high voltage analog optocoupler  - Page 1
Forum section: electronics / projects

author: hussamaldean
author_rank: Unknown
timestamp: January 25, 2019, 12:38:59 pm
content: Hi allI am looking for an analog optocoupler for arduino project which is flyback boost converter which shall boost the input voltage from 12V to about 200V. however, dealing with such high voltage is dangerous and may burn my arduino if voltage divider is used.I looked using google but I couldn't come with some usefulis there any type of those thing ?not very high bandwidth though up to 50KHz is more than enough

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2019, 12:41:28 pm
content: 200V is not a high voltage.Most opto-couplers have an isolation voltage in the kV range.Why do you think it has to have bandwidth of 50kHz? The opto-coupler feedback path should have a much lower bandwidth, than the switching frequency.Please post a schematic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2019, 02:29:49 pm
content: Which one is it, flyback or boost? Flyback is transformer coupled, and an opto is required to get feedback. (No unusual voltage ratings are necessary.) Boost is common ground, and does not need an opto.If you are worried about resistor breakdown, use a few in series. 200V is acceptable for most 1/4W carbon film resistors, but smaller types (including 1206 and smaller SMT chip resistors) should use two or three in series.Tim

author: hussamaldean
author_rank: Unknown
timestamp: January 25, 2019, 02:41:02 pm
content: Quote from: Zero999 on January 25, 2019, 12:41:28 pmthe schematic in the attachmentthe optocoupler will be used as feedback between the output of the DC value and arduino to control the duty cycle of the mosfet/igbtQuote from: T3sl4co1l on January 25, 2019, 02:29:49 pmyou can find the schematic in the attachment

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 07:21:58 pm
content: Horowitz and Hill Art of Electronics Book Release 2 has a complete switcher including optocoupler feedback, if you are interested.

author: TimNJ
author_rank: Super Contributor
timestamp: January 25, 2019, 08:05:28 pm
content: Hi,You don't need anything special to do 200V. Typically, for isolated converters, a TL431 is used on the secondary side. The TL431 is basically just an error amplifier with an integrated voltage reference. It'snota zener diode, though it is often drawn that way.In the attached schematic, the resistors of interest are Ra, Rb, and Rc3.The TL431 continually tries to maintain 2.5V on its REF pin by feeding an error signal back to primary-side controller. The primary side responds accordingly and the error is corrected on the secondary side.To set up the output divider, we know that TL431's REF pin will be always be 2.5V, and that we'd like Vout to be 200V. Let's choose Rb, the bottom resistor, based on a reasonable amount of dissipation in the output divider, maybe 100K to start.Now we know 3 out of 4 variables in our voltage divider equation. Just rearrange to solve for Ra.2.5 = 200 * (100K)/(100K + Ra)Ra = 7.9MegOhmSo if you want 200V on the output, you need a 7.9M + 100K resistor (or another combination of the same ratio).---Rc3 sets up the bias current through the optocoupler's LED. Sometimes it's referred to as the "master gain" resistor. You need sufficient LED bias to ensure the control loop can respond to different events.Rc3 = (Vout - VLED- V431)/ (Ibias), where VLED is around 1.0V and V431 is 2.5V max.Typically, 1-10mA is needed through the LED. If you design around 5mA, then you'd need a 39K resistor for Rc3.Hope this helps.

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2019, 08:19:55 pm
content: TL431 is not a high voltage part, I don't believe that will work.If the TL431+opto LED is "off", say we're at 198V, what is its cathode voltage? It gets pulled up there.

author: TimNJ
author_rank: Super Contributor
timestamp: January 25, 2019, 08:41:10 pm
content: Quote from: floobydust on January 25, 2019, 08:19:55 pmAh yes, of course...head's not on straight, apparently. Thanks for catching that.I think the usual solution to this is to put a zener diode in series with Rc3 so that the worst case voltage across the cathode-anode is <36V. So, in theory, a 180V zener would work fine. You'd have to re-calculate Rc3 accounting for 180V drop across the zener. (The resistance will go way down.)Any reason that shouldn't work? I think you just have to be prepared to dissipate 0.5 - 2W in the zener.

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2019, 08:58:56 pm
content: OP didn't say how much current his 200V power supply has. I think you'll waste some power with an opto-coupler.A geiger counter PSU does not have enough current to light an opto LED.Nixie tube power supplies just use a resistive voltage divider.OP should be fine using that, 200V will not kill the Arduino if the resistors are done right.I also see a HV zener array + optocoupler LED or transistor etc. It's a bit mushy and has a tempco but good enough.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2019, 11:07:16 pm
content: Quote from: floobydust on January 25, 2019, 08:19:55 pmThe simplest approach is to add a zener in series with the TL431, but it might not be workable at such high voltages, especially if it needs to be adjustable.The TL431 can be easily cascoded with a BJT to work at whatever voltage you want it to. The TLV431 could even be used, for lower power dissipation. I haven't done this with a switched mode with optical feedback, but it works really well in a linear regulator set-up.EDIT:It will need some frequency compensation, which I've omitted, but the resistor values will get you close to 200V.

author: hussamaldean
author_rank: Unknown
timestamp: January 26, 2019, 03:25:00 am
content: I am just looking for linear optocoupler for feedback purposewhat type shall I use ?TL431 is enough ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 26, 2019, 07:39:52 am
content: Quote from: hussamaldean on January 25, 2019, 12:38:59 pmHow accurate woul you want thet "about 200V" to be?TL431 has a pretty sharp "Knee" between of and on and with that it delivers a very small window in which there is some analog amplification.TL431 is also the defacto standard for these kind of applications, and very many are being used in PC power supplies and most wallwarts of better quality than the lowliest crap.Do you have some wallwarts lying around?If you crack them open you are likely to find most of the components you need.You can exchange the primary and secondary of the transformer.Adjes the circuit around the TL431 to limit at a higher voltage.But directly powering the optocoupler LED from 200VDC is a problem.A nice approach is to have an extra transformer winding with a 1:40 ratio to the 200V winding.Then you have an auxillary 5V to power the opto coupler.And Again:Houw much current do you need @ 200V?It has been asked before, but I missed the response.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2019, 07:50:31 am
content: Also, TLV431 may be better, because it needs less current.Any phototransistor opto will do, 4N35, FOD817, etc.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: January 26, 2019, 02:28:54 pm
content: Quote from: hussamaldean on January 26, 2019, 03:25:00 amJust a reminder that the TL431 is not an opto-coupler. It is an error amplifier with anopen-collector output. The TL431 is connected to an optocoupler to control/modulate the signal fed back to the primary-side.When you hook up the open-collector output of a TL431 to the cathode of an opto-coupler's LED, the TL431 can adjust how much current it pulls through the LED by turning on or shutting off its output transistor.Typical opto-coupler part number is LTV-817.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2019, 06:44:30 pm
content: Another possibility is an isolation amplifier, such as the ACPL-7900-300E, to monitor the voltage using a potential divider, but it will be more expensive than the TL431 and an opto-isolator and will require a 5V axillary power supply on the secondary side.https://docs-emea.rs-online.com/webdocs/133c/0900766b8133c16e.pdf

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2019, 01:42:02 am
content: Quote from: T3sl4co1l on January 25, 2019, 02:29:49 pmNothing precludes wiring both sides of a flyback transformer to a common ground so feedback can be conventional. A flyback transformer could be used to lower the voltage requirements of the power switch. In the example below, the flyback transformer could be replaced with an inductor and still drive the capacitive multiplier. The capacitive multiplier could be extended to further stages.