Usb isolator schematic9/8/2023 To limit the current through the shunt regulator, a serial resistor is needed. With a 22k and 20k resistor, it can be set to 5.25V It has an internal 2.5V reference and the can be configured with two resistors. Since Zener diodes are not very precise, temperature dependent and you can't get them for any voltage, a shunt regulator TS431CX is used. So the idea is to sink the current only when the output voltage exceeds 5.25V (maximum USB VBUS voltage). One idea would be to permanently sink 10% on the DC/DC output but firstly this would be a waste of energy and secondly this would also reduce the maximum current the slave could use. Without load or at very low load current, the specified 5V output might be exceeded by up to 100% - depending on the specific device. for a 2W device and a 400mA maximum continuous load current. Typically an isolated DC/DC only keeps its specified output voltage with a minimum of 10% load. Now this would be straight-forward if these DC/DC converter modules would be regulated, but they usually aren't. Depending on the type you choose, it can deliver up to 400mA at 5V. To be able to supply the circuit behind the isolation barrier, an isolated 5V to 5V DC/DC converter is added. There is also a placement option (R6) for a pullup resistor on the serial input (for devices which use an open drain output at their transmit pin). Selecting either 5V or 3.3V is done via a jumper (J5).Īnother jumper (J3) can be used to either supply the secondary site from the RS232 connector (pin1) or to supply the device through this pin in case the DC/DC is placed. To also use this with 3.3V circuits, I added a TLV1117 linear voltage regulator which can handle up to 800mA. There are two LEDs connected to CBUS0 and CBUS1 to show receive/transmit and another one to show the FT232 is enabled.Īs the PWREN/CBUS3 output is low active (high in suspend mode), a PNP transistor is used to invert the signal and drive the LED.įor transient voltage protection, TVS diode arrays are added at the input and the output in addition to the typical serial resistors. The FT232 circuit is more or less the recommended default setting. As USB to serial converter I decided to use the FTDI FT232RL which is still somewhat the industry standard. It provides two channels, can withstand 2500V for 1 minute and supports bitrates up to 25MBit/s. I decided to use an ADUM1201 which is a digital isolator by Analog Devices. Regarding isolation, there are several approaches available using either optocouplers or digital isolators. So I decided to built my own isolated RS232/TTL to USB converter which doesn't need an external power supply, fits in an off-the-self case and doesn't cost the world. Of course the according converters are available for these devices, but they usually quite expensive - like close to 100€.īesides, they are pretty appropriate since usually they can't supply the output side of the isolator so this has to be done by the device or by an external power supply. Now actually, you can find these kind of converter on eBay for a few Euros, but apart from the problem that most of them use fake (FTDI) converter chips, they're not isolated.Īs power supplies, electronic loads and the like usually should be isolated from ground for practical and safety reasons, an additional isolation is needed. Besides PCs nowadays usually don't come with RS232 connectors anymore, so a USB to serial conversion is necessary. So you can't connect them to a normal RS232 interface on a PC. There are several power supplies, electronic loads and similar devices which can be controlled through a RS232 DB-9 connector but which can handle only 5V TTL levels instead of +/-15V.Į.g. RS232 TTL USB Isolator RS232 TTL USB Isolator 2.5kV RS232 (TTL level) Isolator with up to 400mA host supply and FTDI USB interface Introduction
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