Re: 453 calibrator

Fabio Trevisan
 

Hello Lop Pol, and all the gurus,

Here's one that got me thinking...
Looking at the output stage, fed by 12V through a 12R Resistor and a saturated switching transistor... Isn't it too marginal to expect 1V across 200 Ohms, through a 2k19 resistor?
Does it really ever reach 1V?
Just making the math: 200R + 2k19 = 2k39 x 5mA = 11.95V! It leaves 50mV for the voltage drop on the 12R resistor AND the Vce(SAT) of the transistor!
Only the average current of the output stage (5mA @ 50% duty cycle = 2.5mA,) through the 12R resistor already eats-up 30mV! and the same resistor still have to feed the oscillator... What's left for the transistor voltage drop? The smaller voltage drop I've seen, for any saturation current curve, for this type of transistor is around 70~80mV... and that's the end of the line of the lowest current graphic!
And Tek claims the calibrator specs to be +- 1% (over a normal temperature range)?
I know Tek wouldn't usually screw up on such a basic thing but... Didn't they stretch it a little bit too far here?
And yet, 2k19 is not a standard value even for E192 series... and Tek lists this resistor as a 0.25% precision resistor !?
I can't understand they went this far in choosing a hi precision non standard resistor, for a design that can't make it on-spec even on paper, let alone real world.
Am I missing something here?

Rgrds,

Fabio

On Tue, May 8, 2018 at 09:26 am, lop pol wrote:


On Tue, May 8, 2018 at 09:04 am, Jeff Urban wrote:


Don't you just love undocumented modifications ?

But still, have a pity party for people who work for ISO compliant
companies.
If they modify something they have a mountain of paperwork. I would just put
a
note inside and out but that ain't good enough apparently.

Actually I do understand why - it has to do with company value and how much
trouble it would be to liquefy those "assets". But like everything they get
ridiculous,

So, let's guess the resistor values our budding ex engineer used.

You get 6 X the output so I would assume the values of those 3 resistors at
the bottom are approximately 6 X of the proper value, err the proper value
for
the rest of us that is. But they might have to be a bit higher than that. I
doubt he was going into 50 ohms, he wouldn't get much. More that is.

Am I close ?
What threw me off the most was.. I checked around there yesterday. R1275 is
supposed to be 180ohm i checked it, it checked good 180ohm I started
scratching my head. I checked it this morning and man did I goof up last
night. It was 1.80K ohm. I need to learn to look at stuff correctly. I can't
be mad about the modification seems they got what they wanted from the
calibrator.

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