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Old 27th Apr 2018, 10:51 am   #21
Sinewave
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Default Re: Homebrew power supply for calibration

I'm giving this some thought again regarding the audio frequencies and amplifer.
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Old 27th Apr 2018, 11:18 am   #22
The Philpott
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Default Re: Homebrew power supply for calibration

For the purposes of getting a reasonable line in the sand for ACV, borrowing an inverter generator (even a small Impax one) would, i would hope, give you an exact and clean sine output. The alternator output is rectified, smoothed, then inverted, and it's all typically microprocessor controlled.. Not much opportunity for error.

Conventional generators producing raw AC won't be any good for this, but my 'best' ACV meters agree with the 230v (50hz) output from the Impax inverter plus or minus 1 volt, so i am satisfied with that.

Dave
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Old 27th Apr 2018, 2:47 pm   #23
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Smile Re: Homebrew power supply for calibration

Maybe one of these then? Pure sinewave inverter which will do nicely on the bench.

http://cpc.farnell.com/mercury/652-1...59?st=inverter

It'll be more than enough to power my variac for simply mA worth of testing purposes, even with a step up transformer attached to it.
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Old 27th Apr 2018, 4:20 pm   #24
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Default Re: Homebrew power supply for calibration

For obvious reasons I've had similar needs and thoughts to those expressed above over the years. As I see it there is the problem of sine wave purity as mentioned several times. The other problem is one of safety; 1000V or more is not to be treated lightly, especially when working alone.

To take the second point first, this might be addressed by having an enclosure with interlock switches or some kind of residual current device, perhaps using the cut-out contacts or even the movement from a Model 8 Mark V meter.

I'm interested in what Ed had to say about an AVO calibration device/rig. I believe that AVO did make one for sale to authorised repair agents but I've never seen it in any price lists. Perhaps Andy might be able to find some information for us?

Another question for Ed is the effect of Variac style transformers on waveforms. The core material is going to have some effect which, without better knowledge, I would expect to vary with load and possibly with transformation ratio.

I would be surprised if the term "Pure Sine Wave Inverter" means much more than that the output "looks" like a sine wave and is unlikely to cause any ill effects on equipment supplied from it for which waveshape is important.

At least in theory it should be possible to filter any alternating waveform with a narrow bandpass filter centred on 50 Hz, but how practical this might be at the order of 1000V is another question. If a sufficiently linear transformer were to be used, the filtering could be at the LV side. This almost brings us back to the audio amplifier technique.

We know that suitable calibrators exist, so there wouldn't be any harm in finding out how the problem is overcome in them. The question of patents could be dealt with if it arises.

PMM
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Old 27th Apr 2018, 4:37 pm   #25
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Default Re: Homebrew power supply for calibration

I don't see safety as a problem, most here I assume are all qualified electrical engineers, so we're able to work safely.

As for the purity of the waveforms, I could use an oscilloscope to see how pure it is. I know that the modified sinewave inverters do appear very squared at the peaks of the waveform.
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Old 27th Apr 2018, 7:48 pm   #26
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Default Re: Homebrew power supply for calibration

Hi Gents, most variacs are operated at a high flux density and in the non-linear part of the B-H curve, so there is usually visible distortion of the waveform. Not as bad as so called "constant voltage transformers though!

The Bradley 125C that I have has a pretty comprehensive set of transformers inside as well as stabilised DC power supplies and a power amp with the op trans in the cathode circuit of this push-pull stage.
Transistors are extensively used in the measuring/ deviation circuits as well as op amps.

The 1258 plug in oscillator unit offers frequencies from 40Hz to 3.2KHz, fully variable with a lamp stabilised (Wien Bridge?) oscillator.
Output voltage range is 0-511v in 100mV steps, max output 5VA.

Quite a comprehensive instrument!

For DC work a simple potentiometer box could be used

I believe I'd heard of an AVO unit, but I don't think I've seen one.

There is a similar unit available for use in checking oscilloscopes.

Ed
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Old 30th Apr 2018, 6:20 pm   #27
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Default Re: Homebrew power supply for calibration

Out of interest, I found a video on youtube where someone was investigating how true a pure sinewave inverter is at producing pure sinewave. It was actually a very clean and nice sinewave.

I should think for my purposes that so long as I'm sure of a clean sinewave, this should be enough for simple FSD calibration, using a meter in known calibration as comparison. This would be a good starting point. I could use AF oscillators at a later stage if I wanted to experiment with that.
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Old 30th Apr 2018, 11:00 pm   #28
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Default Re: Homebrew power supply for calibration

Interesting thread. I thought about this many times, particularly when I dug my HP3478A out of the cupboard and found the calibration battery was dead. Unfortunately I got nowhere with anything which would produce a credible repeatable pure sinusoidal AC output for an affordable expenditure. I looked at Apex microtechnology PA85 "high voltage opamp" as the output stage. This gives +/-225V swing which is pretty good! Unfortunately £137, each and it doesn't solve the "how do I get stable bipolar 300v supplies?" problem either.

Ergo I fished out the £70+VAT to get someone else to calibrate it. Stings but was worth it.
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Old 1st May 2018, 11:29 am   #29
Sinewave
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Default Re: Homebrew power supply for calibration

For collecting purposes, it isn't an economical to get nearly 100 Avos officially calibrated.

On another note, if I've got a Fluke 179, then after three calibrations, I've spent more on calibration than what the meter is worth. An 87V add on a few more years and that's a lot of money spent on cal.

But for what it's worth, I keep the 87V in offical calibration, so that's my official check meter.
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Old 1st May 2018, 11:56 am   #30
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Default Re: Homebrew power supply for calibration

Very true. I'm doing the same with my other meters. I get the best one (WRT accuracy, AC bandwidth) calibrated periodically and then use that and a power supply/signal generator to check the other ones.

Also with an Avo, the calibration tolerance is a little easier than a 5.5 digit DMM so comparative cal with a trusted DMM is probably good enough.
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Old 1st May 2018, 12:16 pm   #31
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Default Re: Homebrew power supply for calibration

From time to time I need clean stable sine-wave mains and it helps if I can have the option of floating it from ground. So I bought a used 'true sine' UPS online (one like this https://www.ebay.co.uk/itm/APC-SMART...-/132570808540). They can be had pretty cheaply once the internal batteries have failed. I added two 12V lead-acid batteries externally and beefed the thing up with a few extra FETs and a bigger fan (there was an online video explaining how to do this) and the resulting setup is one of the most useful bits of kit I have. It also comes in handy if you ever take a stall at a boot sale or fair.

The factory setting is for 230VAC and that is stable to within a fraction of a volt. If you register with the manufacturer you can download control software which allows you to re-programme the supply to deliver 240VAC (you only need to do this once). I looked at the waveform with an audio spectrum analyser and if I remember rightly the total harmonic distortion was under 1%.

Cheers,

GJ
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