One battery or two

[Ohm_My]

I have this designed in my head but I need to know if it will work. I'm wanting to power two devices with different input voltages from a single 3S LiPo battery. I sketched out the design online and would like to know if it will work as is or does the circuit require two batteries?

All comments are welcome and appreciated.

Thanks.

[ledtester]

It'll work if it's ok that the devices share a common ground.

What are the devices?

[Ohm_My]

Device 1 is a ublox F9P GPS evaluation board (https://www.u-blox.com/en/product/c099-f9p-application-board).
Device 2 is a wireless 3dr 915MHz telemetry radio.

[ledtester]

You don't need the 9V step-down to power the ublox board - you can connect the 3S Lipo battery directly to the DC input jack -- have a look at page 13 of the User Guide:

https://www.u-blox.com/sites/default/files/C099-F9P-AppBoard-Mbed-OS3-FW_UserGuide_%28UBX-18063024%29.pdf

Also, the ublox board certainly has voltage regulators on it, and it's Arduino shield compatible, so it should have pins that will deliver +5V. In that case you will be able to power the 3dr radio board directly from the ublox board if it really consumes at most 100 mA.

In short, you don't either of the step-down converters.

[Ohm_My]

Wonderful. I hadn't thought about tapping off of the 5V supply for the Arduino shield but that would certainly work. I was just being cautious to use a voltage regulator for the GPS board even though it does accept a 6-17V DC input.

So it sounds as if I only need to power the GPS board straight from the LiPo and tap the 5V pin on the Arduino shield for the 3dr telemetry radio. It sure makes it easier when a second pair of eyes looks it over and offers up suggestions. 

Thanks for taking the time to look this over and commenting.

[Ohm_My]

Well its back to square one with this. Unfortunately ublox appears to be running way behind on the production of the GPS unit and their is no foreseeable delivery date. I've had the GPS eval unit on order from DigiKey since 8-6-2019 which has been on backorder ever since.

SparkFun electronics sells a somewhat similar unit that is a much smaller package (https://learn.sparkfun.com/tutorials/gps-rtk2-hookup-guide). The unit can be either powered with 5v USB or driven directly with 3.3v. It does not share the same feature that the ublox unit does in stepping down a much higher source voltage.

In getting back to your comment about "It'll work if it's ok that the devices share a common ground" has me somewhat confused. I would assume for all inents and purposes that ground is ground. Please keep in mind that my electronic theory is lacking and that any help is certainly welcome.

[ledtester]

In getting back to your comment about "It'll work if it's ok that the devices share a common ground" has me somewhat confused. I would assume for all inents and purposes that ground is ground. Please keep in mind that my electronic theory is lacking and that any help is certainly welcome.

If you have isolated circuits each has its own "ground" which is simply a chosen place in the circuit which is referred it as 0 volts.

For instance, take two battery powered flashlights. Each has a "ground" but there is no defined relationship between them. The voltage between the negative terminal of one flashlight and the negative terminal of the other can change in time. You can fix their relative voltages by connecting, say, the negative terminal of one flashlight to the positive terminal of the other one and then they won't be isolated from each other.

Sometimes you want isolated circuits. Suppose you have a circuit which monitors the mains power used by a device and reports it back to a computer. You want your computer to be isolated from this circuit to reduce the chance of it getting zapped by mains power. An opto-isolated USB cable can give you that isolation.

Lab power supplies which have multiple outputs usually provide isolated outputs. That makes them a lot more useful because you can then configure them in series or in parallel or just use them as independent supplies.

[Kasper]

On the subject of GPS and 915 MHz. You might want to check out iota tracker.
http://www.iotatracker.com

I think they had a good product and was expecting them to do well but then they shut down!

https://www.reddit.com/r/IOT/comments/80e5l5/iota_tracker_service_shutdown/?utm_source=amp&utm_medium=&utm_content=post_body