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SiriusHardware · 21st Mar 2020, 1:17 pm

When using the PROM reader sketch (which requires the UNO to be connected to a PC so that the serial connection can be used) the UNO will be running on power provided by the PC through the USB connection. I understand 'normal' USB has an upper current limit of 500mA allowed, so that's the absolute limit but the connection between +5V in on the UNO USB connector and the +5V out connection on the single row IDC female connectors is switched by an Si2301DS MOSFET.

The source and drain current capability of this device is above one Amp but its maximum power rating is quite low - not a problem as long as it is switched fully on and there is no volts drop across it since (any current) times (no voltage) = no Watts.

Obviously in the real world the device will have a small amount of 'on' resistance but it should be OK, I think, to draw 200mA through the PCB. If in doubt you could always provide the PROMs with a +5V supply of their own. For example, you could just temporarily tap the power for the PROMs from the +5V pin on the USB socket, bypassing the MOSFET.

21st Mar 2020, 1:17 pm	#323
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: MK14 schematic revisions When using the PROM reader sketch (which requires the UNO to be connected to a PC so that the serial connection can be used) the UNO will be running on power provided by the PC through the USB connection. I understand 'normal' USB has an upper current limit of 500mA allowed, so that's the absolute limit but the connection between +5V in on the UNO USB connector and the +5V out connection on the single row IDC female connectors is switched by an Si2301DS MOSFET. The source and drain current capability of this device is above one Amp but its maximum power rating is quite low - not a problem as long as it is switched fully on and there is no volts drop across it since (any current) times (no voltage) = no Watts. Obviously in the real world the device will have a small amount of 'on' resistance but it should be OK, I think, to draw 200mA through the PCB. If in doubt you could always provide the PROMs with a +5V supply of their own. For example, you could just temporarily tap the power for the PROMs from the +5V pin on the USB socket, bypassing the MOSFET.