Post by thread:[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...). -forrest

On 11 Dec 2007 at 0:42, Forrest Christian wrote: {Quote hidden}> 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...). > 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 eMail: spam_OUTbrent.brownTakeThisOuTclear.net.nz

>> 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...). 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 frequency drive. Or you could do power transfer to an eg optocoupler using a few photovoltaic cells and a suitably powerful LED light source. 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 capacity. Battery could be charged by opto charging. An LED can be used as a low power high voltage (inefficient) photo cell. Russell

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: > <snip> > 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> >

msi1259 wrote: > 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: > >> <snip> >> 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 in singles. --Bob Axtell