Tiny digital ac volt/amp meter

[Mark Hennessy]

I agree that the 100R Carbon Film Resistor is no good, but equally I wouldn't trust a Carbon Composition one. They can, and do, fail S/C is severely overloaded. Metal Film or WW will fail O/C.

No - carbon comp resistors are sold today for exactly this sort application.

Solid carbon resistors designed for high energy dissipation. These small leaded resistors are non inductive and combine high pulse characteristics with excellent stability

The advantages of the composition resistor are its very good high-frequency characteristics and the high capability to overload relative to the component size. These factors mean that these relatively expensive resistors are used in applications like power supplies, welding controls and as “dummy loads”.

Some years ago I built an amplifier that used a capacitor dropper PSU for the control circuit. Initially I used carbon-comp, and it failed after about a year. I replaced it with a 0.6W MF, which lasted for about the same time. I eventually got around to buying a pack of carbon-comp resistors, and the amplifier has been 100% reliable for some 8 or 9 years now. It was the same type of resistor that I initially fitted to the "Wun Hung-Lo" module, replacing the carbon film job.

I suspect that apart from cost, they were regarding the carbon film resistor as a fuse that fails in the event of the dropper capacitor failing short. But, assuming the dubious cap remains OK, I predict - from personal experience - that the CF resistor will fail O/C prematurely. Obviously, it depends on how "spiky" your mains is, and how often you power it up. The sustained power dissipation is trivial - around 0.1W - but it needs to survive the surges. Film resistors don't.

WW resistors of this sort of value need a lot of abuse before failing O/C - the wire is normally welded to the end caps or lead-outs, and in my experience the body usually deforms (or sets fire to something nearby) long before the weld or wire fails. Higher value resistors with thinner gauge wire are no-doubt more likely to fail O/C - the last O/C one I encountered was 10k if memory serves.

Anyway, the matter is academic for me, as the capacitor dropper supply is no more.

[AlanBeckett]

Sorry Mark, but the answer is still no. Carbon Composition Resistors, of any variety, are not allowed as Safety Components in Hazardous Area Equipment simply because they do fail S/C. Metal Film and WW Resistors are allowed simply because they do fail O/C.
High Frequency use and the ability to withstand brief Surges are valid Parameters for choosing them, but Safety isn't.

Alan

[Mark Hennessy]

Where did I claim the resistor was a safety component?

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I advocate changing the resistor because the carbon film original will fail prematurely.

I advocate changing the capacitor for an X2 type for obvious safety reasons.

I advocate adding a fuse, for obvious safety reasons. I highlighted that point in red on the webpage.

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Obviously, an appropriate fuse removes any concerns about the possibility of a carbon-comp resistor failing S/C. The fuse also gives protection from the horrors of the PCB layout.

No properly designed product would rely on an inrush-limiting resistor to also act as a fuse. You need a separate fuse.

Incidentally, I just noticed that I omitted to label the capacitor on the schematic. I've just corrected that. In case it's not 100% clear to anyone else reading this, we're discussing R7 and C4.

[Nick]

Nice project - good write-up and neatly executed.

Couple of small things:

I seriously hope that pin 6 (IC1b) of that TL082 in the reverse-engineered schematic is tied to ground and not floating - recipe for HF oscillation otherwise...

Just a thought - I always include a decent mains filter block in my PSU strips - typically one of the Schaffner series as I like their specs. - doing this isolates my bench from the rubbish coming into the shack - I'm out in the country, so get a very high RMS value - typically 250 - 255VAC on the phase to the shed - we have 3-phase, pretty well balanced generally - one to the house, one to a cottage in the grounds, and one to my shack.

I borrowed a Dranetz for a week and had a long hard look at the harmonics etc. on the line - not pretty. As I do a lot of Hi-Fi type work, I installed a lightning barrier for the shack as a whole (the 3ph to our site goes half a mile across fields and just to make it worse, we're on top of a hill), and Schaffner filters on each PDU (for those that didn't have one already). For the sake of a few quid, its a cheap win.

[Mark Hennessy]

Hi Nick,

Good question re. the op-amp. I can't immediately remember as it was last year when I did the work, but I'll see if I can find the old PCB...

About the filter, I did consider using one. I do have good stocks of them, and there is reasonable space in the box. In the end I decided against once I'd decided to add the voltage monitor output. I'm not sure how good the frequency response of my small mains transformer is - probably not great - but given that I was now in a position to view the mains waveform, I thought it best not to filter it. Of course, one approach might be to use a separate filter (as opposed to one integrated into the IEC socket), and connect my monitor to the incoming side of the filter.

I was also considering adding some MOVs for surge protection, but decided against that for the same reason. If I'd stuck with that capacitor-dropper supply, then MOVs would have helped protect the module.

Oh - another factor was that those filters often use the dreaded "Rifa" X2 caps. At a similar time, the one in my frequency counter let go rather dramatically (A Schaffner FN326-1/07 - sadly, I can't find a modern replacement that fits into the space, so I had to use an unfiltered IEC).

As I mentioned in the write-up, I am planning to rig up a large audio amplifier so I can characterise the behaviour of both monitor outputs w.r.t. frequency. One day...

Cheers,

Mark

[Nick]

Being fair, I've only ever had one Schaffner IEC input filter die on me - it was in the back of an HP 8904A which was NIB when I got it - in 25 years of doing this stuff, its the only one I've had that died.

I've also been lucky on the RIFA (now part of Kemet) front - no explosions there, though I replace any X2 or Y2 caps of that genre I find...

I have a LOT of very large, ultra-low ESR, HV (400-500VDC) RIFA, BHC & EPCOS caps (all part of Kemet now) - offer them for sale from time to time, mainly for tube audio use..

BTW. I really like the quality of the finish on your projects - very smart. Man after my own heart - when I was designing and building small DRSSTCs (Doubly Resonant Solid State Tesla Coils) a few years ago, I was taking the detail to a bit of an extreme... http://4hv.org/e107_plugins/forum/forum_...90019.post

[Nick]

Mark,

I did a bunch of work of very high linearity, high frequency current transformer designs for my DRSSTC projects (which run up to about 500kHz).

Getting an "ideal rectifier" that works with low input voltages is tricky, as is matching impedances...

The circuit below was very close to my final design - I like Rod Elliot's pages, but, as he says, they are the basic stuff. I came across a neglected TI app note SBOA068, "Precision Absolute Value Circuits" that eventually put me on the right path (for my app, anyway!) - it's an excellent read and I can't understand why it's not more widely known about... for your app, you only need IC1a/b - the rest is to do with ultra-high-speed over-current detection.

I'll look up the actual CT I used with the 10R burden resistor/load (R7) - it was tricky finding the right one.

Should you wish, I'm happy to loan you a Pearson CT to check your readings - these are normally enormously expensive, having a flat response into the high MHz regions - they are based on Rogowski coils - I have a few different types and they are essential for accurate work.

   

HINT: I used Linear Technology devices, not just because they are very good, single-rail & rail-to-rail, but also that I use LTspice to simulate stuff and accurate models are obviously included for all the LT stuff in LTspice IV - I've probably got the ASC files for this somewhere if you want to have a play with it.

[Mark Hennessy]

You're doing well! Only a couple of weeks ago we had one go up in a Tek 2465. And another in a Philips 'scope a few months before. Fortunately, they usually smoke for a quite a while, so plenty of opportunity to notice and kill the power before too much of the black stuff has escaped and coated the insides of everything...

Regarding rectifiers, that's an interesting app-note which I don't think I've seen before. The solution you went for (figure 5) is one that I find very interesting. I have used half that circuit (A2) before, as a way of switching the phase of an audio signal - IIRC I first saw it in The Art of Electronics. But combining it with an active clamp is very neat. And I reckon there will be space to build that on my board without it looking too much like an afterthought (I was thinking about using a daughter-board that sits along the LHS of the op-amp if I ran out of space on the existing board). So, that gives me something to experiment with...

I must admit that I liked the "textbook" rectifier because it's what's used in the BBC designed PPM (peak programme meter). Obviously, low-level linearity is not a big issue here. I was also planning to take a look at a few DMM schematics, as even the very cheap ones tend to be pretty good at low frequencies.

Regarding frequency response, I'm not really sure what to expect, and have no fixed idea about what is reasonable. If I get out to a few kHz I guess I'd be happy (for current - I'm expecting voltage to be a little better, but again, no real feeling for what I'll get yet). When making the current measurements, I was planning to use a 1 Ohm shunt resistor or similar to feed into the 'scope. But I'll definitely bear in mind your loan offer if it looks like I'm reaching the limits of what I can easily do using that method

Thanks for your kind words. I'm afraid that most of the stuff on the website is from the BC (before children) era, so these days I don't really get the chance to work on bigger projects. It's taken nearly 10 years to get a half-decent workshop established in this house! The kids are growing up quickly, and I am starting to get a bit more spare time, but I'm finding it harder than ever to get motivated in the evenings. I don't know if that's because the day job is more demanding (which is definitely is!), or because I'm getting older (I definitely am!), or if broadband is getting in the way (I'm sure it definitely is!).

I'm quite jealous of your laser cutter - sadly we don't have such exciting toys here. The possibilities

[Nick]

...I'm quite jealous of your laser cutter - sadly we don't have such exciting toys here. The possibilities

Sadly, I don't have access to it any more - its not mine - it belongs to the school my lads went to - when the last one left about 18 months ago, I lost access to all their nice toys

Currently (!) making up some neon letters as small Christmas presents for SWMBO & others - my, could I do with that laser cutter now...