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 Post subject: Why is this transformer configuration used here?
PostPosted: Dec Mon 02, 2019 7:57 pm 
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Location: Leesburg, Virginia
Power transformer in a General Resistance Inc., 'Dial-A-Volt' model DAV46E. Instrument supposed to dial down to uV. What characteristics made for the selection of this open frame with primary and secondary separated desirable?

The complete unit is housed in the shell on the left. Thermal stability? Primary on the left 120V in via white/black pair. Secondary on the right the two red leads feed AC via the 150 ohm R to a Motorola rectifier chip (can use see the edge). The secondary has a pigtail sticking out; it is a solder connection between a yellow and green leads. Temp sensor? Output is 60V dc on the orange/black leads.

Must be a reason to pick this configuration?


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Mon 02, 2019 10:36 pm 
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W0MPM wrote:
Power transformer in a General Resistance Inc., 'Dial-A-Volt' model DAV46E. Instrument supposed to dial down to uV. What characteristics made for the selection of this open frame with primary and secondary separated desirable?

That transformer has very low capacitive coupling between the primary and the secondary. Why that's important for the instrument I don't know. Perhaps the idea is to have the output floating, to avoid ground loops that might introduce enough 60 Hz hum to prevent accurate measurement of the very small DC voltage differences provided by the instrument?

I guess it's an ovenized zener in that silver box. That's a nice instrument, does it use a Kelvin Varley divider? Are the resistors in an oil-bath? Six digits of accuracy is not easy to get.

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 2:43 am 
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Thanks Steve. In fact the 1N3029B 24V zener lives on the pcb next to the transformer enclosure. But they did go to a lot of effort (well about 4 extra .005% resistors) to elevate the 20V supply to the Kelvin Varley dividers above ground. So your explanation makes sense. Noise. Everything is shielded. There is a shield box goes over the Kelvin Varley set.

I have it doing good down to the 4th digit. Simply an exercise in learning. Not inclined to do more. Replacing all the caps both raised the output closer to the thumbwheel indication and reduced noise. But there is still too much noise/drift to justify expecting the 6th digit. Used an borrowed HP-34465A with data collection graphing all the good stuff to watch the output. Makes me want to have a spare $1000 to spend. ;-)

Stumbled on something else too. After taking the pictures and blowing them up I saw on the power supply a jumper from the top post to the fourth and it was right up against the second. It's an insulated jumper. But the insulation was hard cracked and the usual corrosion permeated between the second post and the wire. Stretched that wire away from the second post and then scraped all the rosin residue off that and some other places. Measured voltages on the power supply all shifted about 1 1/2 volt! Reduced the error from dial reading about 50uV.


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 2:16 pm 
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Interesting stuff. At 6 digits, even dirt on wires can throw things off. I wonder if it was ever actually that good when it was new? Do you know what the actual +/- accuracy spec was?

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 7:12 pm 
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Barry H Bennett wrote:
Interesting stuff. At 6 digits, even dirt on wires can throw things off. I wonder if it was ever actually that good when it was new? Do you know what the actual +/- accuracy spec was?


Nope. This is one of those boxes which even the vast reservoir of useless internet information fails to explain. I see some second hand listings but no data. General Resistance Inc seems to have built lots of these kinds of boxes. Some of the later versions show up but the single schematic I found was a much different newer topology.

Like you I'm somewhat skeptical it was ever useful at 1uV.

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 10:35 pm 
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It is not unusual to find such devices that have resolution that exceeds their accuracy. Sometimes for nulling purposes.

For devices that are expected to be accurate down in the micro-volt region, will have special switches, contacts, terminals and circuit insulation.

Charlie


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Wed 04, 2019 7:08 pm 
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In other words, it has precision, but not accuracy. :wink:

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Fri 13, 2019 1:50 pm 
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My suggestion would be to see what tolerance those carbon comp resistors have and try to find metal film resistors of the same or better tolerance if the originals are out or close to being out of tolerance.


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Fri 13, 2019 3:00 pm 
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In answer to the question that was initially asked, a couple of thoughts come to mind. First, notice that this is a "bolt together" design with two coils which are presumably on laminated cores, and two laminated arms linking them. This is a so-called dual bobbin design which is considered the best for safety (effectively double insulated). The distance between the coils also makes it less likely that high frequency line noise will couple capacitively from primary to secondary. Leakage flux would necessarily be higher with this kind of construction than with a typical E-I core, which is not as efficient but would produce a drooping voltage characteristic which might protect the rest of the unit in the event the output is shorted.

I do not think this was done as a cost-saving measure for a couple of reasons. First, they went to the trouble of shielding it in a can. Secondarily, somebody had to order and gather the parts, then put this thing together with nuts and screws. That would have been more labor intensive and therefore costly than simply ordering finished transformers from an OEM supplier with automated machinery doing most of the assembly.

Two thoughts about using this device. First, it appears that the only regulation--if any--may be a zener diode. All zeners are temperature sensitive to some degree, and subject to aging. Consequently, it is telling you what it is dividing the voltage by--but the voltage is only relative. It appears that you are supposed to connect a precision reference voltage to the "REF" terminals if higher precision is needed. The other is, it's hard to say without knowing the precision resistor values, but it is very possible this device was meant to be used with potentiometer type instruments like volt, millivolt, and thermocouple bridges. Such instruments do not draw any current from the source being measured when they are in balance. Consequently this "Dial-A-Volt" may not ring true unless it is used with that kind of instrument; even a 10-megohm DMM or VTVM could load it down so much that it will become inaccurate at millivolt and microvolt levels.

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