Opto Isolator, resistance between channels

Thread Starter

fastbike

Joined Dec 29, 2020
72
If a circuit uses a multi channel opt photo transistor type isolator, what is the isolation rating between channels on the output side.
Looking at stand DIP type packages with 2.54mm pin spacing (and pins about 1.2mm wide), I'm guessing that the gap of 1.3mm between pins provides much less than the rated isolation voltage of the device.

Would potting the output side in epoxy help to raise the rating (preventing flashover etc) ?
 

ericgibbs

Joined Jan 29, 2010
12,901
hi f,
All the opt-isolators I have used have the Input side on one row of pins and the output side on the other row,
ref this image.
So the isolation spacing is not 0.1inch minus the pin width, its around 0.3inch.
E
 

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crutschow

Joined Mar 14, 2008
27,188
If you need high voltage isolation between outputs (not input to output which is the normal concern) then you should use separate, single channel optos in different packages.
What is your application that you have a high voltage between outputs?
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
What is your application that you have a high voltage between outputs?
It is a control circuit for the FL5150 dimmer, where the GNDA on the device is floating with the mains input. Hence the need to isolate the input PWM.
The circuit is shown at https://easyeda.com/mriksman/fl5150-with-opto-and-opamp_copy_copy_copy_copy and employs a negative feedback to the op amp buffering the input to linearise the response.

1609445433825.png

My concern is that GND and GNDA are only separated by the distance between two adjacent output pins.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
My concern is that GND and GNDA are only separated by the distance between two adjacent output pins.
I'm guessing using two separate isolators, rather than a dual channel device, would be safer. But then you run into CTR matching issues etc.
I'm not familiar with how CTR differs between channels on the same device, vs between two instances of a single channel device.
 

crutschow

Joined Mar 14, 2008
27,188
Why do you need such high linearity?

If you drive the opto input with an op amp current-source, then the opto output current is fairly linear with respect to the op amp input voltage (Ltspice simulation of example circuit below).

1609452065775.png
 
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