular dimmer at least, is clearly not a good plan! When used with "standard" sinusoidal-ish mains, it'll behave much more as expected. But don't forget a fuse!

BTW, now that I have one here, I can see that my earlier suggestion of a dead PP9 case is just about perfect for these. Plenty of space for the unit and a panel-mount fuse holder
mhennessy is offline   Reply With Quote
Old 1st Oct 2018, 9:27 pm   #39
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,403
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I've fitted one of the little displays alongside a dual 13A outlet in my 'office'.

The front-plate and 25mm back-box are standard MK items bought from Screwfix. Though in the past people have reported problems drilling Bakelite faceplates using traditional twist-drills, I had no problems at all using a flat 'woodworker' bit to drill both the main hole and that for the fuse-holder. [I decided that since the sockets are fed from a 40A RCD down 6mm T&E it might be wise to introduce some local current-control].

And yes, 250V on the display is +/- 1% correct. I've got a dedicated 3-phase 100A supply here, connected direct back to the 33KV substation a couple of hundred yards away. Happiness is a low loop-impedance...

--
G6Tanuki. The Future's Bright; The Future's Otter!
Attached Thumbnails
Click image for larger version

Name:	meter.jpg
Views:	108
Size:	27.9 KB
ID:	170171  
G6Tanuki is offline   Reply With Quote
Old 2nd Oct 2018, 8:05 pm   #40
dave walsh
Dekatron
 
Join Date: Jan 2006
Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex.
Posts: 3,857
Default Re: Quick-and-dirty testgear: mains voltage monitor.

I didn't know those LED indicators existed Tanuki That's a very neat holder you've put together!
BTW is your Atavar related to Quatermass by any chance?

Dave W
dave walsh is online now   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 2:23 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.