Thanks for posting. There's always something to learn from doing it differently.
Have you considered interleaving R4 and R5?
Good luck with the design!
I would make D3 a 1N400X series diode to handle the surge current for the electrolytic caps.
U1 needs a 0.1ufd cap directly across it's power pins.
As I mentioned in the main LTZ thread a few days ago, installing a zener in the collector of Q1 can help to prevent excess stress on the chip and to prevent damage.
You will want to minimize the distance (copper resistance) between the M2 and the M3 nodes.
Will this be for you lab use only or will it be transported?
The possible lower temperature in the non A version is a two sided thing: Aging is slower, but this also means it takes longer to get a "stable" state. The difference is not that large anyway. With an additional about 300 K/W and a typical power dissipation of the LTZ1000 (without the heater) of some 30 mW it's about a 10 K higher temperature that is needed for the A version. An extra 10 K will speed up aging by something like a factor of 2 to 4. With a relatively high temperature there is a chance to get the fast processes settled after 1 year or so, so that after that mainly a single slow process will remain active and thus drift gets predictable and allows for extrapolation.
The A version allows a lower power consumption, despite of the higher temperature set-point, unless very good thermal isolation around the non A version is used - but this tends to reduce the performance of the temperature regulation.
The temperature gradients on the board are more due to the power level. With the A version more of the temperature gradient is inside the case and less outside. So the A version can help to build a more compact version.
The logical way to fight temperature gradients (especially those than can change) would be a metal case all around the board and a rather compact design. The natural place for the heater controlling transistor would than be mounted to the case. An always slightly elevated temperature can also help with humidity effects, especially if the unit is always on.
I don't think it is worth using 4 resistors each in stead of a single one. This drives up the costs, but with very little advantage. There is not much to improve one the standard LTZ1000 circuit. The important point is to get the layout right so that not too much copper resistance is included and to keep RF noise out. A suitable stable thermal layout can also be important. If at all one might consider the modification that allows more capacitance at the output and thus more filtering there.
Doubling might be an option for the heater transistor: having a symmetric layout with 2 transistors could be used to reduce the possible temperature gradients due to the heater transistor(s). If well distributed, there heat could even be considered a partial temperature stabilization for the whole circuit. Another point is a limitation of the heater power to prevent damage - just in case.
LTZ1000 non-A
... The main reasoning behind using a non-A LTZ is so I can run at a lower setpoint, also decreasing long-term aging. And also as a side-thought, lower temp setpoint will induce lower thermal gradient on the PCB and in the enclosure.
Comments/questions are welcome as this project is in its infancy.
Hello CalMachine - the LTZ1000ACH has a much better thermal insulation - 400K/W vs. 80 K/W for old LTZ1000CH part - this means you need to pump in less energy in system for -A part vs. non -A to reach same oven setpoint ... ? Are you sure about your above statement in bolt ?
cheers
Butterfly
Because higher temperatures accelerate aging and decrease
long-termstability, the lowesttemperature consistentwith
the operating environment should be used. The LTZ1000A
should be set about 10°C higher than the LTZ1000. This
is because normal operating power dissipation in the
LTZ1000A causes a temperature rise of about 10°C. Of
course both types of devices should be insulated from
ambient. Several minutes of warm-up is usual.
There will be little to gain from quadrupling the resistor count, particularly with PWW, the power levels are very low and will have no significant effect on them as far as aging goes. In the case of film/foil, having a very small mass for a resistor element, the effects, however small, would be multiplied due to the very small mass. In either case, once the unit has achieved equilibrium and stays at a relative constant temperature, everything will settle down to a minimum drift with time, all it takes is patience, it will not happen in months, it takes years for it to happen. Even accelerated aging has its drawbacks.
You can just call me the Ronco Showtime Rotisserie Oven.. I'm just going to set it and forget it
I've attached my most recent schematic and PCB revision. Further comments/questions/advice is welcome
I've attached my most recent schematic and PCB revision. Further comments/questions/advice is welcome
Hello,
if you really use the BC639 (and not the 2N3904) be aware that the wiring on your PCB is wrong.
(the BASE is not the middle pin on BC639)
with best regards
Andreas
It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.
It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.
series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%
moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.
regards.
It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.
series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%
moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.
regards.
Accuracy won't be enhanced by default, only after characterization of each resistor and matching those with opposite properties.
Resistor-accuracy doesnt matter in this application, only long-term-drift, absolute tempco and matching-tempco of some resistors. Noise is sufficiently low due to resistor technology (pww or foil).
It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.
The PCB lines are quite thin in areas where they don't need to, this can add to trace resistance, especially around the 120 Ohms. The sensing lines to the reference usually go right up to the chip. With the 4 mA reference current it's mOhms that can matter.
The temperature sensor should be placed where it matters, so more like at the 13K/1 K divider, but not so close to the LT1013.
The 13 K / 1 K divider is rather close to the heater controlling transistors - this might not be such a good idea, as the transistor is a variable heat source. One can not use the heat from the transistor to compensate for environmental temperature change, as the heat from the transistor and the zener is approximately proportional to the current and the current is more like changing like a square root law with temperature. So for the transistors there are two good options: one is far away from anything critical - that is what most circuits use. The other good option would be mounted to a metal case that surrounds the whole circuit or at least the LTZ1000 part - in this case the transistor would be a secondary heater for the outer shell. However the contribution of the transistor would not be very large, so not sure it is really worth the effort.
Similar, if no extra driver transistor is used the heat from the LT1013 can be more of a possible problem.
It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.
series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%
moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.
regards.
Accuracy won't be enhanced by default, only after characterization of each resistor and matching those with opposite properties.
assuming a usual gaussian curve distribution of resistor tolerances, say 100 of 0.1% each, i would say
the composite (parelelled) would be pretty near 0.01%
and of course, tempco balancing would need characterization, at least 2 bins +ve and -ve tempco.
regards.
....
How do you feel about the idea of making a cap, like for the LTZ1000, but for the heater transistor?
... The most important is the heater resistor -ratio TC- but again absolute value is not too important; you do want to see some good stability and repeatably on those two, any problems on the heater ratio TC will certainly show up on the Vref output. ...