There was an interesting spark when the grounds of the two computers connected via the shield conductor of the cable. You can buy USB optoisolators that are rated and tested and guaranteed, as you point out this is not a good place for homegrown experiments. It is optoisolated. With well-known chips for optoisolation.
How is the PCB layout? Is the space between the sides of the optoisolators devoid of traces to meet creepage specifications? Did you read the article about the project itself?
A difference in ground potential is not necessarily an issue at all. Most modern ungrounded electronic devices with metal chassis laptops, tablets, hifis, etc etc can float you well above local ground potential when you touch them.
Ever felt a tingle from your laptop when you touch it and a ground at the same time? Safety issues only arise when the internal resistance of such a source is sufficiently low to drive significant currents. Uh, you should realize that any device plugged into the wall with just a 2-wire no ground wire cord does exactly this: the chassis is coupled to both hot and neutral through stray capacitance, and tends to float at around the halfway point.
With the capacitance in the line filters in modern SMPS, it amounts to microamps. Maybe they attribute the tingly effect to owning an Apple product, I dunno. Another offender is electric blankets: they capacitively couple to you, and if you reach out to touch a grounded object or a partner not under the blanket, you get a tingly feeling in the fingertips. Especially to said partner. Vengeance shall be mine!
You Americans got the nice end of the deal. It become more of a problem for a high frequency SMPS. Since OP did not specify the cause of the voltage, I do not try to make any non-safe assumptions. If there are sufficient current tohave sparks, then it is more than tens of uA. The operating frequency of the SMPS is irrelevant, since that frequency does not appear on either the line or the output. SMPS are relevant here only because they usually require much more filtering on their AC input lines than their linear brethren, so typically have much more capacitance between line and chassis ground.
The SMPS operating frequency has nothing to do with that. The leakage current is redirected current from EMI filter common mode choke and Y caps back to Earth ground. I have done my share.
The one my friend has a Earth connection and the leakage current flowing into the Earth causes the unbalances that trips the GFI.
These pesky devices are required in new home construction. They attempt to detect arcing and flip when they sense a wave on the line that looks like arcing. Anything that puts the frequency and characteristic that looks like arcing on the line will trip the bastards.
A crummy switching wall wart will do it. I had to build a filter for the AC plug line by looping it through a ferrite ring 6 or 7 to stoop the nussance flipping. So very few know the issues associated with ground potential. This is an age old problem in electronics that date as far back as the tube era.
Today this is exassorbated by laptops and wiring to electrical outlets, breaker boxes and variations in XO grounding. As for the USB issue. It is complicated by a laptop to laptop or desktop. Presently there are no methods of ensuring both devices are subject to the same ground potential. The Optoisolator is the most eloquent solution to this problem. Includes easy to use selection table. Null modem is a communication method to directly connect two DTEs computer, terminal, Several wiring layouts are in use because the null modem connection is not covered by the RS standard.
A null modem cable is a RS serial cable where the transmit and receive lines are crosslinked. In some cables there. By: Leah Zitter Contributor. Dictionary Dictionary Term of the Day. Techopedia Terms. Connect with us. Sign up. Term of the Day. In this type of connection, the transmission and reception lines are connected crosswise, enabling two-way data transmission.
Today, null modems are used primarily to exchange data between older computers or laptops. These machines may not have a USB port or network card. For these types of machines, a null modem is the only option when data transmission is required. As we have seen, null-modem connections are made possible through the use of a null mode cable. This is the simplest solution to connecting the two computers.
But what is a null modem cable? The cable is comprised of three lines. One wire serves as the signal ground with the other two lines fostering the transmitting and receiving of your data.
Based on the software used in this connection, some kind of authenticating handshake may be necessary. RS cable wiring scheme by Microsoft has become the industry standard, using a cable made up of seven wires. As we stated previously, a null-modem cable connection will not provide high-speed data transfer. Using a null modem emulator allows effective and higher-speed data transfer using your null modem.
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