I need to connect two phones with USB-B Micro ports through a relay. I need the relay because I need to control when the phones are connected. I made two USB-B Micro cables with wires on one end. One cable is configured to be the host and the other is configured to be the server. When I connect the ends of the two cables directly to each other, the phones connect to each other. However, when I route the cables through the relay, the phones won't connect. I checked the resistance of the lines to make sure that the relays are closed and they were (<1 ohm). Any idea why I can't route my USB lines through the relay?
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\$\begingroup\$ Just a comment on terminology: We say a switch or relay contact is open when it is not making contact - a light switch is open when it is "off", and not conducting. When the switch or relay contacts are conducting, we say they are 'closed". You appear to be using 'open" to mean "conducting". This seems to be a common confusion for beginners. \$\endgroup\$– Peter BennettCommented Aug 11, 2021 at 20:50
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\$\begingroup\$ Thanks. I fixed my question. \$\endgroup\$– PetSvenCommented Aug 11, 2021 at 20:58
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\$\begingroup\$ USB is fast enough that a piece of wire isn't just wire anymore. Not just resistance but other things like inductance and capacitance. Transmission lines (things like antennas and the like). There's a lot behind USB, even just the cable selection. When you're going fast in an F1 racecar, not just any old strip of road will do. Road conditions really matter. \$\endgroup\$– DKNguyenCommented Aug 11, 2021 at 21:28
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\$\begingroup\$ Show a picture of what you wired. \$\endgroup\$– user1850479Commented Aug 11, 2021 at 22:18
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1\$\begingroup\$ It's not that you can't run USB through a relay, but that because USB is a high-speed (>>100MHz) interface you can't just use any old relay, you're getting into the realm of RF voodoo where a wire is not just a wire anymore and every picofarad counts. There are RF relays out there that can handle USB speeds (these ones are good up to USB3 speeds). Relays good to 500+ MHz should work (USB2 is a bit forgiving), but solid-state USB switches/muxes are likely smaller and cheaper. \$\endgroup\$– SamCommented Aug 11, 2021 at 22:50
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3 Answers
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I think a relay would introduce a large enough impedance discontinuity in the transmission line to ruin signal integrity and prevent USB2 from working.
There are USB switch chips, here's an example. Focus is on low capacitance to get good signal integrity. It could also work with any low capacitance analog switch IC, but if the chip is specified for USB use, it has more chances of working.
If you want a "low-tech" solution, put a USB hub between the two phones, and use the relay to switch the hub power on/off.
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Any idea why I can't route my USB lines through the relay?
Sure you can. Just you better use special relays, called "RF relays". Many USB test instruments use Teledyne RF relay like this one
Or COTO relays, series 9800 or 9200.
The COTO relays use reed contacts, and some models are properly shielded to form a 50-Ohm controlled impedance, so two relays can form a 100-Ohm differential path.
Or Standex-Meder relays, smaller, but same idea.
If one need to switch USB lines (or other high speed signals in automated test equipment) in a clean leakage-free and bias-free manner, they use RF impedance-controlled relays.
answered Aug 12, 2021 at 4:14
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There’s a few things that may be happening.
It could be wired incorrectly. Make sure D- goes to D- and D+ goes to D+.
USB standard requires a 90 ohm differential impedance transmission line. Adding a relay in the middle of the transmission line may ruin your signal integrity.
The magnetic field from the relay may be interfering with the signal. There could be other interference entering the signal too because there is no shielding where the relay is.
