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Old 8th Apr 2018, 3:26 pm   #1
Sinewave
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Default Homebrew power supply for calibration

I wonder if anyone has ever thought about making a homebrew PSU for calibration purposes?

Ideally 1000 volts AC RMS, with a rectifier for testing DC.

It would have adjustable output, to set for different voltages that you'd like to use for calibration. Perhaps to keep things simple, a calibrated known volt meter could be used with the output to ensure the accuracy of the set voltage.

I've done some searching, but haven't found anything much in the way of what I'm looking for, apart from some valve power supply circuits.

Any thoughts or ideas welome!

Thanks.
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Old 8th Apr 2018, 3:37 pm   #2
Ed_Dinning
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Default Re: Homebrew power supply for calibration

Hi, a lot of this type of gear is easily made, but there were AVO devices available (there was a stripped down one at Golbourne today).
Basically an isolating transformer with output taps and a variac for adjustment will work fine.
You can then switch in a rectifier and cap filter for the DC.
I may even be able to find a suitable transformer for it if you PM me your requirements.


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Old 8th Apr 2018, 5:06 pm   #3
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Default Re: Homebrew power supply for calibration

Fluke made an AC/DC calibrator that ran to those voltages.
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Old 8th Apr 2018, 5:32 pm   #4
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Default Re: Homebrew power supply for calibration

Indeed, but the purpose of this homebrew is to keep well away from the Fluke/Time electronics etc prices - otherwise I'd just buy one off the shelf.
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Old 8th Apr 2018, 5:38 pm   #5
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Default Re: Homebrew power supply for calibration

You could use one of those saturable reactors on the primary, that will stabilize your AC and DC

I actually had a 1KV dc voltage source made by Philips for in house use, that used a string of zeners and a BU205.

For the DC, you could look at a shunt regulator, perhaps even go tube, the 85A2 is stable within 0.01% if ran at 3mA. Those tubes can be picked up for 50C on swapmeets if you look around.
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Old 8th Apr 2018, 6:36 pm   #6
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Default Re: Homebrew power supply for calibration

Quote:
You could use one of those saturable reactors on the primary, that will stabilize your AC and DC
It will sort of stabilise it, but it also has a large effect on the waveshape and as most AC meters respond to the peak and are scaled to read the RMS equivalent assuming a sine wave it won't be good for calibration.

In fact you may want to avoid mains waveforms completely. An audio generator into a power amplifier into a mains transformer backwards?

David
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Old 8th Apr 2018, 7:02 pm   #7
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Default Re: Homebrew power supply for calibration

Quote:
In fact you may want to avoid mains waveforms completely. An audio generator into a power amplifier into a mains transformer backwards? David
What sort of accuracy are we looking for here? It had not occurred to me that mains waveform distortion could cause Vrms errors that would be significant to most hobbyists. I have one of those 'ferromagnetic' mains stabilisers and that certainly does seriously mess with the waveform.

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Old 8th Apr 2018, 8:38 pm   #8
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Default Re: Homebrew power supply for calibration

Mains waveforms can be misleading by over 5%
The sort of rounded-squarewave out of a CVT can put you 10-20% off So if you calibrate your AC voltmeter this way and use it to measure the mains, you could have an error between 24 and 48 volts, which is probably enough to cause problems.

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Old 8th Apr 2018, 8:46 pm   #9
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Default Re: Homebrew power supply for calibration

The idea is to use a meter in calibration, to check the output of the 'calibrator' which is used to generate the voltage.
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Old 8th Apr 2018, 10:26 pm   #10
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Default Re: Homebrew power supply for calibration

I've had similar ideas - luckily, I'm too short of time to get around to building anything, but I've got notebooks full of sketches that would probably work OK.

Starting with a clean sine wave is a must. Multimeters respond differently to non-sinusoidal waveforms, so how can you be sure the "DUT" is behaving the same as the meter used during the initial calibration?

Next, a power amplifier using the ubiquitous LM3886. This is good for around 50W into 8 ohms - which is 20V RMS. Other chips are available, but I've got lots of these lying around...

Between the oscillator and power amp is where you can put your calibrated amplitude control. Many ways to do this - a 10-turn pot with a dial on might well be good enough. Obviously there would need to be some range switching too. For low voltage mV ranges, an attenuator (divide by 1000) at the output of the amp kills the power amp noise at the same time.

For the high voltage output, a mains transformer in reverse would be good for 250V AC. If you want 1000V, either have a custom transformer made, or use 4 mains transformers with their secondary windings in series! Crude but effective - there's no current demand, so these can be small 6VA units perhaps... Alternatively, I half-wondered about an ignition transformer from an oil boiler or similar. Lots of options available.

But there is a problem with this - the output from the transformer will not be an exact integer multiple of the carefully calibrated output from the amplifier. Some form of feedback would be tricky to arrange. So perhaps the best bet is to forget about the precision 10-turn pot (Time Electronics 1030-style) and incorporate an uncalibrated adjustment that works in conjunction with a decent AC voltmeter built into the unit (like the Time Electronics 1044). Easier said than done at 1000V AC, needless to say.

That was as far as that "thought experiment" got, but it did include a DC option (obviously not via the transformer) and also a DC+AC option (as some multimeters have issues measuring AC when DC is present). Variable frequency was also included, as multimeters all have different roll-over points. Square wave too (for checking for True RMS detection), though I dread to think how that would look after a mains transformer. In practice, I'm sure I'll never get around to building it, as I can already run most of these tests using gear I already have (like a function generator and a bench amp), but I would like something better than a variac for doing high voltage AC tests...

Another thought is to ditch the transformer and just use high voltage transistors. Got any old line-output transistors lying around? Would be quite feasible to do, but a bit scary during development. Getting the DC supply to this in a safe-ish fashion would be a challenge in its own right. Of course, using a pair of these to provide a differential output would halve the DC voltage required, and I suspect that would be an essential requirement, given that a BU508 is only good to 1.5kV. But that means that neither output is grounded, so both are hazardous.

Usual disclaimers about safety: this stuff gets scary quickly
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Old 8th Apr 2018, 10:33 pm   #11
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Default Re: Homebrew power supply for calibration

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Originally Posted by Sinewave View Post
The idea is to use a meter in calibration, to check the output of the 'calibrator' which is used to generate the voltage.
Then you need to make sure that the meter being calibrated against has the same sort of response and correction arrangement that the meter being calibrated has. Make sure you don't mix sinewave corrected and true RMS meters.

David
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Old 8th Apr 2018, 10:38 pm   #12
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Default Re: Homebrew power supply for calibration

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Originally Posted by Sinewave View Post
The idea is to use a meter in calibration, to check the output of the 'calibrator' which is used to generate the voltage.
OK, but the question of required accuracy (reproducibility ?) remains; as a hobbyist, I think I'd be happy with +-1%, 2 probably OK, 5 sounds a bit less good and 10 would be too much.

As I no longer have ready access to meters which have traceable calibration, I generally check 3 meters against each other and if they're all within a gnat's whatsit, that's OK.

B
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Old 8th Apr 2018, 10:40 pm   #13
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Default Re: Homebrew power supply for calibration

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Originally Posted by mhennessy View Post
I've had similar ideas - luckily, I'm too short of time to get around to building anything, but I've got notebooks full of sketches that would probably work OK...
Sounds like you've got some good ideas.

I had thought of making this kit take off at between 60 to 100 volts, then going higher.

In the garage I think I have a 500-0-500 or 600-0-600v transfomer, or both.

Say the 500-0-500, could be used for the 1000v range, if it is floating a bit higher than 500v then circuitary could deal with that.

Some range switches as you say and a 10 turn pot would be idea per range.

The ranges could anything like this

0-100
0-300
0-600
0-1000

Something like that. Then some thought into a rectifier for DC.

In fact, it may work, though crude, using my variac on the input of a 500-0-500 tx, noting that the rectifier will give the peak value of the sine.

Also, as suggested, the issues with non pure sine, disortion on the waveform. All things to consider.
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Old 8th Apr 2018, 10:44 pm   #14
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Default Re: Homebrew power supply for calibration

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Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by Sinewave View Post
The idea is to use a meter in calibration, to check the output of the 'calibrator' which is used to generate the voltage.
Then you need to make sure that the meter being calibrated against has the same sort of response and correction arrangement that the meter being calibrated has. Make sure you don't mix sinewave corrected and true RMS meters.
It may be worth me using a true RMS and Averaging Rectified meters for this, or developing a way around.
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Old 8th Apr 2018, 10:45 pm   #15
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Default Re: Homebrew power supply for calibration

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Originally Posted by Bazz4CQJ View Post
Quote:
Originally Posted by Sinewave View Post
The idea is to use a meter in calibration, to check the output of the 'calibrator' which is used to generate the voltage.
OK, but the question of required accuracy (reproducibility ?) remains; as a hobbyist, I think I'd be happy with +-1%, 2 probably OK, 5 sounds a bit less good and 10 would be too much.

As I no longer have ready access to meters which have traceable calibration, I generally check 3 meters against each other and if they're all within a gnat's whatsit, that's OK.
I supposed this would come down to the accuracy of the calibration check meter, which could be anywhere between 0.05% to 1%.
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Old 9th Apr 2018, 2:58 am   #16
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Default Re: Homebrew power supply for calibration

This paper https://www.google.co.uk/url?sa=t&rc...TWn-psLa3zS5pvdescribes an inverter which seems to claim (it is 3am so I'm sleepy!) it could produce 340V pk-pk of pure sine wave for a cost of $65. Some ideas worth stealing?

B
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Old 9th Apr 2018, 10:20 am   #17
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Default Re: Homebrew power supply for calibration

I'm just completing a variable AC power supply using two back-to-back transformers for isolation. The nominal output is 0-240VAC but I can switch to a lower voltage tap on the second transformer to increase this to - I reckon - a safe maximum of 300-350V.

For calibration purposes I suppose current levels aren't a large issue (only a few 10's of mA?) but I could also switch another, purpose-wound, transformer into circuit to extend this supply to anything I required.

For DC voltages I use an SMPS converter connected to a 0-30V DC supply which results in a 0-300V DC output.
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Old 9th Apr 2018, 12:50 pm   #18
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Default Re: Homebrew power supply for calibration

I'd be interested in the details of that converter you use.
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Old 9th Apr 2018, 4:12 pm   #19
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Default Re: Homebrew power supply for calibration

Quote:
Originally Posted by Bazz4CQJ View Post
This paper https://www.google.co.uk/url?sa=t&rc...TWn-psLa3zS5pvdescribes an inverter which seems to claim (it is 3am so I'm sleepy!) it could produce 340V pk-pk of pure sine wave for a cost of $65. Some ideas worth stealing?
Thanks, worth a read.
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Old 9th Apr 2018, 4:43 pm   #20
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Default Re: Homebrew power supply for calibration

I lashed up a system forty odd years ago, to be used with a known calibrated meter. it consisted of a 14 watt Williamson power amplifier with an old single ended KT61 output transformer secondary connected to it. The primary then produced about 300 volts at a range of audio frequencies when the set up was driven by a Levell TG152D oscillator. DC was available with a couple of BY127 diodes and a suitable cap.

Not your required 1000 volts but it worked well for calibrating a range of DVMs.

Last edited by barrymagrec; 9th Apr 2018 at 5:05 pm.
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