Optocoupler - wide input voltage range - Page 1
Forum section: electronics / beginners

author: PeLa
author_rank: Contributor
timestamp: January 07, 2021, 10:02:27 am
content: Hi everyone,I am hoping that one of you can help me with something that I initially thought would be quite simple... but then found out that it's not... for me.I have an optocoupler that receives a signal from a sensor and forwards it to a microcontroller.I would like to connect different sensors with a relatively wide range of signal voltages (3.3V - 24V).Therefore, the optocoupler should turn on at sensor Voltages above 3.3V. The maximum sensor Voltage is 24V.The LED in the optocoupler has an absolute maximum forward current rating of 60mA. 2mA is enough to reliably turn it on.This is the optocoupler:https://www.vishay.com/docs/84181/tcmt4100.pdfIdeally, I'd like to keep the component count to a minimum and make it work over a temperature range between 20 and 60C.If you have any ideas, I'd be all ears!Best regards,Pete

author: DC1MC
author_rank: Super Contributor
timestamp: January 07, 2021, 10:12:27 am
content: Put a resistor in series that gives you the smallest current to trigger it reliably at 3V3, let's say 2mA, at 24V it should not produce more than 50mA.Ca. 800ohms should be OK.DC1MC

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2021, 10:25:32 am
content: Use a current regulating diode:http://www.linearsystems.com/lsdata/datasheets/J500_J501_J502_J503_J504_J505_J506_J507_J508_J509_J510_J511_Current_Regulator_Diode.pdfYou can also make one yourself using a JFET.Doesn't get any simpler.

author: DC1MC
author_rank: Super Contributor
timestamp: January 07, 2021, 10:46:45 am
content: Quote from: Benta on January 07, 2021, 10:25:32 amI would say a resistor in series is simpler

author: PeLa
author_rank: Contributor
timestamp: January 07, 2021, 11:23:48 am
content: Can't argue with that! Thanks mate!

author: jmelson
author_rank: Super Contributor
timestamp: January 07, 2021, 05:26:07 pm
content: Quote from: DC1MC on January 07, 2021, 10:46:45 amBut, the simple resistor will not work. OK, let's try some values. Most optocouplers need about 5 mA to trigger reliably. So :(3.3 - 1.7)/0.005 = 320 Ohms (3.3 v minium supply minus 1.7 V drop of the LED)Now, how does this work at 24 V? (24 - 1.7) / 320 = 70 mA! Most optos have a maximum LED current of about 20 mA.Now, if you find a sensitive opto that can reliably turn on with much lower current in the LED, the resistor can be made a larger value.Working backwards :(24 - 1.7) / 0.020 = 1115 OhmsNow, at 3.3 V you'd have : (3.3 - 1.7) / 1115 = 1.4 mA If your opto will turn on reliably at 1.4 mA, then this will work.Jon

author: DC1MC
author_rank: Super Contributor
timestamp: January 07, 2021, 05:49:38 pm
content: Well, the OP said that the reliable turn-on current is 2mA and I've based my calculations on this value. In any case, before going on further, a quick test with a resistor will actually take minutes.

author: wraper
author_rank: Unknown
timestamp: January 07, 2021, 06:16:30 pm
content: Quote from: jmelson on January 07, 2021, 05:26:07 pmSimple BJT optocouplers do not trigger to begin with. You have a CTR (current transfer ratio) spec which depends on LED current and is not linear, and usually drops to a very low value at low LED current. You have an analog output in a nutshell.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2021, 06:58:27 pm
content: Quote from: PeLa on January 07, 2021, 11:23:48 amIt seems several other posters can. Also, you'll need a 1 W resistor at least, which generates quite some heat.Thanks for theby the way.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2021, 07:00:16 pm
content: How well the opto-coupler triggers or not, depends on what's connected to the other side.The minimum CTR of the TCM4100 is specified at 50% at 5mA. It will be lower at lower currents: look at the data sheet.A resistor is definitely the simplest option, but not ideal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2021, 08:00:27 pm
content: Easily furnished with a current source.JFETs and CRDs aren't so popular anymore, but depletion MOSFETs are readily available. Indeed, a DN3545 will give you a whopping 450V of input range, albeit not continuous given the power dissipation at that extreme (4mA * 450V = 1.8W).Also, if it works reliably at 2mA, it's probably wise to double the operating point to 4mA, since CTR drops with age and temperature. Or use a somewhat more sensitive or stable type opto, or use a lower threshold on the output side (which can also be sharpened with a current source load, or an amplifier), assuming you don't need much speed of course (optos are very slow and need every mA they can get, to go fast).Examples:https://electronics.stackexchange.com/questions/368365/opto-coupled-eia-422-receiver-understanding-suggested-circuitshttps://electronics.stackexchange.com/questions/355159/wide-voltage-range-input-regulation-24v-3-3vhttps://electronics.stackexchange.com/questions/262011/mosfet-current-source-current-limiter-request-for-reviewTim

author: Gyro
author_rank: Super Contributor
timestamp: January 07, 2021, 08:33:25 pm
content: Why not spread the dissipation with an intermediate clamp.... Series resistor, Zener clamp (maybe 8 - 12V), second series resistor, Opto.It would still be a cheap and minimal solution, which would extend the input range without punishing the Opto LED too much at the upper end. Cheaper than a FET.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2021, 08:51:29 pm
content: Quote from: Gyro on January 07, 2021, 08:33:25 pmAn MMBFJ112 from ON Semi costs around $0.10 and it's just one part. Compared to three parts plus mounting cost.BTW, the DN3545 is around $0.70. Hmmm.I dunno.

author: Gyro
author_rank: Super Contributor
timestamp: January 07, 2021, 08:58:59 pm
content: If it's smd, and the board isn't crammed I think I win. When I was designing high volume consumer stuff, I'dmove heaven and earth toleap on an opportunity to save an easy $0.10 on the bom.The resistors and additional placement are down in the noise, it's the actives cost and sourcing that matter.

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2021, 09:06:41 pm
content: Good. You win.'nuff said, the OP has seen the options.

author: Gyro
author_rank: Super Contributor
timestamp: January 07, 2021, 09:09:32 pm
content: Quote from: Benta on January 07, 2021, 09:06:41 pmSorry, I didn't mean it that way.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 08, 2021, 12:11:01 am
content: US$0.1206ON Semiconductor NSI45030AT1Ghttps://lcsc.com/product-detail/Constant-Current-Regulator_ON-Semiconductor-ON-NSI45030AT1G_C150066.html

author: Smokey
author_rank: Super Contributor
timestamp: May 27, 2023, 10:25:49 pm
content: Bump for some interesting solutions to a common(ish) problem.