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'[EE] Any relay closure input optoisolators?'
A few months ago, someone posted a link to a photovoltaic coupler which
is useful in that it produces a voltage on the output which can actually
be used to drive a FET or similar... I didn't realize such a beast even
existed (having only been aware of the 4n26 and some more modern
derivatives, including logic-level outs)... and is cool enough to have
been added to my useful devices list...although I haven't had a chance
to use one in a real application.
So, today I'm working on an optoisolated input. These inputs are
typically going to be contact closure, but need to be electrically
isolated from my circuit for various reasons. In circuits I have
previously designed, I just used a 4n26-like device with appropriate
passives around it and made the customer provide a suitable external
power supply to provide input power to the isolator... I.E. they're
really 24V optoisolated inputs. But..... it occurs to me that there
just might be some relay closure input optoisolator that I'm not aware
of that uses some magic (aka significantly advanced technology) to be
able to not need a separate power source on the relay closure side.
So, the question is this: Is there such a beast? That is a truly
isolated relay closure input which does not require a separate power
supply on the relay side - oh and one final requirement, it has to be at
least somewhat reasonably inexpensive ($30/each input modules do not
qualify). If such a beast exists, it would make my life a lot
easier.. If not, well, back to the optoisolator selection guide
(hmmm... 3.3V logic level output, wide input current range...).
|On 11 Dec 2007 at 0:42, Forrest Christian wrote:
That was probably me that mentioned the IR PVI devices. Like you say, kind of a
cool device, but not quite what you need this time. Quite possibly there is something
out there that will do exactly what you want but I'm not aware of it.
One question that needs to be asked is, if the input device is a set of relay contacts,
then are they not already isolated enough?
Ignoring that question and carrying on looking for a solution, you might consider
using a transformer to provide isolation. A circuit could supply a small AC voltage to
the primary side (eg. 5V @ 10mA), your isolated switch or relay contacts would be
used to short the secondary. The change in current in the primary would be reflect
the on/off state of the input.
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
>> So, the question is this: Is there such a beast? That
>> is a truly
>> isolated relay closure input which does not require a
>> separate power
>> supply on the relay side - oh and one final requirement,
>> it has to be at
>> least somewhat reasonably inexpensive ($30/each input
>> modules do not
>> qualify). If such a beast exists, it would make my
>> life a lot
>> easier.. If not, well, back to the optoisolator
>> selection guide
>> (hmmm... 3.3V logic level output, wide input current
If powering from the output side is acceptable then for
$30/module it would be easy enough to roll your own if there
were a few involved.
Bent's suggestion of shorting a transformer secondary is a
time honoured and successful one.
The "transformer" could be a ferrite core with suitable high
Or you could do power transfer to an eg optocoupler using a
few photovoltaic cells and a suitably powerful LED light
You could use less power than an opto needs by eg having an
input side oscillator that is powered by optical means and
capacitively coupled back to the output. Switch closure
stops or slews the oscillator.
Opto powering a processor ...
Battery may be present but not evident.
A 100 mAH battery will deliver about 1 uA for 10 years with
infinite shelf life.
Even a modern Alkaline cell has a 7 year shelf life to
rather more than 50% of original charge.
A few hundred mAh of battery would power a processor that
wakes every, say, 10 mS and checks switch closure.
Signalling as seems best.
If switch closure is not of high duty cycle then the same
battery embedded and powering an LED may suffice.
1 mA per 0.1% on time per 100 mAh of 10 year battery
Battery could be charged by opto charging.
An LED can be used as a low power high voltage (inefficient)
Years ago, I worked on a ECG pickup that vibrated about a 3" diameter
piezo-electric disk by applying a frequency to the outer diameter and
connections to the center of the disk were rectified to power the
patient side of the ECG leads. I think the designer had previously
worked on Sonar.
Forrest Christian wrote:
> So, the question is this: Is there such a beast? That is a truly
> isolated relay closure input which does not require a separate power
> supply on the relay side - oh and one final requirement, it has to be at
> least somewhat reasonably inexpensive <snip>
Not opto, but how about a transformer? Put the contacts on the secondary
winding and drive the primary. When the contacts close, the primary
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
> Years ago, I worked on a ECG pickup that vibrated about a 3" diameter
> piezo-electric disk by applying a frequency to the outer diameter and
> connections to the center of the disk were rectified to power the
> patient side of the ECG leads. I think the designer had previously
> worked on Sonar.
> John Snider
> Forrest Christian wrote:
>> So, the question is this: Is there such a beast? That is a truly
>> isolated relay closure input which does not require a separate power
>> supply on the relay side - oh and one final requirement, it has to be at
>> least somewhat reasonably inexpensive <snip>
These devices are easily confused with standard opto-isolators.
These devices are actually photovoltaic devices. Their input is a small
current driving an IR
emitter, but the internal secondary is a series of PV cells wired in
series so that the output is a
voltage. The voltage is dependent on the current of the input, but on
most devices it ranges from
3 to 30VDC. The rub is that the output current is almost NIL, but it
will drive FETs directly.
Look at Digikey's P/N TLP191BF-ND; it is a fine device that I use
frequently, made by Toshiba,
the TLP191B. While its spec indicates 7V, driving 5V thru a 330-ohm
resistor at the input gives
a 20+ volt output, enough to drive any MOSFET I know of. Costs $2.70 USD
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