PC817 opto - reverse voltage on the out. transistor? - Page 1
Forum section: electronics / beginners

author: naut
author_rank: Contributor
timestamp: January 26, 2017, 07:40:59 pm
content: Hi,the datasheet for the PC817 opto-coupler reads: Emitter-collector voltage 6V.Does this mean that if the emitter of the output transistor is connected to ground and 6V is applied to the collector, the transistor will burn?Can it be continuously operated in reverse with a lower reverse voltage, e.g. 5V?https://radiodetali.com/media/catalog/product/8/7/875_pc817.pdfthanks

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 26, 2017, 07:49:53 pm
content: If you apply GND to the collector and 6V+ to the emitter it will burn. Used "normally" it will withstand 35V.

author: SeanB
author_rank: Super Contributor
timestamp: January 26, 2017, 08:14:59 pm
content: Run in reverse it will typically act like a zener diode, and will break down ( depends on the device you have as to exact voltage) at around 7-9V reverse bias. Providing you limit the current it will do no short term damage to the transistor, but running long term will damage the junction, and the transistor will not really work as an optocoupler in reverse, the coupling will be poor, and with long term reverse bias will just drop with time.

author: naut
author_rank: Contributor
timestamp: January 26, 2017, 11:54:49 pm
content: In my circuit the opto-coupler will connect to a input socket for TR/TRS jacks. I'm mainly thinking of reverse polarity protection because the user could insert a cable with the wrong polarity, not very likely, but still...So for now I think that I would either insert a rectifier in between, or maybe I could find a opto-coupler with bidirectional transistors on the output. The opto-couplers with triac/thyristor outputs won't work because they stay on after the input channel is off. Perhaps an opto with a j-fet output might work, I'll have to look it up.

author: oPossum
author_rank: Super Contributor
timestamp: January 27, 2017, 01:41:05 am
content: Bidirectional MOSFET output optocouperWithstands 60V in both directionshttps://toshiba.semicon-storage.com/info/docget.jsp?did=17036&prodName=TLP222A

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2017, 09:02:32 am
content: How much current does it need to pass?A diode is a sensible option if you don't need it to work when reverse biased, just to protect it against damage. If the voltage drop is unacceptable, then the diode can go in reverse parallel with the opto-coupler.MOSFET opto-couplers are more expensive than BJT opto-couplers. If you're only switching small signals (under 100mA) then the TLP175A might do, which is cheaper than the TLP222A (linked to above) but the on resistance is much higher at 50 Ohms.http://docs-europe.electrocomponents.com/webdocs/129d/0900766b8129d5be.pdf

author: naut
author_rank: Contributor
timestamp: January 28, 2017, 12:08:14 am
content: QuoteNot much. It's only for logic signals. The opto-coupler transistor will be emulating a foot-switch. A lot of musical devices have foot-switch inputs that enable the user to e.g. switch presets with a foot-switch. I'm assuming that the they use 5V levels or lower.QuoteThat's how I will probably do it, cheaper and simpler PCB design. I'll only have to write in the user manual that if the user plugs in a cable and nothing is happening, it means that the cable has the polarities reversed or that the foot-switch input on his/her device has the polarities reversed. Although I reckon that the music industry has a polarity standard, i.e. Tip = positive, Sleeve = Ground/Negative.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2017, 11:47:39 pm
content: If the voltage drop is not an issue then how about a bridge rectifier on the BJT opto-coupler?

author: SeanB
author_rank: Super Contributor
timestamp: January 29, 2017, 09:29:37 am
content: As it emulates a switch why not use a mechanical contact in a 5V relay. That way it will work with both logic level systems and with those that use 12 or 24V relay inputs as well. If you want longest life use a relay with 2A contact ratings, I doubt there are many musical controllers with a higher current in the switch side. on the contact side put a 22n 275VAC class X capacitor in series with a 100R 0.25W resistor across the contacts, so there is both snubbing for high current applications and contact cleaning for low current applications. 275VAC class X because those are both low cost, rugged and reliable ( unless a transparent RIFA product, but those are long discontinued) and will switch mains.Just use an isolated jack ( so it does not contact the casing if metal) and run the wiring and PCB layout inside as if you are doing IEC mains isolation, with 3mm spacing from the control logic traces, 2mm spacing between switch traces and use a 300V rated connector for it, with 300V rated wire ( preferably twisted) connecting to the socket in the case. That way a fault in the other side does not kill you or the users of the equipment and as a bonus less chance of noise being induced via this connection.

author: naut
author_rank: Contributor
timestamp: February 01, 2017, 05:37:29 pm
content: QuoteThanks for your comprehensive response, but I can't use a relay because one of the functions of my circuit will be to translate a drummers click track to foot-switch presses, that will kill the relay pretty fast since click tracks are usually quarter notes, so e.g. at 120 BPM that's two clicks per second.

author: BrianHG
author_rank: Super Contributor
timestamp: February 01, 2017, 06:03:56 pm
content: How much is too much cost? The TLP222 at digikey is 1$ each for 1. Most likely cheaper elsewhere. There are more expensive SS-Relays which will do high voltage AC & DC, but I bet internally, they have a reed relay + mosfets.Reed relays exist which are designed to switch 120bpm 24/7, 365 days a rear, but they cost $$$.If the 60v of the TLP222 isn't large enough, let me know, there may be a bridge solution, but now we are going way up in parts and price.\Too bad vactrols are obsolete. They do cost more than the TLP222, but you might be able to get one with 100v AC/DC switch, but, they behave like a large 1-10meg resistor when off, a 1-10 ohm resistor when on.If you do go with the TLP222, I would recommend 2 zener diodes, back-back, on the output to protect the 60v limit of the optocoupler's output. In fact, using this trick with a dual, or 2 of the TLP222 outputs in series, you would now have a 120v switch. You can also take things in the other direction, parallel output to get higher currents.

author: naut
author_rank: Contributor
timestamp: February 02, 2017, 10:48:41 pm
content: @BrianHG, I don't think that there's a need for anything fancier than a pc817 opto-coupler for my circuit. I've measured the voltage levels on the foot-switch inputs on some of my music equipment (synths, guitar efx), they're either 3.3 or 5V. So a pc817+diode or pc817+bridge rectifier is sufficient.Other than a opto-coupler there is also the option of coupling an LED with a photo-resistor (inside heat-shrink tubing).