Passive high-voltage dummy load

[Skywave]

I think most of us have been in the situation where we have a mains transformer of the type suitable for a valve radio and need to know "what it's good for" - in terms of voltage and current - for that repair or design job that is being undertaken. If you're lucky, you'll know some of the essential data - in particular, what the HT winding is capable of in terms of voltage and current - usually on account of information stated on the transformer. But when that isn't the case, what do you do? 'Suck it and see'? Rely on 'calibrated eye-sight'? My typical resort is to successively connect an assortment of different power resistors on the HT winding and monitor the resultant current and voltage. But easy though this may be to do, it has its drawbacks. On account of the inconvenience of needing a large number of power resistors of various values (which will get hot), there is also the risk of damage to the test equipment in use and the transformer under test from accidental short-circuits. And the real risk of receiving a very nasty HV shock is ever present! :omg:

Over the years I have accumulated a large number of such transformers and it is a great convenience to build a database catalogue of their significant specs. to help me rapidly locate a suitable transformer when needed. Faced with the prospect of using the method outlined above, I decided it was time to build a test unit to make this job a lot easier, now & in the future. Such a unit also has a use for testing a suspect mains transformer when said item is fitted in a radio which is undergoing repair.

This is my write-up of what I ended up with. It's certainly not going to win a constructor's competition, but it did cost me very little in terms of specific expense: in particular, only the meter. Everything else came from the proverbial 'junk box'. The reason I am producing this Post is simply to show that such a unit can be designed & assembled by anyone for little cost: the design can be simplified or expanded according to one's needs and resources.

Basic usage.
The whole of the HT winding of the transformer-under-test is connected to the unit's two input terminals: so a 350v - 0 - 350v transformer has no connections to the centre-tap. The mains voltage is applied to the transformer primary and a variable stepped resistive load inside the unit is internally connected across this winding. The resultant a.c. voltage and corresponding a.c. current are monitored on a meter as the internal load is varied. The meter is front-panel switched between voltage and current.
The unit is designed for a maximum input voltage of 1,000v r.m.s. and a max. input current of 100 mA continuous, 200 mA for 'soak' testing. The unit contains its own 230v.a.c. fan.

Photographs are below.

To prevent this Post from becoming a total monologue, I'll stop now. Further remarks may be added later, depending upon this Post's response (if any). :D

[attachment=3479][attachment=3480][attachment=3481][attachment=3482][attachment=3483]

Al. / Dec. 5, 2011 //

[pwdrive]

Ok Al...spill the beans, I'm interested, nice build quality, well done.

Lawrence.

[Andy M0FYA]

Please tell us more!
There looks to be a lot of gubbins in it, so it must do all sorts of clever things...........
Andy

[Michaelr]

Very useful Al, Have you got any form of temperature monitoring of the transformer under test.

Mike

[AlanBeckett]

...... and a max. input current of 100 mA continuous, 200 mA for 'soak' testing.

Al. / Dec. 5, 2011 //

Al,

Is that the right way round?

Alan

[Andrewausfa]

Gosh! From someone who gets excited when he makes the simplest of bulb and battery circuits work, I always find it amazing and compelling that chaps such as yourself come up with stuff like this. The whole design and execution thing.

Andrew

[Yorkie]

This is my write-up of what I ended up with. It's certainly not going to win a constructor's competition, To prevent this Post from becoming a total monologue, I'll stop now. Further remarks may be added later, depending upon this Post's response (if any). :D

Al. / Dec. 5, 2011 //

I disagree that it wouldn't win a constructors competition! It looks well designed and very well built, with safety in mind, as befits its purpose with high voltages bouncing around. It's always far easier to replicate something that someone else has designed, than to conceive a design of your own, do the R&D and exerimentation, wherever possible, using such components and materials as are to hand, and to then build, test and commision it.

As to 'writing monologues', don't let that concern you! I for one, would be interested in a sketch of the circuit Al.

Posts on this or any other forum aren't force-fed to people who'd rather not read them - they're self-selected. We're all endowed with free will, and just as people have a right to write about things that are of interest to them so long as they comply with the few sensible rules of the forum, others have an inalienable right to ignore them. If a post is of interest, people will read and respond to it, but if not, the thread simply fizzles out. My basic outlook is that I write about things that interest me, in the hope (rather than the expectation!), that they might interest and encourage a few others, but if not, it's taken nobody's time but my own.

What infuriates me most, is on certain forums where it is clear that members wish to continue a discussion, all too often, a moderator steps in and without warning, says something like: "I think we've covered it all now - some useful comments, but I’m closing the thread now". What's that all about then? Is the moderator trying to safeguard members from opening a post that they might find boring? I don't need anyone to perform that dubious sevice for me or to 'hold my hand' and act as 'parentis in locus' - I'm a big boy now and strange as it might seem to moderators in certain forums, I can excercise that judgement all by myself.

So my simple message to anyone on here is that if you have something that you feel is worthwhile to write about, don't be inhibited!

Anyone who has read down to here had done so because they want to - not because they have to:D

[Robert G0UHF]

Very well put David (Yorkie) I completely agree with your comments.

Al (Skywave) please post a schematic of the circuit and elaborate further. You've obviously put a great deal of hard work into the project and ended up with a very useful piece of test equipment. Just excellent!

Kind regards

[Skywave]

Thank you gentlemen for your encouraging responses. And so to the details . . . .
First, a simple diagram explaining the initial concept:
[attachment=3484]

Then an expansion of this:
[attachment=3485]

And finally to the detailed drawings:

[attachment=3486]
RL is the variable load resistance. This compromises a number of individual resistors and 3 'resistor modules', as detailed in drawings 2 and 3.

[attachment=3487]
The rotary switch for RL - which for convenience is drawn as a straight line - must be a MBB type. At each end of the switch there is one no-load position. C1 is included simply to reduce the arcing at the switch contacts.

[attachment=3488]
The 'resistor modules'

[attachment=3489]
Rectifier unit & its PSU; mains input arrangements

[attachment=3490] [attachment=3491]
Two sketches illustrating component layout. (Useful as a maintenance aid).

Comments.
Metering: I did consider having separate meters for voltage and current. However the size of the available case made this not possible.
The scaling of the milliammeter, ideally, would have been 100 mA and 200 mA. But this would have meant awkward and confusing scales on the one meter. For most transformers, 100 mA FSD is adequate, with the occasional need for 200 mA. On the grounds of maximum allowable dissipation of heat in various resistors in RL, the continuous current rating becomes 100 mA, but on test, currents up to 200 - 250 mA were found to be 'acceptable' without overload-heating of those resistors. This makes the unit suitable for 'soak testing' certain transformers.

The chassis, which is plated brass, is simply a salvaged sub-chassis from a scrap TF144G signal generator.

There is no provision for monitoring the temperature of the transformer under test - apart from using the 'finger on the laminations' approach!
I did contemplate fitting additional circuitry (based on an analogue multiplier) that would calculate the total power dissipated and use that result as another meter-displayable function. Lack of chassis space prevented this. (Maybe a feature in a later Mk. II)

Calibration: Two pre-set pots. are provided: one for FSD voltage, one for FSD current, with 'select-on-test resistors' to give appropriate scaling.

Usage: Basically, obvious: by adjusting the value of RL and switching between voltage and current, the drop in voltage as the current is increased can be observed. From that, a graph could be drawn from which the transformer regulation characteristic can be derived.

Application: For selection of a transformer for a specific application, the particular rectification circuit and the necessary filter circuit needs to be known, since the mean current demand on any given transformer depends on that configuration: usually bi-phase or bridge rectification; capacitor or choke input filter.

Finally, my 'get-out' clause! I'm not a professional designer of electronic equipment, so there may well be short-comings or over-sights in the design. As such, any comments in those directions will be appreciated. At present, I do not regard this unit as a 'finished & proven' unit: 'input' from others and time will lead to that result.

Al.

[Skywave]

Very well put David (Yorkie). I completely agree with your comments.

Quite: I totally agree with every word & sentiment.

Al.