Selecting power

Hi everyone, I asked diodes about this circuit but now I have another question.
I am using stepper motor and some sensors. Stepper motor needs 24v and the other devices needs 5V and 3.3V.
I will use this PCB with adapter. I won't plug USB cable, I will use USB cable just once and It will be for uploading code. Then I will use adapter for power all circuit.
If i need to change something in the code (if i need to speed up the stepper or change something else), I will plug cable and try to change. While USB cable plugged, I guess I need to unplug the adapter for protection.


I used 2 diode for protection. For example if USB cable plugged, VCC won't power VOUT or when 24V adepter connected the circuit and USB cable not connected, V_USB won't powered.

  • What happen if USB and 24V adepter connected at the same time? Will something blow up ?
  • How can my system work when both are connected? For example 24V will power only stepper and USB will supply 3.3V regulator. When I see code is working correct, I will unplugged USB and 24V will power all system (Motors, 5V, 3.3 Regulator etc.) (I don't know much about electronic components so is there another solution for my circuit?)

The attached schematic is for an Arduino based GRBL(CNC) controller that has two external power supply options: USB and 24v. INTERNALLY the board has three voltage levels:

5v for the Arduino ATMEGA328p chip / logic
12v to power a laser
24v to power a Spindle Motor

When supplied with USB only: the Arduino/logic are active. Laser/Spindle inactive
When Supplied with 24v: Logic, Laser 12v and 24v Spindle are active.
When Both USB and 24v are supplied concurrently, Logic, Laser 12v and 24v Spindle are active. No harmful over-supply occurs.
CAMXTOOL_V3'5_Sch(rev0).pdf (228.3 KB)

I understand the differences between your application and the GRBL board. And I have not answered either of your two questions. But hopefully this gives you some ideas to try.

Dan
San Jose

Thank you, this is huge schematic it really helps. I don't understand one thing, USB's power called 5V and 24V Adapter's power called 5V too. How does the arduino know which one to get power from? Which one going to supply Arduino/logic's 5V when both connected ?

If this schematic work like you mention, my schematic will work fine too right? Again thank you.

I do not see a 24v adapter in the schematic. Only an input jack labeled "DC in 24V" meant to receive power from an outside power source such as a battery or an adapter.

Assuming you mistook the power supply section (shown below) for an adapter, I have pointed out the three voltage levels below.

You can safely connect USB and 24v at the same time.

If you disconnect 24v, but leave USB connected, the 5v parts of your circuit will stay on, while the 24v and 12v sections will be off.

If you connect 24v, and remove the USB, all three voltage levels will be on.