Part 2 LTZ1000A of Andreas
Mechanical
All is placed in a aluminium Euro-Card case EG2 of ProMa with dimension 168*103*56 mm.
http://www.reichelt.de/Proma-Gehaeuse/GEH-EG-2/3//index.html?ACTION=3&GROUPID=5199&ARTICLE=50424&SHOW=1&START=0&OFFSET=16&Top side from left to right:
D-Sub connector for the main output with neighboured pins for the Zener output.
Next to the D-Sub is the auxiliary connector for a daughter board
Below the D-Sub you can see one of the auxliary 4 mm banana plug connectors (Hirschmann).
Both connectors are placed intentionally on the same height of the reference.
I cannot understand why on most instruments the positive (red) plug is always higher
(and hotter) than the negative (black) plug which is then cooler. Especially when having
a device with a power consumption that makes it necessary to use a fan.
In the middle the reference section with the parts around LTZ1000A. They are all placed into a TEKO 3710 metal shield with top cover.
http://www.reichelt.de/Teko-Stahlblech-Gehaeuse/TEKO-3710/3//index.html?ACTION=3&GROUPID=5202&ARTICLE=34042&SHOW=1&START=0&OFFSET=16&The teko shield has the intention to equalize thermal gradients from outside to the reference section.
And calm down air currents.
All resistors (pre-aged before use) are in the upper half of the shield.
The LTZ1000 is thermally shielded with some foam.
Since the PCB is not plated through I could not settle the LTZ1000A directly on the PCB.
So the legs of the LTZ are surrounded by additional foam to keep air currents away.
This can be better seen on the closeup (IMG_4671.JPG) which I posted May 29, 2013 (page 12) in this thread.
The BF245C FET for the current source is above the LT1013.
The NTC for temperature sensing of the board temperature is near the LTZ
(between the FET and the LTZ).
The air gap between the top plate of the TEKO and the rest of the cirquit
is filled with additional polystyrene foam material (plate with 10mm thickness).
The right half of the aluminium case is filled with 12 AA cells in 2 battery holders
which are fed by a simple constant current charger during charging.
Between the TEKO inner shield and the battery holder some parts of the voltage regulator can be seen.
Bottom side:
On the bottom side you can see that the zener voltage is Kelvin sensed at the LTZ1000A
and going directly to the neighboured pins of the D-Sub connector only being connected
to the output filter capacitor.
There are many additional capacitors and some cirquit modifications which where not planned
from the beginning but which are already included in the cirquit diagram.
The base emitter capacitors at the LTZ are connected as close as possible to the LTZ1000 pins.
On the right side lower the low noise 14V power supply around the LT1763 is built.
Unfortunately the Aluminium case has not much place on the side below the pcb.
So there can be no metal plate for the LTZ-Section on the lower side.
There is only room for a 4 mm polystyrene foam sheet for thermal isolation.
By the way: for first firing of the cirquit I did not use the LTZ1000A but replaced the
zener section with a self built "refamp" consisting of a ordinary zener and a transistor.
Just to look whether the current regulation will do it's job properly, especially
when powering up/down.
Shielding and guarding:
ince I use a battery powered design where all components are within the aluminium case,
and no mains line can introduce any common mode noise to the cirquit,
the topic of shielding and guarding can be much simplified:
I have only a guard and dont need a outer shield (connected to eart ground)
All parts are "inguard".
A 2mm banana plug connected directly to the EG2 aluminium case can be used to connect
to the guard pin (or if not available to the negative pin) of the multimeter or calibrator.
Of course I have a large metal plate connected to earth ground on my desk which I use as outer shield
during my measurements. The guard of the LTZ is isolated against shield by some bumpers.
problems observed with the cirquit:
A short ciruit to the (unbuffered) output will set the heater setpoint to a large value.
This shifts the output voltage of the LTZ1000A. (Hysteresis probably due to the die attach).
Fortunately I could remove the hysteresis by simply power cycling the reference for several times.
When measuring immediately after charging Im observing some shift to the output voltage.
Probably this is due to thermal gradients going across the PCB due to the "hot" AA-cells.
todo list will follow ...