LM399 based 10 V reference

[dietert1]

Looks good. I'd have five proposals:
- Select a metal enclosure and make board to fit the mounting holes
- LM7818 instead of LM7812 for better LM399 oven performance, mount to enclosure
- Solder or screw terminals instead of supply socket
- Add separate output Gnd terminal
- If PTC heater won't be used, move temperature sensor closer to the 10 V to 7 V divider resistors

Regards, Dieter

[Birb]

Thanks for your feedback!
Perhaps I was slightly unclear - the PTC heater will remain in use, but I do not need the 5V regulator to be placed together with it.
However, using a higher voltage might cause the 5V regulator to dissipate a larger amount of energy and that may cause heating issues - I'll see.
Actually, the temperature sensor will have some wires to extend it to be inside the enclosure for the resistors (I have some insulating fiber/ foam material)
As for a metal case, I will probably use an extruded aluminium one.
The ground terminal is likely connected through external wires.

The socket was chosen because some 24V lithium battery packs, as well as most power plugs use this type of connector to connect.
I could consider to break it out and use wires directly, I suppose.
Though I haven't figured out a good way to keep it on continuously.

[iMo]

It has been discussed here many times - the voltage regulator inside the box or close to the reference circuitry is not the best idea in case your input voltage will vary (ie a battery or mains transformer). That is because the heat dissipation of your 7812/18 will vary with the input voltage changes..

[Birb]

Then perhaps I will separate the PCB into two, to try and isolate the heat better.
Or I may need to figure out a different power solution?
Though using a higher voltage rated PTC may be able to allow me to continue to heat the resistors without adding the 12V to 5V conversion, at the expense of higher temps for the resistors and current draw.

[iMo]

There are two options (when using linear regulators, what is usually recommended with Vrefs):

1. to leave your voltage regulator on the Vref board as-is in your design, AND to add the second one in front of it and have it mounted outside the Vref board
2. to have a single voltage regulator outside the Vref board (thus not thermally coupled to the Vref circuitry).

Mind the chip temperature of ANY linear voltage regulator is aprox:

Tchip = (Vin-Vout)*Iout*Rthja+Tambient, where the Rthja is the thermal resistivity of the junction to ambient (a constant).

[Birb]

Thanks for the feedback.
By second one, do you mean the 12V to 5V linear regulator? Thanks.
At 12V the temperature is around 60C
I haven't figured out how to effectively reduce the setpoint, but it may include adding a layer of insulation between the PTCs and the resistors, so the resistors are kept at ~50C or lower (preferably)
Is this more optimal?

[iMo]

Thanks for the feedback.
By second one, do you mean the 12V to 5V linear regulator? Thanks.

The second option means your "input to 12V" regulator (the 7812) should be off of the voltage reference board.
Also the 7805 powering your PTC will change its temperature as well - its in/out voltages will be "constant" but the current will change.
Simply look at the above equation and try to apply it to your design accordingly..