Opto-isolated zero crossing detector

The impedance only 2kohms.

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John Devereux

Yes, but the variation in input impedance of the opamp inputs (and hence input biasing and offset currents) will negate the balancing of the 500:1 resistor dividers. To get accuracy from this circuit the impedance of the opamp has to be orders of magnitude greater than 1M.

Mark.

On a sunny day (Thu, 15 Jul 2010 17:53:22 -0700) it happened Jim Thompson hit the wrong keys in :

It is a good cicruit, and the issue was not medical grade. If you want attention that much say something sensible. Greetings Osama

On a sunny day (Fri, 16 Jul 2010 12:33:06 +0100) it happened "markp" wrote in :

Bollucks.

[...]

Hi Mark,

Don't think so - it just needs to be orders of magnitude greater than

2k, which is a lot easier :) As a worst case imagine there is a 1M resistor across one of the 2k. It only makes a ~0.2% difference to the ratio.

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John Devereux

OK. Are you sure that input offset currents are not going to cause any problem? If the common mode causes the inputs of the opamp to both be above

2.5V then to get positive input bias and offset currents this has to come from the 1M resistors. Is that not going to perturb the accuracy?

Mark.

Actually forget that. Since I'm strapping one of the outputs or the centre tap to ground, the point of accuracy will be when both inputs are at ground. As long as I don't strap these to any other potential and my logic ground is close to the strap potential it looks like it would be OK.

Mark.

I can apparently get him to say totally absurd things, as he does regularly these days. We should cut him some slack, the poor old git.

John

[...]

It is true that the 1M resistors need to be matched to 1% or 0.1%, as JL has shown. So if you think you could get

--

John Devereux

Use a CMOS opamp or comparator.

Since you're apparently going to ground one end of the transformer or the center tap, this just has to work. All three cases poke nice signals into the comparator, even if the voltage dividers use 5% resistors.

John

It wouldn't even pass UL, you narcissistic egomaniac. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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               Friday is Wine and Cheeseburger Day

If, as he says, he will always ground one end of the transformer or the ct, 20% resistors would work fine.

John

Let's see, current limiting resistor, into bridge with 5V zener and opto. Maybe a resistor across the zener.

Discrete representation (high transistor count of chip would scare you :-) of what I do in off-line-powered appliance controller chips...

Of course "The Bloviator" will claim too many parts, at the same time not providing component values on his own "designs".

You can build my representation as shown... and it WILL work.

Simplistically, from a years ago (1978) disco design...

Also mentioned recently here in...

Message-ID:

Adequate for non-critical Triac synchronization ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

Make that...

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I have this (perhaps) amusing NON-isolated contribution, but anyone using it does so _entirely_ at their own risk. ;-)

1M -->

Cute. But applying name substitution, JT would say...

As long as we're doing cluckworthy circuits...

___

If R is big, 1M maybe, you can eliminate C. Or replace R+C with two more xor gates if they're unused, delay-line thing.

Let the pecking begin!

John

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Funnily enough I did something very similar for another project, that is use a bridge rectifier with a capacitor to create a low voltage 'off-line' DC supply for a micro, then used the neutral wire as a reference into the micro with a current limiting resistor. What actually happens is the low voltage DC supply goes up and down wrt neutral, the micro sees + and - edges when it crosses the neutral potential. That works fine, LTSpice simulated and built. But it's non-isolating.

Mark.

done

Spehro chose to use a resistive divider at his circuit's AC side, which makes sense to me. In your above case, the input is tied via 1M to the AC, with no divider at all. I'm not nearly as comfortable with that approach regardless of the rest of it.

Jon