Instrument Isolation - Page 1
Forum section: electronics / general

author: sub
author_rank: Regular Contributor
timestamp: February 02, 2012, 08:16:16 am
content: In a PC-connected instrument one would normally like some level of isolation. At the same time, though, one needs to get power to the hot side of the isolator. Is a second mains transformer or an isolating switchmode supply the normal way to manage this, or is there an easier way that I am missing?

author: ejeffrey
author_rank: Super Contributor
timestamp: February 02, 2012, 08:39:05 am
content: An independent power supply for the instrument is certainly the simplest way to get isolation. If it is important to be bus powered, or if you need multiple isolated zones, you can use small low voltage isolated DC/DC converters.

author: Psi
author_rank: Super Contributor
timestamp: February 02, 2012, 09:56:21 am
content: for example, this prebuilt unit will give you an isolated 5V rail at up to 200mAhttp://search.digikey.com/us/en/products/VBSD1-S5-S5-SIP/102-1360-ND/989815Or this one, which can do 1Ahttp://search.digikey.com/us/en/products/CC6-0505SF-E/445-2480-ND/920440

author: Hideki
author_rank: Frequent Contributor
timestamp: February 02, 2012, 03:23:02 pm
content: It depends on the type of interface.USB can be powered from the USB connector with at least 100 mA.Ethernet is already isolated by the magnetics.

author: Unknown
author_rank: Unknown
timestamp: February 02, 2012, 09:12:51 pm
content: Extra transformer windings, isolated DC-DC converters and USB power are all common solutions. AD also has a line of RF isolators (iCouplers?) that incorporate both an isolated DC-to-DC converter and isolation for signals. They're faster than your average opto, but also more expensive.Quote from: Hideki on February 02, 2012, 03:23:02 pmUntil you touch the ground shield of the Ethernet jack, or use F/UTP. Also you should evaluate whether the isolation rating is sufficient for your application. I don't believe Ethernet was designed for high common mode voltages.

author: amspire
author_rank: Super Contributor
timestamp: February 03, 2012, 11:18:20 pm
content: Many of the new Ethernet connectors with built in transformers, like the one on the Arduino Ethernet shield, are rated at 1500V.The catch is the winding feedthrough capacitance can be as high as 60pF, so they would have a limited ability to cope with fast transients, or a strong RF common mode signal.Richard.

author: Neganur
author_rank: Unknown
timestamp: February 05, 2012, 11:18:32 am
content: Stumbled across this component from Analog Devices, maybe it's useful for you.I think Alm mentioned something similar.ADUM4160: Full/Low Speed USB Digital IsolatorCopy & paste of the features list:Fully USB 2.0 CompliantLow and full speed data rate: 1.5 Mbps and 12 MbpsBidirectional communicationShort-circuit protection for xD+ and xD? lines3.3 V and 5 V (dual mode power configuration) operation

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2012, 12:25:13 pm
content: A cheap isolated supply is available if you can find some old network cards that use a 10baseT coax cable. they all have a converter inside that provides 9V from a 5v input, that can easily supply around 50-100mA with 1500VAC isolation. I have a whole pile of old ISA network cards that I use when I want one.

author: wkb
author_rank: Super Contributor
timestamp: February 05, 2012, 06:06:27 pm
content: Quote from: SeanB on February 05, 2012, 12:25:13 pm10base2. These converters typically come in a morphed DIL24 style case. Look for a DP8392 (if memory serves) in their neighbourhood, that is the chip commonly used on these Cheapernet-style cards.