When do I need USB isolation?

[Mercator]

Hi,
Currently I am working on a project that involves controlling a DC-motor. The circuit can be powered from 12V up to 24V and the motor current can reach up to 10A. The system looks like this: USB-Interface -> Microcontroller -> Mosfet driver -> H-Bridge -> Motor. The current motor voltage is measured and fed back to the microcontroller.
It is reasonable to assume that several of those units will be connected to one computer later on.

Since I am using the MCP2200 USB to UART converter I could isolate the USB-interface by isolating the UART with some optocouplers and use the USB to power the MCP2200. The project is supposed to be low cost and I don't really know if USB-isolation is needed. Would this method of isolation be beneficial or even necessary in this case?
   

[ConKbot]

From a practical and safety point of view, you will probably be fine with just series resistors on signal lines to keep fault current from being excessive if something happens, as long as you are using an isolated DC power supply (isolated from mains,  it can still be earth referenced)   You could run into ground loops and other noise from multiple grounds, but then it would depend on the input specs of the MCP2200, usb being differential and all.

That being said, isolation makes sure that when mosfets go bang, and stuff gets shorted in wierd ways, that an inductive spike from the motor cant go up your USB line and blow out a motherboard.

There are other methods of doing that (fusing/clamping various portions of the circuit separately) , but isolating would be a nice "bigger hammer" approach which more then covers your bases.

[konfu]

I usually use an ADUM 1201 AR chip that isolated the RS232 signal to about 1000V. It's just 2.5 EUR ... not really a bid deal when it comes to security.

There is also not much needed for the chip to run: two 100nF capacitors and done. Nice little thingy

In my opinion the question should be "is it worth spending 2.5 EUR to be safe?" It's always a question of whether you want to set up 10 or 10,000 of these units.

I usually set this up a little modular:

I use a 2-row 4pin header (so 8 pins in total).

Pin1  --> VCC IN  o | o   VCC OUT   --> Pin8
Pin2  --> RX IN    o | o   RX OUT     --> Pin7
Pin3  --> TX OUT  o | o  TX IN        --> Pin6
Pin4  --> GND IN  o | o  GND OUT  --> Pin5


This will allow for two options:

1. Jumper-Mode:
  I can adjust RX/TX connection:
  - GND and VCC are being connected left to right in any case
  - put in the RX/TX jumpers left to right will make it a 1:1 connection (2-7, 3-6)
  - put in the RX/TX jumpers top to bottom will make the connection a cross-over one (2-3, 6-7)

2. Isolator-Mode:
  - I can put in an isolator riser board with ADuM1201. I'm just using as small PCB with a female 2row, 4pin header. Plug it in instead of the jumpers and you're isolated.

I am sorry not to have some photos ready to show how I'm doing it but this is really flexible and neat. If you seperate in and out of this connector by using upper and lower side of the PCB you're a little safer. This for sure won't protect against 1kV but at least of someone doing something weird with the circuit

[Matje]

Hi,
Currently I am working on a project that involves controlling a DC-motor. The circuit can be powered from 12V up to 24V and the motor current can reach up to 10A. The system looks like this: USB-Interface -> Microcontroller -> Mosfet driver -> H-Bridge -> Motor. The current motor voltage is measured and fed back to the microcontroller.
It is reasonable to assume that several of those units will be connected to one computer later on.

Since I am using the MCP2200 USB to UART converter I could isolate the USB-interface by isolating the UART with some optocouplers and use the USB to power the MCP2200. The project is supposed to be low cost and I don't really know if USB-isolation is needed. Would this method of isolation be beneficial or even necessary in this case?

Strictly speaking isolation would not buy you much in this case, as opposed to measuring equipment connected by USB where ground loops and stuff are always a concern.

I would put it in anyway. Without it you may easily blow out an USB port on the computer (or likely a pair since they often seem to be implemented that way in the actual hardware). And nowadays there is usually no way to repair that, so after some tests gone wrong you may be forced to replace at least the complete mainboard. That would likely cost more than implementing isolation and be more hassle too.

Plus there is some safety benefit too, your motor control circuitry may get shorted to mains power somehow (if you use mains power) without endangering the person at the computer.

[Mercator]

Thank you for your answers.
Although there are no concrete plans yet, the product is designed for production in larger numbers. So I will design isolation in. Better save than sorry.

Thanks again.

[Dreso12]

Analog devices have a really large catalog of isolators for USB, depending on the maximum speed the prices are not so expensive or really expensive. So if your design can afford it I would suggest you using a low speed version of this isolators, you will save quite a lot of money in your board.