Secure gateway with sms receiver: stability?

[gohan]

@alowhum the 2A are most likely to be peak current and since they are coming directly from the USB port, the mcu is bypassed so you should be fine.

[mfalkvidd]

@gohan I don't think so. Most usb chips can provide a maximum of 500mA.

[gohan]

The 5v rail should be on its own... I have to check on my nano

[mfalkvidd]

@gohan unless the usb chip negotiates more than 500mA with the host computer, it is not allowed to drain more than 500mA. It's in the usb spec. This avoids burning your motherboard in case it is not built to deliver more. Without this negotiation protocol, expensive repairs would have to be made when newer devices are connected to older computers.

[gohan]

On old computers the USB chip in the motherboard will just shut down if more than 0.5A are drawn, on modern ones there are charging USB ports that can supply 1 or 2A and others that just supply the 0.5A even if you connect a power bank that can draw 2A but will just slowly charge at half amp

[zboblamont]

@gohan I suspect @mfalkvidd is correct as I understand it. Certainly there are USB protocols and specific devices which permit negotiation of different constraints including voltage, but this is highly unlikely in this instance....

[mfalkvidd]

@zboblamont actually, it seems we are both correct. According to Maxim Integrated (which should be a reputable source) :

In USB 2.0, it is not strictly legal to draw power without enumerating, although much of present-day hardware does just that, and in violation of the spec.

[gohan]

@mfalkvidd open energy for all USB devices :grin:
I haven't looked at any specs, I just reported what I have seen in 18 years working with computers :wink:

[Anticimex]

@gohan the USB spec require proper device descriptors and enumeration to permit more than 100mA current draw. To go above 500mA you also need to indicate charging capabilities and have termination resistors with certain values. But this all of course require the host to enforce this, which some chips don't. So it might be possible to violate,which is why older pc:s might so this (in some cases at the expense of the motherboard).
But for safety reasons anyone developing USB drivers should make sure to follow the spec properly, since hosts cannot be "trusted" to just cap the current if you draw too much.
I have developed such drivers when I was working for Sony Mobile. Was some time ago now, but the spec have probably not changed significantly in recent years in that regard.

[mfalkvidd]

To get back on topic, the schottky diode between VUSB and the Nano 5V rail is a MBR0520 which has a maximum average current of 0.5A but can handle a non-repetitive peak surge current of 5.5A.

The FT232R usb chip is not capable of negotiating more than 500mA, so drawing more than 500mA will be in violation of the usb spec.

So if you have an original Nano, or a clone with equal or better specs (not likely), it might be able to handle the 800L. But I would not bet on it. All gsm modems I have tried have been very flaky with less than 2A supply.

[alowhum]

So I created a gateway with the 800L.. and as predicted it used too much power. I've now put it on a separate node, and seems to work fine.