Approach to Temperature Regulated Project - Page 1
Forum section: electronics / projects

author: EPAIII
author_rank: Super Contributor
timestamp: September 04, 2023, 11:53:42 pm
content: Up front project description:I am trying to make a DC Voltage Reference for checking and calibrating ANALOG VOMs. Here are my present design goals:1. It is a one-off device for my own use. Not to be produced in any numbers: just one, I hope.2. It should not need calibration, at least not initially. So, it should be self calibrating.3. It should provide a wide range of DC Voltages from well under 10 VDC to, at least 300 VDC.4. Analog VOMs are usually specified in the 3% to 10% accuracy range. But I suspect that, at least the better ones are capable of 1% or perhaps even 0.5%. So I want a DC accuracy that is ten times better than that or 0.05%.5. Since it will be used as a reference Voltage source only, the output current will be low, <=10 mA.6. It will be designed for use in an ambient temperature of about 20C +/-5C or 68F +/-10F. This is a temperature that normal room air conditioners can reach in the summer and heaters can reach in the winter.Decisions I have already made:1. 50xx Voltage Reference chips will be configured to provide 10V and 2.V steps following the TI Application Report SBA203. I have 33 of the 5010s and 5 of the 5025s already purchased and delivered so supply chain is not an issue - hopefully.2. Individual mini banana jacks will be used for each 10V and 2.5V output step. This avoids problems with actual mechanical switches. And it allows 94 output Voltages from 2.5 to 310 VDC in 2.5V steps.3. Unit will be powered from AC line power, not batteries.4. Unit will have a digital display of the AC line Voltage. Since it is only used to confirm that the AC line is in working tolerance, this only needs to be accurate to 1% or 3%.5. Unit will have several (3?) LED indicators to show the various parts of the circuit are functioning.6. The reference circuit will be in a temperature controlled area of the enclosure. At present my goal is to maintain the temperature +/- 1degC using a Arduino or similar controller. But a smaller percentage would be better. There should be a digital display of that temperature.7. The actual operating temperature will be somewhat above the 20C/68F ambient mentioned above. This will allow it to be controlled with heaters so cooling will not be needed.8. The outside enclosure/box will probably be aluminum. The VR string, heating resistors, and temperature sensor will probably be surrounded with an inner enclosure that may have some insulation.9. A display (LCD?) will show the temperature and how long it has been stable.Design:The TI Application Report provides the design of the regulator string. It states that this design has been used for much larger strings of 1000 or more stages for much higher Voltages so I anticipate no problems there.I have a preliminary power supply arrangement for 325+ VDC and 12 VDC. I plan to add a 5 VDC regulator to that as needed. I hope to use the unregulated 15 VDC for the heater current. I will start another thread to discuss that separately as I want a sanity check on my approach.I have not looked into the actual temperature control yet other than the two general thoughts of using a micro-processor (Arduino) and some resistors distributed about the mother board for the heat sources.Edit: I have NOT designed the enclosure yet. I do have several preliminary versions of the front panel, but even those are incomplete. I feel certain that the front panel will have some insulation between it and the mother board which will be behind it, supported by the banana plugs. This insulation would be there to prevent body heat from effecting the circuit as my hands work the controls or change the connections. End EditMy First Question:I have successfully designed a number of projects, both professional and personal, which incorporated both a power supply and operational circuits (digital and analog). Those aspects of this do not worry me. What I am a bit unsure of is the temperature control. I hope to find and copy a circuit and code for whatever device I pick to use for this. But, what worries me is the temperatures involved and the amount of heat already generated inside this box (5-10 Watts by rough calculation) and the amount of heat that will be lost to the ambient (room temperature) space around it. I have no experience with thermal design other than gross measures like "a big heat sink" or "a fan will cool it". I have a significant investment in the 50xx ICs and don't want to smoke them.While the 50xx devices are rated up to 125C, I do not want an operating point anywhere even near that point. I am thinking 28C/82F, but have no actual engineering reason for that number. It is simply 8 degrees C higher than my ASSUMED ambient temperature. Again, the unregulated 15 VDC times 1/4 A (0.25 mA) is 3.75 Watts. But that is just a wild guess/assumption. Is 3.75W enough to heat my circuit to 28C/82F in a reasonable time and keep it there?What I need is a design procedure that will, at least get me in the ball park in the thermal design of this project. As I see it, the biggest mystery and the starting point is the amount of heat that will be lost from the VR circuit inside the box to the outside ambient air. I need a way to calculate or at least estimate this so I can build the outer box and any inner enclosure and have space on the mother board for the needed resistors/heaters. And then I need to arrive at a first estimate of the operating temperature.Or am I totally off base.All suggestions and comments are welcome and appreciated. But please stay away from "no, no, no, the chip you need is xxxx" or "just one set of banana jacks and a digital Voltage control". I am PAST those decisions - PLEASE! I am asking aboutthe temperature control.PS:I do know I can probably get away without the temperature control. I know we are talking about PPM/degC and only a few degrees C if it is used indoors. But I do want to include it. Bells and whistles, and all that whop-tee-do, you know! Good knowledge and experience for future projects.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 05, 2023, 02:38:10 am
content: Quote from: EPAIII on September 04, 2023, 11:53:42 pmAll that will depend very much on your insulation design, so you should mock up something, using whatever you intend for insulation and measure it.Everythingthat passes through the insulation, represents heat loss.Sounds like you have a lot of wires, so either very fine wires, (labour intensive and fragile) or a flat-flex cable that is deliberately long and zig-zags thru insulation layers ?If you search for OCXO, you may find useful info on watts, insulation and stabilize times.