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I am trying to design a system for a sap flow sensor with a heated needle in a tree. This particular sensor has a heater resistor of 50 ohms, and the application calls for a 3V input and ~0.2 W power.

I am also using a data logger that requires a 12V input to log the sensor output. I can connect the 12V battery to the logger no problem, but I am trying to find the right way to handle the 12V to 3V conversion from the 12V battery (we're using batteries like these and will be replaced and recharged as necessary)

I am starting to understand that a voltage regulator may be the right avenue, but I do not have experience building or buying these. The manufacturer that sells the sensors also can supply us with voltage regulators, but these are $500. These seem really expensive and beyond the scope of our budget. After a quick google search I see already-built voltage regulators that are less than 5-10% this price. What gives? Am I missing something here?

I am also learning that these voltage regulators release heat based on the voltage drop and supplied current and some are more efficient than others. The sensor/needle will need to be constantly heated (i.e. supplied the above 3V) for ~18 hours each day and will not receive power for the remaining hours at night. These sensors will ideally be continuously logging for weeks to months and left in a forest. I will be monitoring these sensors periodically, but not every day. I don't really know how to evaluate purchasing or building cheaper voltage regulators for my application. I just want to be sure nothing get super hot or melts(?) for the duration of our research project.

Does anyone have any suggestions how best to tackle my problem? For me, it is as much a technical problem as it is a budgetary problem. We require 10 set ups, so 10 voltage regulators at $500 each is too much for us to justify before looking into other options (e.g. other sensors, other research questions, etc).

Please let me know if any other information not in here would be useful.

Thank you!

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  • \$\begingroup\$ If you want something very power efficient with very low standby power you could use something like this: ti.com/tool/tps62175evm-098 You could build your own with the design documentation given or you could even buy the boards directly from TI. A single resistor change would lower the voltage from 3.3V to 3V. You have no output ripple or voltage tolerance specified, but it may be important to take into consideration. \$\endgroup\$ – John D Apr 5 '18 at 16:12
  • \$\begingroup\$ To use 25% of voltage linear regulators will be at best 25% efficient but step-down buck regulators are cheap and efficient 80~95% from Amazon or Bangood but adding post or courier charges in mind. Ambient air temp must be specified and accuracy of internal temp and then tolerance of temperature regulation. Insulation will significantly save power and with good insulation regulate to 1~2 'C, so Define the physical design. Combining all functions with data logging and RAM can be done with rPi or various Arduino kits at very low cost if you research web. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 5 '18 at 16:13
  • \$\begingroup\$ Revise question, and start with a simple spec to define all inputs, processes and outputs in a hierarchical way. or as IBM called it (HIPO). Do not specify hardware unless absolutely necessary, only interfaces , functions and environmental stress factors ( rain , lightning) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 5 '18 at 16:17
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You can use a linear voltage regulator, or a switching DC-DC converter.

  • Linear: low efficiency, low noise.

0.2W at 3V is 67mA. A linear regulator will draw the same current, ie 67mA from the battery, and dissipate 0.6W as heat. The heat won't make the regulator burn, as the power is very small, but it still wastes a large part of the battery energy.

Your battery is 3.4 Ah, and it's lead acid, so it is better not to discharge it too deep. Let's go with 50% discharge. So half of 3.4 Ah is 1.7 Ah, divided by 67mA gives 25 hours battery life. Not that good.

  • Switching DC-DC converter: high efficiency, higher noise.

Let's use a crummy switching converter, with 75% efficiency. 0.2W on the output means 0.2/75% = 0.27W from the battery, which on 12V is 22 mA.

This is 3x less current, so you get 3x longer battery life.

The deciding factor should be how much power the data logger uses. If it uses a lot more than the heating element, then optimizing this is a bit moot... however if the data logger is very low power, then it's worth it.

Since this is a flow meter, the accuracy will depend on how accurate the heating power is, which depends on the accuracy of your regulator, so you need to keep this in mind.

I mentioned noise, because switching converters do generate a bit of high frequency noise. If the data logger is properly designed it won't have any problems with this, but it is still a good idea to test the setup before going into the woods.

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Voltage regulators (linear regulators) are very cheap components (a few $). Because your application is battery powered it would be better to use a more efficient solution (linear regulators just turn excess power to heat) like a small dcdc converter module. DCDC converters in that power range are also very cheap but offer a much higher efficiency. Just google "dcdc converter module" or something similar and you will find what your looking for.

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After a quick google search I see already-built voltage regulators that are less than 5-10% this price. What gives?

For your application, small linear voltage regulators (LDO) would work and are quite cheap (at least not 500$ a piece), but you've found out that they do heat up rapidly as the voltage drop increases so they are not recommended here. Then you would need a step-down buck converter, as suggested by Tony Stewart, which does not work the same as LDO and thus do not heat the same way. There are a lot of parts suited for your needs mostly from Texas Instruments, Microchip and STMicroelectronics. They have a low-cost, require few external components, hence easy to use, and do the job under working conditions specified in their respective datasheets.

For example, I would suggest something like the LM2594 series from Texas Instruments that can provide up to 500 mA, way more than what your sensor requires :

Current determination

The matter with this converter is the relatively poor efficiency with low output current. At best, you would get something like 70% conversion efficiency, thus your battery would provide :

Battery output current

instead of 16 mA, and your buck converter would dissipate almost 0.1 W. This should be taken into consideration for how long your battery will be able to power your sensor as well as your logger.

You can browse TI's product line for buck converters and specifiy your input and output parameters, or even look for low-current high-efficiency converter. They shouldn't cost more than 10$ a piece, even with external components. What will be difficult however is to find a suitable package regarding your options for self-made circuitry. Surface Mounted Devices components might not be as handy to use as through-hole ones. Good luck!

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