safe to connect USB with DC IN selected

Combining the power of the ATmega2560 MCU with onboard Ethernet, a microSD card slot, an efficient switchmode power supply, and a small prototyping area. [Product page]
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effgee
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Joined: Tue Aug 07, 2012 2:41 am

safe to connect USB with DC IN selected

Post by effgee » Mon Sep 17, 2012 4:55 am

probably a stupid question, but better than toasting my ethermega.

I want to run 24VDC into the VIN pin of my ethermega, and select the jumper to DCIN.
Once I've done that, can I safely connect a USB cable to program the device ?

I've just been running off USB power so far, but need to break free from my USB cable.

effgee
Posts: 64
Joined: Tue Aug 07, 2012 2:41 am

Re: safe to connect USB with DC IN selected

Post by effgee » Tue Sep 18, 2012 10:57 pm

(bump)

can anyone answer or comment on this;

Can I safely connect my PC to an ethermega with 24VDC on the Vin pin, while the ethermega is getting its 5V from 5V_PSU (jumper 5 across pins 2&3) ?

The schematic suggests its OK, but I don't want to risk my PC. (If I thought only the mega was at risk I'd have done it already).

cef
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Re: safe to connect USB with DC IN selected

Post by cef » Wed Sep 19, 2012 11:16 am

As long as the jumper is set to 5V_PSU, you should be fine, with all of the project powered off the power supply.

BTW: Note that 24V is close to the top end of the regulator on the EtherMega (28V). Always wise to check that your supply is putting out what you think it should be, before plugging stuff in.

effgee
Posts: 64
Joined: Tue Aug 07, 2012 2:41 am

Re: safe to connect USB with DC IN selected

Post by effgee » Thu Sep 20, 2012 1:22 am

thanks cef.

I'm confident the 24V supply is below 28V, I'm more worried about earth loops and conflicts between the 5V usb signals and the 5V supply on the mega. On reflection I guess it should be no different than plugging a pc into a printer in terms of voltage differences

cef
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Re: safe to connect USB with DC IN selected

Post by cef » Thu Sep 20, 2012 4:41 am

If you're worried, hook it all up and before you plug the USB connection in, then check for a voltage difference between the ground on the Mega/your 24V supply and the PC ground using a volt-meter.

The only issue I can think of is if the AC ground pin is connected to the DC side on the PC and on the 24V supply (ie: say +12V on the PC and 0v/Gnd on the 24V supply).

Note: If you're using a buck-converter to get 24V from another voltage, and that voltage is from your PC (eg: 12V) then confirm that the ground pins on the in/out of the buck-converter are common (ie: the same), otherwise you will have issues.

SkullKill
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Location: Perth, WA, Australia

Re: safe to connect USB with DC IN selected

Post by SkullKill » Mon Oct 29, 2012 4:29 am

Jumper on "DC in"
EtherMega connected to 12V PSU

Laptop not connected to mains,
laptop connected via usb to mkII AVR to ISP of EtherMega (AVR not powering the EtherMega).
Laotop connected via usb to EtherMega .

the above setup was fine for me.

effgee
Posts: 64
Joined: Tue Aug 07, 2012 2:41 am

Re: safe to connect USB with DC IN selected

Post by effgee » Sun Nov 11, 2012 12:24 pm

SkullKill wrote:
the above setup was fine for me.
Yes, all worked out for me too, with 24V supply to the EtherMega.

I'm only getting ~4.7V on the 5V rail, and AFAIK I'm not really drawing much current. Seems to run OK though, just wondering what others see on their 5V rail.

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