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Old 16th February 2012, 03:24 AM   #21
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Very informative link. I was just looking over some linear output optic isolators. Maybe its as simple as scaling your sense resistor so at max current the led is at max brightness.

If you want to measure current on the low side, ST makes some nice little fixed gain opamps for sense resistors. See the TSC101.

A wise man once told me "Design around Digikey. Otherwise you'll design around this awesome part that you can't buy because nobody stocks it."
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Old 16th February 2012, 03:28 AM   #22
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Originally Posted by smoking-amp View Post
Wouldn't it be a lot simpler to just put one opto set in the serial USB link?
I guess there was (is?) some backward compatibility issue with analog impedance on the USB line, but can't that be dispensed with now?
The USB link has power and ground so it isn't easy to isolate.
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Old 16th February 2012, 04:01 AM   #23
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One would think that someone would have come out with an IC interface for USB that performs the isolation functions by now, could just go serial USB to serial port or parallel port.

The Xantrex power supplies I have use two uP's with optos (serial port) between them to do the isolation functions for digital PC control. An internal floating supply powers the outboard oriented uP. An XHR-600 off Ebay gives you floating 0-600V for plate supply with PC control and current and voltage readbacks (16 bit A/D and D/A). An XT-250 gives 0-250V for screen supply with the same features. You can run the whole show from the PC using the readbacks.
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Old 16th February 2012, 04:16 AM   #24
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Originally Posted by smoking-amp View Post
One would think that someone would have come out with an IC interface for USB that performs the isolation functions by now, could just go serial USB to serial port or parallel port.

The Xantrex power supplies I have use two uP's with optos (serial port) between them to do the isolation functions for digital PC control. An internal floating supply powers the outboard oriented uP. An XHR-600 off Ebay gives you floating 0-600V for plate supply with PC control and current and voltage readbacks (16 bit A/D and D/A). An XT-250 gives 0-250V for screen supply with the same features. You can run the whole show from the PC using the readbacks.
There are isolated USB and RS232's -- I used some from Maxim about 10 years ago. There are also ADC/DAC cards which are isolated. It gets expensive, however.

At any rate, it's very important to keep the two sides separate since power supplies can issues.
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Old 16th February 2012, 04:30 AM   #25
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Originally Posted by jackinnj View Post
The Application Note from Vishay appears to be updated from the version which I have -- see figures 8 and 10.

http://www.vishay.com/docs/83708/appn50.pdf

The material is equally applicable to the HCNR200 -- there's a spice model for this part on the Avago datasheet (Avago Technologies Semiconductors Analog, Mixed-signal and Optoelectronic Components and Subsystems) -- used to be Hewlett Packard's semi biz.
Right, a high side servo with opamp powered off the high side. Not particularly simple. They illustrate isolating 2 electrical domains that each already have VCC and ground. Devising a floating high side power supply that doesn't influence the measurement is I think a bit of a challenge. Still might be worth playing it out though...
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Old 16th February 2012, 05:17 AM   #26
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Originally Posted by jackinnj View Post
There are isolated USB and RS232's -- I used some from Maxim about 10 years ago. There are also ADC/DAC cards which are isolated. It gets expensive, however.

At any rate, it's very important to keep the two sides separate since power supplies can issues.
I think there are 2 distinct issues here; one is the possibility of the computer interface being energized through some colossal failure inside the tracer. The other is how to do current measurement safely.

Implementing isolated high side current measurement seems nice, but doesn't completely eliminate the HV risk.

Twisted pair Ethernet is balanced and transformer isolated.

Here is an iteration of the regulator using transformer isolation of the current measurement, plus current limiting and voltage readback. I think there is still an issue of C2 and C3 needing to be scaled for the measurement current.
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Old 16th February 2012, 12:45 PM   #27
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Default USB Isolator

Analog makes a dedicated USB isolator chip ADUM4160 | Full/Low Speed USB Digital Isolator | Digital Isolators | Interface | Analog Devices
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Old 16th February 2012, 01:17 PM   #28
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they have an evaluation board for the ADUM4160 as well --
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Old 16th February 2012, 02:23 PM   #29
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Quote:
Originally Posted by jackinnj View Post
The USB link has power and ground so it isn't easy to isolate.
Use an isolated power supply for the USB interface.. If you are using opto of magneto isolation for data, you don't need the ground.
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Old 16th February 2012, 02:26 PM   #30
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Current measurements can be safely be made by return sensing and using something like the AD629 (High Common-Mode Voltage Difference Amplifier)
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