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Topic: Powering 5VDC shield electronics. (Read 874 times) previous topic - next topic

Brian Neltner

Aug 03, 2011, 11:07 pm Last Edit: Aug 03, 2011, 11:13 pm by Brian Neltner Reason: 1
Dear forum,

I have a project where I need a high current 5VDC (well, not that high of current, perhaps 100-300mA).

In previous projects, I explicitly did not power the arduino via the +5V pin, because according to my read of the schematics, if VIN/2 is less than 3.3VDC, then GATE_CMD will be zero, resulting in an electrical connection from USBVCC to +5VDC.

My read is that if VIN is supplied with 6.6VDC or greater by either PWRIN or a shield, then the USBVCC will disconnect from +5VDC and the onboard +5VDC regulators (why there are two in the schematic I don't have a clue) will provide +5VDC, which should work fine while plugged into USB. However, if you are directly supplying +5VDC to the arduino using a shield, without applying VIN > 6.6VDC, then if there were a small voltage difference between USBVCC and +5VDC, it would cause a massive current loop through the system's two competing power supplies.

In the past I've had to handle this by using a switching regulator to take 24VDC in my shield down to 7.5VDC supplied to VIN. Then I took +5VDC from the arduino regulator to produce the somewhat high current 3.3VDC I needed on a shield.

In this case, I'd like to avoid such a convoluted route. I have perhaps 100-300mA of shield electronics running at 5VDC. I would like this to all be powered from the regulator on the arduino. I already have a separate 5VDC power supply to run the analog half of my electronics for noise reasons, so it would be annoying if I had to put on both at 7.5VDC supply, and *two* 5VDC regulators.

The most optimal way to do this, of course, would be for me to build my own 5VDC high power switching power supply on a shield, and power the arduino, the digital half of my electronics, and the analog half of my electronics each from separate low impedance connections to the same filtered supply. But I think this breaks the Arduino badly when it is plugged into USB simultaneously. Is this still true? Is there some clever workaround?

Any thoughts?


Let's see if I get this straight: you want to connect over USB (for serial communication) but you also want to power the Arduino from an external 5V power supply connected to the +5V output pin. Right?

I doubt there will be a massive current loop anywhere because a) there is a 500mA PTC in series with the USBVCC power supply, and b) internal wiring resistance on the PCB traces will serve to cause current load sharing between the two supplies.

It's still not the greatest idea IMHO as if your external supply decides to go rogue (or your hand slips on the voltage knob) and climbs up to 6V, 7V, whatever then you could damage the USB port on your computer (Method #5 of destroying your Arduino).

I'd consider going with an external 7V supply and using the DC power jack.

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Brian Neltner

Sorry, I accidentally submitted the post before I finished writing it. I think it's complete now.

What I have is an external 24VDC power supply. On my board, I am building a switching regulator based on (for instance) the LM2675 to get some on-board fairly high power voltage. I can make this voltage 5V or 7.5V. Obviously I don't want to attach 24VDC directly to the arduino due to the power dissipation issues in the arduino on-board linear regulator.

If I use 7.5VDC to power the arduino, then I need to pull +5V from *somewhere* to power my shield electronics. In the past, I've pulled it from the arduino +5V regulator through the +5V connector. However, I'm nervous about adding an extra 100-300mA of load to that regulator. So, unless that regulator can handle more current than I'm thinking, I'd need a separate +5V supply on my board connected by a common ground. That's not the end of the world, but it means having 2 LM2675s and a linear regulator per board, one for the 7.5VIN, one for the +5V used by the shield's digital electronics, and optionally an additional 5VDC linear regulator fed from the 7.5VDC switching regulator to provide a clean voltage for my high resolution analog electronics.

Because the ADCs and DACs are both 8 channel 18-bit and 16-bit (respectively) devices, it's important that the noise on my +5V line for the analog electronics be clean. Ideally, I'd have less than 0.1mV of peak to peak noise from the digital electronics, although the power supply rejection should make that easier to attain. I'm using thermocouples, so even 1mV of error on the conversion is equivalent to an error of 24C. This is readily achievable, but for instance attaching my supply voltages between my 20 channels of 100mA digital outputs to the analog chips is almost definitely a bad idea =) The linear regulator from 7.5VDC should work reasonably well though.

So I guess what I'd like is if I could power the digital electronics on my shield from the Arduino's 5VDC output pin, and then just have two regulators on my shield -- a 7.5VDC switching regulator, and a 5VDC linear regulator special for my analog electronics. My board is already quite crowded, so I'd like to avoid adding more. Unfortunately, the datasheet for the Arduino Mega 2560 does not specify a maximum power for the on-board 5V regulator.


If I use 7.5VDC to power the arduino, then I need to pull +5V from *somewhere* to power my shield electronics. In the past, I've pulled it from the arduino +5V regulator through the +5V connector. However, I'm nervous about adding an extra 100-300mA of load to that regulator.

In our lab tests on the Ruggeduino the 300mA on the 5V output is no problem when the input voltage is less than 9V (and 7.5V is even better). I imagine you will get similar performance from an Arduino and think it's worth trying for the simplicity of it.

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Brian Neltner

Oh, that's a neat little board. I see what looks like a ton of input protection resistors on the output pins, but where are the resettable fuses? I've seen SMT fuses before (and used them for critical stuff), but I'm amazed that they sell a resettable version in a small enough package to make it feasible to put on a board. Very cool!

I am curious though why you wouldn't have replaced the 5V linear regulator with a switcher. For instance, my favorite switching regulator is this one:


which can handle 1A of output power, requires only five fairly small external components, and can accept input voltages of up to 40VDC with no damage, and operates at 90+% efficiency.

Just a thought for your next revision. I love those suckers!

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