![]() Is this even a good idea? even if we get GND disconnected too, will noise be transfered over the data lines? or do most receiver chips reject noise on the data lines? Can i use a isolation transformer and just connect ground to the output (and leave V unconnected)? or will this not work? (if just connecting GND wont work, how about a large resistor between secondary V and GND and use the GND this way?) maybe current sensing on the output and disconnect ground after 1 sec? how can we leave ground connected but disconnect it as soon as the usb device is sucessfully connected? (since if gnd is completely disconnected most devices wont connect sucessfully) So in the end just D+ and D- should be connected from input to output, V and GND will come from external power input if possible disconnect GND too from input and use GND from external power disconnect V from input and just use external power 3 usb connectors, one input, one output, one external power This way, adding to the previously mentioned electrical safety, I obtained +24V with respect to ground without loading the virtual ground op-amp.I try to build something similar to the Ifi Idefender The immediate solution to this problem was to incorporate a DC-DC converter, but not any converter: as the virtual ground op-amp was already sinking a great amount of current for its capabilities and the external power supply could still sink a lot more, an isolated DC-DC converter came in handy: at the input of the converter (Vin+ and Vin-) were the full 48V from the external supply, and at the output (Vout+ and Vout-) were the +24V rail and the virtual ground. ![]() ![]() However, during development, a few functional blocks were added and the required ICs only supported supply voltages up to 30V. For this particular case, I was tempted to try an op-amp-based virtual ground, which is (very very basically) obtained by buffering the output of a resistor divider (provided the buffer op-amp has enough current capability). As the external supply was a simple 230VAC to 48VDC SMPS, there were some ways to divide the 48V into three rails (+44V, 0V, and -4V). I was developing some test equipment that required positive and negative supply voltages for the intended operation of some ICs. In addition to NeuroEng's comment on your question, which pretty much explains the functional part of isolated DC-DC converters, I can also give you an example from one of my old projects. In such cases, there is nothing preventing you from linking the input and output negatives and they work just fine. I have often used isolated DC-DC converters because they generally perform really well and if what I am working on is low quantity, spending the extra money is not a big deal. So if you have a 12V to 12V isolated DC-DC converter, you can connect the + of the input to the - of the output and get 24V when measured from the - of the input to the + of the output. This is also why a lot of these power supplies are referred to as "medical grade".įor non-medical applications, the biggest advantage I have seen is that since the outputs are floating relative to the inputs, there is nothing to prevent you from chaining the input and output together to create higher voltages. The idea is that if something would go wrong on the AC side, there can be no spillage of high voltages or currents to the DC side. Plugged-in medical devices are constraint by regulations that require isolation. There are a few reasons for using isolated power supplies (or isolated dc-dc converters) but the most common I know of is for medical devices.
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