Author Topic: Sensing (interfacing) high voltage with arduino or any Microcontroller  (Read 5494 times)

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Offline LaszloTopic starter

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Hi all,

I have a project in mind which involves sensing high voltage dc on/off signals. The voltage which is being sensed is in between 80-120V dc. I am not concerned about switching speed at all, as the signals I'd like to monitor do not change their state very quickly.

My question is what is the best way of reducing the voltage to TTL logic level and provide isolation in between the two system.

There would be above 60+ signals which I need to monitor at the same time, so using a buck converter is out of option I think  :)
I had a look at reducing the voltage down by a drop resistor, but then I realized that this solution is rubbish due to the lack of isolation in between the MCU and the high voltage source.
My other idea was to use an optocoupler and a resistive voltage divider to bring the current/voltage down to a safe level, but I'm not so sure whether that would work in between the required ranges correctly.

Then I found this SN65HVS882 IC.
https://www.designnews.com/electronics-test/interface-high-voltage-onoff-signals-low-voltage-mcus/51223495829507

According to its datasheet, I would still need an additional limiting resistor to use it on the voltage required but looks like an elegant solution with everything included. Has anyone ever worked with one of these devices? Are they as good as the article states?

Also, is there any other solution for this problem? I am sure that I am not the first who had this in mind, but I can't really find much on the internet other than the usual opto-coupler+relay solution.

As always, all inputs to the question are much appreciated.

Cheers,
Laszlo

« Last Edit: May 29, 2019, 06:19:20 pm by Laszlo »
 

Offline bson

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You could divide them down and clock them into 74HC166's cascaded (Q7-to-DS).  Then read them out as a bit stream.  You can isolate the serial interface rather than the inputs using optocouplers; you need the serial out (final Q7), serial clock (CP), and latch edge (PE#).  Maybe master reset (MR#) if you're ambitious.  Of course you also need a supply rail.
 

Offline rstofer

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In most industrial applications, they run the high voltage into an optoisolator.  You would need a resistor to limit the LED current and wide variations in voltage make this an ugly computation.  You need to provide 'enough' LED current at the minimum expected voltage and not too much current at the high expected voltage.  I didn't try to work it out...

On the output side, you can probably use the MCU internal pull-up resistors although I would probably use 1k to Vcc.  See below about resistor size.

Clearly, this solution takes more real estate than a simple divider but I just don't like having high voltages around the MCU.

I would put all of the optos on one or more external PCBs and use an I2C or SPI IO Expander to read the output of the opto.  Unless the IO Expander has internal pull-ups, you would need to provide that 1k pull-up.

The thing about 1k pull-up is that it draws 3.3 mA when pulled down.  Times 60 signals, we're talking about nearly 200 mA.  Maybe 10k will work just as well.  Check the datasheet for the opto and the MCU or IO Expander.
 
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Offline ptricks

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resistor ---> zener diode----> resistor---> optoisolator will work

Roughly, 2K  resistor, 12v zener , 330 ohm resistor  , 4n25 works from 14V-120VDC
« Last Edit: May 29, 2019, 11:29:51 pm by ptricks »
 
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Offline LaszloTopic starter

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resistor ---> zener diode----> resistor---> optoisolator will work

Roughly, 2K  resistor, 12v zener , 330 ohm resistor  , 4n25 works from 14V-120VDC

I've made a sample simulation in LTspice. Is that what you were recommending?

 

Offline ptricks

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I've made a sample simulation in LTspice. Is that what you were recommending?

The 330 ohm needs to be in series with the opto.
The first resistor the 2K limits current to the zener.  The zener clamps the voltage to 12VDC (80-120VDC range on input will always be 12VDC), and the 330 ohm limits current to the LED inside the optocoupler, so your always powering the optocoupler from a preset voltage.
 
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Offline LaszloTopic starter

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #6 on: September 04, 2019, 02:47:20 pm »
Hi,

So, I've built a prototype circuit using voltage dividers and Zener clamps and a sharp PC817 optocoupler. The output of the transistor is driving a 74HC4050 hex High/Low-to-level shifter and its output is being fed to the Arduino's digital I/O. The whole circuit is taking 265uA from the input 110Vdc, so I was quite happy about that. The optocoupler is switching 5Vdc which is a 9Vdc battery voltage regulated to 5Vdc via a 7805 linear regulator.

The circuit works reasonably well, but only if I pull the Emitter of the Opto's transistor down to the ground via a 1M resistor. Otherwise, the output of the 74HC will always be high, irrespective of the state of the optocoupler. I've tried searching for info but I could not find anything similar.

Looking through the datasheet of the 74HC, and it only mentions that the logic device should have a 100-ohm resistor in series with the input. Would that mean that the 100-ohm resistor needs to be in series with the input of the 74HC or the output?

Could anyone point me in the right direction regarding the 74HC4050, and how it should be driven from an optocoupler? I've looked at the datasheet, and some other forum questions, but I've just got further confused. I've read through the relevant chapters of AOE, but this ic never gets mentioned in that book.

On the attached screenshot, the pink line is the output of the Opto, and the dark blue trace is the output of the 74HC4050. The attached circuit is what I've built on the breadboard.

I've found a topic regarding a similar issue but it is still not clear whether I should add the 100-ohm resistor or not and where should I add it.
https://www.eevblog.com/forum/projects/74hc4050-vs-sn74lvc125a/msg825657/#msg825657

Cheers,
Laszlo

 

Online ledtester

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #7 on: September 04, 2019, 09:16:07 pm »
1 M is a huge value for a pull down/pull resistor - most pull-ups in logic circuits are less than 10K. With a 1M pull-down you'll get a logical state change with only microamps flowing through the transistor.  I am not an EE, but it seems the circuit would be prone to false triggering in the presence of noise.

Going with a smaller resistor value means you would have to expend at least a couple of milliamps through the opto-coupler led to fully turn it on.
 
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Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #8 on: September 04, 2019, 09:42:22 pm »
I have a project in mind which involves sensing high voltage dc on/off signals. The voltage which is being sensed is in between 80-120V dc. I am not concerned about switching speed at all, as the signals I'd like to monitor do not change their state very quickly.

So what is your of/off threshold voltage, and acceptable tolerance for that?  Absolute maximum and minimum voltage?  s there a good common ground between the high-voltage and the processor circuitry  or will there be significant common-mode voltages?  How far are these high voltage signals from the processor circuit?  Optoisolators may not be necessary (although they still may be a good idea.)  We can't help you optimize a circuit without knowing these parameters.  I
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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #9 on: September 04, 2019, 10:01:01 pm »

Looking through the datasheet of the 74HC, and it only mentions that the logic device should have a 100-ohm resistor in series with the input. Would that mean that the 100-ohm resistor needs to be in series with the input of the 74HC or the output?
...
I've found a topic regarding a similar issue but it is still not clear whether I should add the 100-ohm resistor or not and where should I add it.
https://www.eevblog.com/forum/projects/74hc4050-vs-sn74lvc125a/msg825657/#msg825657

Where do you see the advisory about the 100R resistor? I've looked at a couple of datasheets but couldn't find any reference to it.

In the 74hc4050-vs-sn74lvc125a thread is this the reference to a 100R resistor you are referring to?
Quote
Try connecting a 100R resistor between the inputs of the gates you're interfacing with and 0V via a 100pF capacitor.

I think that's just to compensate for the high speed of the 74LVC when feeding it a slower varying signal.

 
 
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Offline Mr. Scram

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #10 on: September 04, 2019, 10:03:32 pm »
How would using an optoisolator work? I'm probably misunderstanding something but I can't imagine that being anywhere near linear.
 
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Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #11 on: September 04, 2019, 10:10:34 pm »
How would using an optoisolator work? I'm probably misunderstanding something but I can't imagine that being anywhere near linear.

The OP said
Quote
I have a project in mind which involves sensing high voltage dc on/off signals.

Linearity not required.
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Offline Mr. Scram

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #12 on: September 05, 2019, 12:01:28 am »
The OP said
Quote
I have a project in mind which involves sensing high voltage dc on/off signals.

Linearity not required.
As it turns out I was right while being an idiot.  ;D
 
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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #13 on: September 05, 2019, 02:50:41 am »
Another reason to avoid using large pull-up/pull-down resistors is that the effect of the input leakage current of a digital input pin becomes significant. At room temperature the leakage current for the 74HC4050 is only a fraction of a microamp, but over the entire temperature range it can be as high as a couple microamps. 1 uA of leakage into 1 M ohm is 1 V which is dangerously close to the 1.35V maximum for a low logic level at an input.

Could anyone point me in the right direction regarding the 74HC4050, and how it should be driven from an optocoupler?

I would try something like this:

827277-0

The idea is to pick R1 and D1 so that you get 2 to 10mA through the led when a high voltage is present on B+. You can create a higher voltage zener by putting two or more in series. You can also move R2 to the emitter to invert the output signal.

 

Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #14 on: September 05, 2019, 07:10:16 am »
If you aren't too particular about the on/off voltages, you can simply use a two-resistor voltage divider feeding the opto.  Say a 10K to the high voltage, and a 270 Ohm resistor to ground.  Connect the 10K/270 junction to the opto diode.  Opto current will start to flow around 40V and it will linearly increase to 10mA at 100V (roughly).  The 10K resistor will dissipate close to 1.5W at 120V input. so you will need a power resistor.  You can adjust the threshold voltage and opto current by playing with the resistor values.  Put a capacitor at the opto input if you want filtering.  If you use a high-sensitivity opto you can increase these resistor values significantly, reducing dissipation and current drain.

The zener circuits will give you a more abrupt on/off transition.

So another parameter that would be good to know is the source impedance of the high-voltage source.  How much current can you draw from these sources?
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Offline LaszloTopic starter

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #15 on: September 05, 2019, 02:24:43 pm »
So another parameter that would be good to know is the source impedance of the high-voltage source.  How much current can you draw from these sources?

I am trying to design it in a way so it would draw the least amount of current from the source. Under 0.5mA was my target.

Where do you see the advisory about the 100R resistor? I've looked at a couple of datasheets but couldn't find any reference to it.

Page 4 Figure 6. In here; https://www.nexperia.com/products/analog-logic-ics/asynchronous-interface-logic/voltage-translators-level-shifters/series/74HC4050.html

1 M is a huge value for a pull down/pull resistor - most pull-ups in logic circuits are less than 10K. With a 1M pull-down you'll get a logical state change with only microamps flowing through the transistor.  I am not an EE, but it seems the circuit would be prone to false triggering in the presence of noise.
Going with a smaller resistor value means you would have to expend at least a couple of milliamps through the opto-coupler led to fully turn it on.

The same issue, it would draw way too much current from source if I do that. I was/still hoping that I could make the 5V rail noise-free enough to eliminate false triggering. I've also considered using and LM324 to drive the 74HC4050, but I haven't tried that option yet.

So what is your of/off threshold voltage, and acceptable tolerance for that?  Absolute maximum and minimum voltage?  s there a good common ground between the high-voltage and the processor circuitry  or will there be significant common-mode voltages?  How far are these high voltage signals from the processor circuit?  Optoisolators may not be necessary (although they still may be a good idea.)  We can't help you optimize a circuit without knowing these parameters.

  • 85-120Vdc is when the signal is considered to be on
  • Grounds are fully separate on HV/LV
  • The HV signals and LV will be on the same PCB, with suitable separation in between them. Physical distance is negligible.
  • I just can't think of anything else other than the opto, to provide full galvanic isolation and keeping the price at a reasonable level

To summarize, you think I should be driving the driving the opto with more current, so it would be more stable? Is that really the only option? The circuit does work on breadboard, but I haven't tested it on the field yet. Before I start soldering, I'd like to make sure that it will have a fighting chance to perform.

Thanks for all the input!

Laz
 

Offline Ian.M

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #16 on: September 05, 2019, 03:18:50 pm »
Sharp PC817X datasheet

You are running the opto in your circuit well below the recommended minimum current for If of 1mA.  You are on the dotted portion of the transfer characteristic curve (datasheet fig.6), below 0.5mA, which I would read as probably being extrapolated data, not to be relied on. 

If you decrease the pull-down in an attempt to improve reliability, device to device variation of the opto's CTRR between batches and even on the same reel is probably going to bite you in the ass, and result in a significant number of failures.  CTRR also declines with age as the LED output decreases and declines with increasing ambient temperature, so  if your design  doesn't have enough excess If margin, a specific individual opto that works on the bench today, in the field may not pull the output high reliably on a hot day a few years later.


A major design difference is whether the input channels need to be individually isolated, or whether they can share a common ground (with each other, not the MCU side). 

If the latter, it would be economic use comparator ICs to provide input buffering to allow you to use much higher divider resistances and thus reduce the loading on the voltage sources you are sensing.  It should be possible to go as low as 50 to 100 uA divider current without issues, if you are careful about layout, put slots under the upper divider resistors to increase creepage distance and clean the board thoroughly after assembly. Use an isolated DC-DC converter to power the comparators (and the opto-LEDs) from the Arduino side (but beware of the Arduino regulator's limited 5V output current).  Add a voltage reference IC to provide a stable and well defined threshold voltage for all the comparators to switch at.   

If you need more than about  four input channels, and you don't need a vary fast response, consider using a SPI I/O expander on the HV side of the isolation barrier and optoisolate the SPI bus.  There's three lines going MCU => SPI device, and one the other way, so that only needs one quad and one single optocoupler.  If your chosen I/O expander has a reset pin its probably worth hooking it up to the spare channel of the quad opto, or you could use that for a second /SS signal to access an extra I/O expander for more channels.  It may be worth swapping the single opto for a double to bring the expander's IOC interrupt output (if it has one) to the MCU side so you can hook it up to an interrupt input so your code doesn't have to poll the expander continuously.
« Last Edit: September 05, 2019, 03:49:00 pm by Ian.M »
 
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Online ledtester

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #17 on: September 05, 2019, 03:43:23 pm »
Is this a one-off or will be it mass produced?

Besides driving the LED at a higher current you can reduce the value of the pull-up/down resistor by using a Darlington configuration on the low-voltage side, or use a opto-coupler that has a Darlington output.

Another way to reduce to power draw on the high voltage side is to have your microcontroller use an opto-isolated signal to briefly switch in the measuring circuit when it wants to know the state of the high voltage side. A measurement should take less than a millisecond, so you can save 1000 times the power if you only need to sample the high voltage side once a second.
« Last Edit: September 05, 2019, 04:10:31 pm by ledtester »
 
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Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #18 on: September 05, 2019, 04:08:03 pm »
So another parameter that would be good to know is the source impedance of the high-voltage source.  How much current can you draw from these sources?
I am trying to design it in a way so it would draw the least amount of current from the source. Under 0.5mA was my target.

You might consider a more sensitive optoisolator.  I quickly searched Digikey and found this Toshiba TLP2703 :  https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/TLP2703-TPE/TLP2703-TPETR-ND/6051974.  This one seems to work well with 0.1 mA input current.  The output is a Darlington stage so the output saturation voltage won't be as high as a single-transistor output, but with 0.1 mA input current the output will sink at least 0.5 mA.  Use a 47k pull-up resistor feeding your CMOS Schmitt trigger and things should be good.

This device will let you scale up the input divider resistors to meet your requirements.

There may be better optos, perhaps ones with built-in logic-level Schmitt trigger outputs.  You should do some component research.

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #19 on: September 05, 2019, 08:33:54 pm »

Where do you see the advisory about the 100R resistor? I've looked at a couple of datasheets but couldn't find any reference to it.

Page 4 Figure 6. In here; https://www.nexperia.com/products/analog-logic-ics/asynchronous-interface-logic/voltage-translators-level-shifters/series/74HC4050.html

I think it is just showing you what the input protection circuitry looks like for the device, not that you need to use a 100R resistor on the inputs.

827721-0
« Last Edit: September 05, 2019, 08:35:41 pm by ledtester »
 
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Offline LaszloTopic starter

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #20 on: September 09, 2019, 04:47:23 pm »
Hi,

Thanks for pointing out the different type of optos, I am new to electronics design, so most components are new to me, as well as most design quirks.  :-[
I've done a little search, and a 4N32M looks also suitable (and I can get those cheap in small quantities.) This is going to be a one off trial solution, so price is a big factor for me to consider.

After looking around some more, I've seen some similar circuit with a flyback diode before the optocoupler led. Can I go around this by adding an additional TVS diode after the first stage of the voltage divider?

Cheers,
Laszlo


 

Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #21 on: September 09, 2019, 06:01:49 pm »
Something like this should be pretty robust, and is about as simple as you can get.  The 6.2V zener will protect the optoisolator against overvoltage (over-current) and reverse-voltage.  You could add a capacitor in parallel with the Zener for noise suppression, say a 0.1uF part.  I have no idea if the optoisolator I picked is appropriate, but it does work in the circuit.  A high-sensitivity opto would be best.  To adjust the on/off switching voltage, change the value of R1 (the 150K resistor).  This circuit will draw less than 1mA from your high-voltage input.  You want to follow the opto with a cmos schmitt trigger.

I have the input and output grounds connected together, just to make the simulation easier.  You would obviously want to keep these isolated.
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Offline fourfathom

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #22 on: September 10, 2019, 12:54:20 am »
Something like this should be pretty robust, and is about as simple as you can get.

Just to be clear, my circuit above is far from precise, and will have a very sloppy on/off threshold.  Your specifications are still quite loose, so I am assuming that any additional precision is not necessary.  If you do need more precision, the previously suggested circuits that had zener diodes as voltage dropping elements will provide a sharper and better-defined switching point.

Also, the zener diode and resistor values as shown will provide a good degree of protection from voltage spikes, etc, but it's entirely possible that some outsized transient might cause damage.  Again, without a specification we don't know if more is required.
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Offline LaszloTopic starter

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #23 on: September 10, 2019, 02:27:28 pm »
Hi,

Thanks for the input Fourfathom, it's much appreciated.

Your specifications are still quite loose, so I am assuming that any additional precision is not necessary.

Precision is not necessary. The signals are coming from various limit switches, auxiliary microswitch contacts, relays and long as the speed of switching is within 0.5s I am happy, There is the chance of odd transients, hence I was thinking a TVS is necessary.
This thing will only provide an indication that certain equipment was energized, preferably by taking as little current as possible from the source. This is an absolute must requirement because it's going to be used on rolling stock and trust me, battery power is not something I want to drain off  :)

I've been working on this for quite some time now, and I've tried multiple solutions, like;
  • Driving the Arduino straight from the Opto - what a stupid idea that was   :palm:
  • Drving a relay from an LM324, and have the relay switch in between 10k \$\Omega\$ to ground and 5vDc to the Arduino digital input. - This worked like a charm but uses too much power due to the relays and it still needed an LM324+the opto for isolation.   
  • Using an inverting Schmitt trigger, but I didn't like the idea of inverted logic.
  • And finally the 74HC4050, and this seems to be the best  solution so far

I know that a single voltage divider would work in this case, but I would like to minimize the stresses on components, so I think multiple levels of attenuation with two voltage dividers+2 zeners will be something I will implement. This is where I was struggling the most because there just isn't anything I could find regarding good design practices with multiple stages of "signal" attenuation. I'd be extremely grateful if someone could point me towards a book or something where I can try to understand more about this topic. I've got Horowitses AOE, but I can't seem to find much about this in that book.

Cheers,
Laszlo
 
 

Offline mariush

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Re: Sensing (interfacing) high voltage with arduino or any Microcontroller
« Reply #24 on: September 10, 2019, 03:03:49 pm »
If you want fewer components, you could look into LR8 linear regulators: http://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdf

They can work with minimum 12v, max 440v, and they can output as little as 1.2v ...

Has a 1.2v reference voltage, so if you want it to output 1.2v you only need a single resistor between output and adjust (datasheet says 2.4k) and maybe a couple capacitors at input and output.

The datasheet says  typical 0.3mA, max 0.5mA output current (through the adjust voltage resistor divider and the load), so it may be a bit higher heat produced if you have some optocoupler that needs less than this to operate.

They're cheap at 50 cents each and they're available in to92, dpak, to243 ... yeah a zener is cheaper at less than 10 cents but maybe a regulator will be more rugged and have some additional protections inside and tolerate heat better.


« Last Edit: September 10, 2019, 03:27:00 pm by mariush »
 
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