How to Use Optocouplers for Galvanic Isolation

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In response to a Types of Grounding video, Tech Consultant Zach Peterson was recently asked about getting signals across a primary-to-secondary ground gap while in a system that requires galvanic isolation. Zach explores this and much more in this video.
0:00 Intro
0:48 Mind the Gap
2:38 Examining the Datasheet
3:59 The Recommended Solution
5:31 Introducing the Optocoupler
7:54 Optocoupler Placement
9:05 Optocoupler Specifications
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Опубликовано:

17 июл 2024

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Комментарии : 11

@googlesucks1376 Год назад

Also - medical guys love the Icouplers - in fact, Lady Ada sells a USB isolater based on them. They work great, but the ones that isolate power put out about as much RF noise as do some the spread spectrum radios I've designed. There's even an Analog Devices appnote about it. For power isolation, much better to use the old 422/485 system isolators that run at 19kHz via actual magnetics....

@wamwam8016 Год назад

Typical analog optocouplers are NOT the same as to what I mentioned. They require power on BOTH sides of the circuit. What I mentioned is a VERY SIMPLE design where the high side optocoupler only has the LED input with NO POWER REQUIREMENTS on that side. So if you're trying to monitor a voltage/current on a system you want totally isolated and has no power supply available - this works really well...We have safety systems that we monitor that can have NO POSSIBLE INFLUENCE - only an LED and resistor is allowed.

@myetis1990 Год назад

Hey Zach, thank you for the useful topic! Mostly I 've seen the optocoupler on the secondary side of the flyback smps for the feedback and also seen a TL431 , could you please explain how to use it with the optocoupler ?

@Zachariah-Peterson Год назад

That TL431 regulator is basically a precision voltage source. I think you would use it as a precision reference for an error amplifier. If it's for secondary side regulation, then you would have that error amp circuit on the secondary side driving an optocoupler, and then the signal on the primary side would be passed to a controller for the flyback.

@googlesucks1376 Год назад

There is also a way to make a simple linear opto isolator circuit . Note that if you try and run an optocoupler Bipolar transistor for an analog signal, it will be be drastically impacted by temperature. But what you can do - And I've prototyped this - is to use optocouplers in the same package then use one as the input, but take the other an use it in the feedback loop of an opamp. That way as the optocoupler substrate changes temperature BOTH opto's will respond in a similar fashion therefor "servo" ing the output. I tested this on a protopcb I CNC'd using an LT1211 single supply opamp and a TLP opto while on a hot plate that took it up to 100C. I then went down as quick as possible to around - 25C. I was using a DAC recorder and it was incredibly linear. Note to Zach - I can send photos of the set up and a screenshot of the charts. It's really cool. And no - I cannot take credit for it - evidently all the oldtime audio guys have known about this for decades. I did about 300 design with the late Gary Stanfill of Vega Wireless/Schaffer-Vega - the first commercial wireless mic/instrument interface company. Those old guys knew a lot of tricks...

@nulldev42 Год назад

I believe I saw these linear optical devices in the late 90s. It looks like a LOC110 or IL300 performs this same functionality. It has a single LED input and 2 matched photodiodes (one for servo feedback and one for output).

@wamwam8016 Год назад

@@nulldev42 Naw - doesn't work quite the same. On the high side you need to provide an opamp with a power supply. See this www.vishay.com/docs/83708/appnote50.pdf As I mentioned in a previous post, that would breech a safety barrier since it's possible that the opamp could influence the safety buss... Yea, we looked at those years ago and were prohibited from using them.

@Zachariah-Peterson Год назад

Do me a favor, send it over to youtube@altium.com

@NathanSweet Год назад

Probably should mention current handling on the photoresistor side.

@Zachariah-Peterson Год назад

Sure, that applies to any design with a transistor though

@wamwam8016 Год назад

Here's a schematic and link to a LTSPICE model of what I'm talking about www.ajawamnet.com/anaopto.jpg www.ajawamnet.com/lintempcompslowpulse5-lt1211-5.asc - LTSPICE circuit. try it... really cool. And note - the RTN/GND on the high side doesn't have to share with the low side GND ref... even cooler...