Shorting external regulator and internal regulator


I have a hardware question to submit to the community. I want to power the Spark Core from a car battery so I’m using an external step-down regulator with a range up to 30V.

When I connect the Core through USB to debug using Serial, I am wary of connecting together the outputs of the 2 voltage regulators (external 12V to 3.3V and internal VUSB 5V to 3.3V). I removed D1 (the diode from VUSB to the input of the internal regulator), soldered a wire from the VUSB pad to an external diode then to the input of of external regulator.

Do you agree with my caution or do you think nothing bad will happen if both regulators are connected together? Is there an easier way of doing this?


If you are worried about conflicts, maybe you could just disconnect the 12V when you plug in the USB ?
I would not give you insurance one the other way, but probably would be ok.

@jvanier, do you have a schematic you can share. I find it much clearer to work with. :smile:

Sure :smile:

The issue about disconnecting the 12V is that I need to see what’s happening when the Core is actually connected to the car. So that’s not really an option for me.

Well, how would it be if you just disconnected the positive , and left the ground (negative) connected to the car?

If I understand correctly you are concerned about the external 3.3V regulator interfacing with the on-board 3.3V LDO regulator of the Spark Core?

If you mean the 12V and plugging into USB at the same time, that should also be fine based on your above schematic. I’m glad you removed D1… that’s the real issue. If you power the on-board 3.3V LDO and an external one at the same time, they will fight each other for the correct 3.3V value and cause oscillations.

Since you removed D1, the only way to power the on-board 3.3V LDO is via the VIN pin directly now. Don’t do that and your circuit will work just fine :smile:

I would also add a large bulk 100uF or more and small decoupling capacitor 0.1uF to the 12V and GND (white and black wires). Depending on what 3.3V SMPS you used, you may want to include some capacitance on it’s output to GND. Just look at it’s datasheet and see what it recommends.


Awesome reply @BDub.

It confirms my thought that 2 regulators should not have their output connected in parallel even though they output nominally the same voltage.

I’ll add a large bulk decoupling capacitors on the input as you suggest. This regulator is from Adafruit and the datasheet says that small input and output decoupling capacitors are included.

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