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Old 16th Mar 2019, 3:29 pm   #1
Skywave
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Default A DC voltage calibrator.

An extract from 'Radio Constructor', July 1972: a d.c. voltage calibrator.
I'm submitting this abridged article for a peer review: cct. is attached.

General concept.
The basic idea is that by comparing two d.c. voltages of the same polarity that appear across a load and adjusting one voltage so that the two voltages are equal, zero current flows into the load. If that load is a voltmeter, its calibration can then be checked with no current flowing in that voltmeter - so the voltmeter does not load the calibrating voltage source.

Circuit note.
The designer has chosen to to use a start-up circuit to get sufficient voltage to initiate the firing of the two stabilizer valves, which once they have fired, the cct. then returns to its 'normal' configuration. The only reason he has chosen that arrangement is simply because his voltage source - the half-wave rect. plus the voltage from the transformer - is insufficient to trigger the starting voltage of the two stabilizer valves if the string of load resistors are then still in circuit.

There are various other comments I could add, but all of the above will suffice by way of an introduction.

► Your thoughts and comments on this design are cordially invited.

Al.

Edit. Sorry about the small size of the cct. I couldn't find a way of making it appear larger here.
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Old 16th Mar 2019, 4:14 pm   #2
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Default Re: A d.c. voltage calibrator

Hi Al,
It’s the June 1972 issue page 650. I am trying to work out why testing meters with different sensitivity (resistance) wouldn’t affect the reading.
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Last edited by Nuvistor; 16th Mar 2019 at 4:21 pm. Reason: Attempt to make a clearer statement
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Old 16th Mar 2019, 4:36 pm   #3
60 oldjohn
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Default Re: A d.c. voltage calibrator

Its here, https://www.americanradiohistory.com...RC-1972-06.pdf P650 #12.




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Old 16th Mar 2019, 5:06 pm   #4
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Default Re: A d.c. voltage calibrator

Thank you, John - that's a great help.

Al.
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Old 16th Mar 2019, 5:35 pm   #5
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Default Re: A d.c. voltage calibrator

Quote:
Originally Posted by Nuvistor View Post
It’s the June 1972 issue page 650. I am trying to work out why testing meters with different sensitivity (resistance) wouldn’t affect the reading.
I think the idea is as follows (from the circuit diagram, I've not looked up the article yet).

There are 2 potential dividers. The one on the left is powered from the stabilised supply (gas filled valves) which is assumed not to change. Of course if you draw current from any of the taps on that divider chain then the voltage there would drop

The one on the right is unstabilised, you can adjust the voltage across it with the variable resistor at the bottom of the diagram.

You connect the voltmeter-under-test to the appropriate tap (selected by the 2 pole switch) on the righthand divider. Of course this pulls the voltage down (due to the current drawn by the meter). And it wasn't an accurate voltage anyway.

You now adjust the variable resistor I mentioned so that the meter shown on the diagram (which is NOT the meter under test, it's essentially a centre-zero galvanometer) reads zero current.

At this point

(a) No current is being drawn from the left hand potential divider (since said current would flow through the galvanometer which is showing zero). So the voltages on that divider are accurate

(b) The voltage on the right hand divider (and thus across the meter under test) must be the same as the voltage on the left hand divider (if not, then current would flow through the galvanometer). Thus the voltage across the meter under test is the same as that from the left hand (stable) potential divider.

Yes you have to re-balance the circuit (adjust the variable resistor) if the meter-under-test resistance changes. But if you do that, the voltage across the meter-under-test will be accurate.
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Old 16th Mar 2019, 5:41 pm   #6
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Default Re: A d.c. voltage calibrator

Thanks Tony, yes I can understand with that explanation.
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Old 16th Mar 2019, 5:45 pm   #7
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Arrow Re: A d.c. voltage calibrator

Quote:
Originally Posted by Nuvistor View Post
I am trying to work out why testing meters with different sensitivity (resistance) wouldn’t affect the reading.
I quote the designer's words from the article:
"The necessary precaution (for accurate calibration) is to ensure no current flows from the calibrating voltage."
and . . .
"Since the voltmeter being calibrated is connected to the 'backing off' supply chain (RHS of the cct. diag), the voltage across the latter will be affected by the resistance of the meter: hence the necessity for RV2, the 'zero adjust' control."

I'm a bit concerned about his 'backing off voltage': RHS of the circuit diagram: it isn't stabilized. Surely that needs to be stabilized as is the 'calibrating voltage' on the LHS of the drawing.

Al.
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Old 16th Mar 2019, 5:50 pm   #8
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Default Re: A d.c. voltage calibrator

It only needs to be stable for the length of the test, which would be a few seconds, any drift would be seen in the m/a meter.
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Old 16th Mar 2019, 5:52 pm   #9
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Default Re: A d.c. voltage calibrator

The whole point is that you adjust the 'backing off voltage' with RV2 to be equal to the calibrating voltage for each measurement. And you do so in a way that means no current is drawn from the calibrating voltage.

So the calibrating voltage is what you expect. Thus the backing off voltage (which is the voltage across the meter under test) is also known accurately. The fact that the backing off voltage will change if the resistance of the meter under test changes is irrelevant. You connect the meter, balance the circuit with RV2 and then you know the voltage across the meter under test.
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Old 16th Mar 2019, 6:05 pm   #10
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Default Re: A d.c. voltage calibrator

I was thinking more of the mains voltage varying but that would be slow and the test would be done quickly. The reservoir cap should hold it steady enough for the test.
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Old 16th Mar 2019, 6:09 pm   #11
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Default Re: A d.c. voltage calibrator

And if it doesn't you'd notice that the 'galvanometer' was no longer on zero.
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Old 16th Mar 2019, 6:29 pm   #12
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Default Re: A d.c. voltage calibrator

Quote:
Originally Posted by TonyDuell View Post
And if it doesn't you'd notice that the 'galvanometer' was no longer on zero.
Yes
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Old 16th Mar 2019, 9:19 pm   #13
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Default Re: A d.c. voltage calibrator

Well, thanks for your thoughts and comments: much appreciated. I now have a better understanding of how and why this circuit works.

I'm toying around with the idea of making my own derived version of this. (The basic concept will remain unchanged). "Why?" you might well ask: indeed, a good question. The answer goes like this . . .

1. My main interest in vintage (and not so 'vintage') electronics is test equipment: collecting it, using it, repairing / modifying it - and designing / building my own.
2. Over the many years I have accumulated lots of 'vintage stuff' - such as H.V mains transformers, chokes, electrolytic caps., valves, etc.: more than I'll ever need.
3. This item - if a successful build is achieved - will be a useful addition to the kit I already possess.
4. And finally - like many others here, it's a hobby - I do it for fun. And that prompts for another topic for another day.

Al.
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Old 17th Mar 2019, 9:04 am   #14
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Default Re: A d.c. voltage calibrator

I think Tony Duell nailed it with his description. In fact, I'm sure I dimly remember something very similar cropping up in a DC Circuit Theory lecture many, many years ago -- and the opposite problem of measuring the voltage from a cell without drawing any current from it, in a school Physics lesson even longer ago. (A metre of resistance wire was involved; stretched taut along the usual polished hardwood base with a millimetre scale alongside it.)

When the circuit is balanced, and the galvanometer is reading zero, there is no potential difference across the galvanometer. This means the voltages on the LH and RH divider chains must be equal (and the voltages indicated on the LHS are accurate, due to the voltage stabiliser valves). But there is also no current through the galvanometer, so all the current flowing through the voltmeter under test must be coming from the RH divider chain.

Therefore, we know accurately what voltage is across the meter, without having to worry about it drawing any current from -- and therefore disturbing -- our accurate voltage source.
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Old 17th Mar 2019, 9:36 am   #15
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Default Re: A d.c. voltage calibrator

Mr. Wheatstone's bridge... We had square ones too, to look like the circuit in our old school lab.
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Old 17th Mar 2019, 10:29 am   #16
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Default Re: A d.c. voltage calibrator

Just like that, only much older-looking; in a darker wood, and with shaped edges (did they have routers in those days, or was it all done by hand with a chisel and sandpaper?) and a brass plaque bearing the instrument maker's name (probably Griffin and George). The resistance wire might have been replaced once or twice, since it would tend to get scraped and flattened in use as you searched and then pressed down hard to get a good connection, and this could affect the evenness of its resistance ..... but then again, it was just a secondary school, not the NPL, so maybe this was insignificant compared to other sources of error.

The underlying maths is "two things, each of which is equal to a third thing, must be equal to each other". The meter voltage and the RHS voltage must be equal to each other, since they are in parallel. The RHS voltage and the LHS voltage must be equal to each other, since the galvanometer is showing no current. Therefore, the voltage across the meter must be equal to the LHS voltage with no current flowing from it.
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Old 17th Mar 2019, 10:42 pm   #17
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Default Re: A DC voltage calibrator.

That device Julie mentioned may have used a Wheatstone's bridge as a part of the process, but in fact that was a potentiometer. You require a standard cell for accuracy (1.0186 if memory serves for a Weston Cadmium standard cell) but you must not draw any current. So, first you standardise an ordinary power cell (typically the old Leclance), then use that as the reference. The unknown is measured against that.
These developed amazingly during the last century until instruments could automatically and continuously monitor potential with accuracy.
The height of electromechanical excellence was the Kent Multilec which I have referenced here previously where everything was handled by a mechanism, from standardising the leclance, to self balancing the bridge, and displaying the temperature (in our case) with a pointer, whilst controlling the heat source to the kiln in question.
I suppose the developers would just not comprehend the little super accurate chip ridden devices we have at our disposal today.
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Old 25th Mar 2019, 4:13 pm   #18
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Question Re: A DC voltage calibrator.

I've built a breadboard version of this d.c. calibrator to investigate it and possibly to develop it further. One design aspect of it that concerns me are the 2k5 resistors in the potential divider chains: each 2k5 resistor is rated at 1-watt and yes, each of those resistors does indeed dissipate 1-watt (approx.) On test, at switch-on, it is necessary to wait for quite some time before the null meter stabilizes at zero. This appears to be because those resistors are changing value whilst they are heating and that change of value is enough to require that delay.
But if the value of those resistors are proportionally increased, keeping their ratios the same, that power will be substantially decreased. (Halving the potentiometer current will produce a quarter of the dissipated power). Obviously, fitting higher power resistors would help, but at present, I don't have any such 2k5 resistors in stock.

Can anyone foresee any problems in doing that design change?

Al.
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Old 25th Mar 2019, 9:32 pm   #19
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Default Re: A DC voltage calibrator.

It should work fine, since by design there is no current is flowing into or out of either resistor chain when the measurement is being made, so nothing is disturbing the voltage. On the RHS, all the extra current drawn by the meter under test is flowing through the potentiometer when the bridge is at null.
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Old 26th Mar 2019, 10:37 am   #20
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Arrow Re: A DC voltage calibrator.

Quote:
Originally Posted by julie_m View Post
. . . by design, there is no current is flowing into or out of either resistor chain when the measurement is being made, so nothing is disturbing the voltage.
Thank you for your reply.

Your first sentence: "there is no current flowing into or out of either resistor chain when the measurement is being made" cannot be correct, since there must be a current flowing through the meter under test, otherwise it would have no deflection! And that current will come from the RHS potentiometer chain. If that current is of such a magnitude that it 'pulls down' the supply voltage to the RHS potentiometer, then the so-called 'set zero' pot. will need to be adjusted to restore the 250 v.d.c., with the micro-ammeter indicating equality (or otherwise) of the two HT voltages.

A particularly attractive feature of this design is that the sensitivity of the meter under test (in terms of Ohms per volt) becomes irrelevant (within practical limits) for that above-stated reason. With the current flowing in the RHS potentiometer, a quick calculation reveals that a meter with a sensitivity as low as 1 kΩ/v. will be suitable for a reliable calibration check.

Al.
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