Opto-isolated zero crossing detector

[markp]

Hi All,

I have an sinusoidal AC signal between 30V and 160V AC and between 200 and
600Hz in frequency, and I need an opto-isolated zero crossing detector. The
AC is actually from a transformer output but I don't want to add any more
windings to it. I do however have a centre tap on the AC output. Does anyone
have any suggestions?

Thanks!

Mark.

 

[Spehro Pefhany]

Hi All,

I have an sinusoidal AC signal between 30V and 160V AC and between 200 and
600Hz in frequency, and I need an opto-isolated zero crossing detector. The
AC is actually from a transformer output but I don't want to add any more
windings to it. I do however have a centre tap on the AC output. Does anyone
have any suggestions?

Thanks!

Mark.

Maybe a bridge rectifier, resistor or FET current limiter, and an
optocoupler. Depends a bit on how close to the zero you want to go
(should the pulses vary in width with voltage, for example, and at
what voltage threshold should the output go "true").



Best regards,
Spehro Pefhany

 

[Joerg]

Hi All,

I have an sinusoidal AC signal between 30V and 160V AC and between 200 and
600Hz in frequency, and I need an opto-isolated zero crossing detector. The
AC is actually from a transformer output but I don't want to add any more
windings to it. I do however have a centre tap on the AC output. Does anyone
have any suggestions?

Why opto? Can't you use a modem transformer that gets a divided-down
signal at the input and has the proper isolation specs? Then use a
regular zero-crosser of your liking on the others side where things
don't have to run isolated.

From a size point of few such a signal transformer and an optocoupler
aren't grossly different. A little, maybe.

 

[[email protected]]

Hi All,

I have an sinusoidal AC signal between 30V and 160V AC and between 200 and
600Hz in frequency, and I need an opto-isolated zero crossing detector. The
AC is actually from a transformer output but I don't want to add any more
windings to it. I do however have a centre tap on the AC output. Does anyone
have any suggestions?

How close to "zero" does it have to detect? The output has to be isolated
from both the primary and secondary?

 

[Paul Keinanen]

I have an sinusoidal AC signal between 30V and 160V AC and between 200 and
600Hz in frequency, and I need an opto-isolated zero crossing detector. The
AC is actually from a transformer output but I don't want to add any more
windings to it. I do however have a centre tap on the AC output. Does anyone
have any suggestions?

Use two optoisolators with the photodiodes in antiparallel connection.
When neither phototransistor conducts, you are close to zero.

 

[markp]

Maybe a bridge rectifier, resistor or FET current limiter, and an
optocoupler. Depends a bit on how close to the zero you want to go
(should the pulses vary in width with voltage, for example, and at
what voltage threshold should the output go "true").

Yes, that sounds like an option. In fact I only need a square wave output, I
can use the edges to detect zero crossing.

So I'm thinking now of a diode with a constant current source (standard two
back-to-back PNP transistors circuit) then the optocoupler diode. The
constant current source will allow more gain at lower voltage so getting
closer to zero when switching, and would limit the current at higher
voltages.

Mark.

 

[markp]

Why opto? Can't you use a modem transformer that gets a divided-down
signal at the input and has the proper isolation specs? Then use a
regular zero-crosser of your liking on the others side where things
don't have to run isolated.

From a size point of few such a signal transformer and an optocoupler
aren't grossly different. A little, maybe.

It's an option, but may be a little more expensive. I'm sure it can be done
with just an optocoupler.

Mark.

 

[markp]

Ooh, you could use a variant of one of my favorite trick circuits, the
totem-pole optoisolator:

ftp://jjlarkin.lmi.net/ZCD.JPG


John

Interesting, the idea of back-to-back optocouplers is nice. I've realised I
only need a square wave output because i can use the edges to detect zero
crossing, so a diode follwed by a constant current source (2 PNPs) and then
the optocoupler diode might do.

Mark.

 

[markp]

How close to "zero" does it have to detect? The output has to be isolated
from both the primary and secondary?

The output from the transformer is essentially floating, but can have either
output connection, or the centre tap, strapped to earth. So the feedback has
to be isolated from the output side and produce a signal referenced to the
secondary input drive ground.

Mark.

 

[markp]

Use two optoisolators with the photodiodes in antiparallel connection.
When neither phototransistor conducts, you are close to zero.

Thanks, John suggested something similar. I only really need a square wave
output though, I can use the edges to detect zero crossing.

Mark.

 

[Joerg]

It's an option, but may be a little more expensive. I'm sure it can be done
with just an optocoupler.

Sure it can be. But the accuracy suffers greatly. You can only send so
many milliamps into an optocoupler and then it signals "out" well before
the real zero-crossing happens.

 

[markp]

The Supertex depletion-mode mosfets are interesting as current
limiting devices here, back-to-back LND150s maybe. But resistors would
be more reliable if, say, really big transients might be possible.

One nice thing about the push-pull opto thing is that it doesn't need
a lot of current into the optos, so you can use big current-limiting
resistors, or the LNDs, off the transformer, as long as you don't get
extreme and add too much time delay.

You could also do a cheap opamp-based diffamp to sense the transformer
voltage, if you don't need true isolation. That devolves to four
resistors (two dividers down to logic ground) feeding a comparator.

John

I do really need isolation. The reality is the output is a centre tapped
winding on a transformer, and I need to be able to tie either the centre
tap, or one of the outputs, to ground depending on application.

The LND150 looks interesting, they can used as constant current sources at
high voltage with a single resistor it seems. My concern with just resistors
is this has to work from 30VAC (possibly less) to 160VAC, and to get the
gain needed to switch quickly within a few volts would mean lowish value
resistors, and then at high voltages would dissipate power. This is why I
thought a constant current device would be better.

Having said that, replacing your resistors in your zero crossing circuit
with LND150s (using the intrinsic diodes to conduct when the polarity is
reversed) and using your push-pull approach with schmidtt buffer to reduce
noise might do the job! Thanks for suggesting that.

Mark

 

[Joerg]

Interesting, the idea of back-to-back optocouplers is nice. I've realised I
only need a square wave output because i can use the edges to detect zero
crossing, so a diode follwed by a constant current source (2 PNPs) and then
the optocoupler diode might do.

You can also buy optocouplers that have those back to back diode inside
the same device.

 

[Joerg]

Thanks, John suggested something similar. I only really need a square wave
output though, I can use the edges to detect zero crossing.

But keep in mind that the "zero" indication will be quite wide, you
won't be able to nail that to fractions of a degree unless you watch
when it comes back and calculate the middle via a uC timer or something.

 

[[email protected]]

The output from the transformer is essentially floating, but can have either
output connection, or the centre tap, strapped to earth. So the feedback has
to be isolated from the output side and produce a signal referenced to the
secondary input drive ground.

How close to zero?

 

[markp]

This should work.

ftp://jjlarkin.lmi.net/ZCD_2.JPG

You don't need galvanic isolation, you just need a little common-mode
rejection.

John

Thanks for this. I think this circuit might work, but I worry about the
opamp/comparitor (which I think might have to be an instrumentation type)
being very high impedance and the possibility of noise picked up in that
area from elsewhere. I'd rather have true isolation to be honest.

The original back-to-back optoisolator variant looks good, possibly with FET
current limiters in place of the resistors and I may use an opamp configured
as a schmidtt buffer with a fixed mid-point reference rather than a gate. In
any case I'll try to do some LTSpice simulation if I can get the models or
create them.

Mark.

 

[John Devereux]

Thanks for this. I think this circuit might work, but I worry about the
opamp/comparitor (which I think might have to be an instrumentation type)
being very high impedance and the possibility of noise picked up in that
area from elsewhere. I'd rather have true isolation to be honest.

The impedance only 2kohms.

 

[markp]

The impedance only 2kohms.

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.

John Devereux

Mark.

 

[John Devereux]

[...]

The impedance only 2kohms.

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.

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.

 

[markp]

message
I have an sinusoidal AC signal between 30V and 160V AC and between
200
and
600Hz in frequency, and I need an opto-isolated zero crossing
detector.
The
AC is actually from a transformer output but I don't want to add any
more
windings to it. I do however have a centre tap on the AC output.
Does
anyone
have any suggestions?

[...]

This should work.

ftp://jjlarkin.lmi.net/ZCD_2.JPG

You don't need galvanic isolation, you just need a little common-mode
rejection.

John


Thanks for this. I think this circuit might work, but I worry about
the
opamp/comparitor (which I think might have to be an instrumentation
type)
being very high impedance and the possibility of noise picked up in
that
area from elsewhere. I'd rather have true isolation to be honest.

The impedance only 2kohms.

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.

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.

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.