Reading Isolated Signal

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Hi,

 

I am using a wide input TEN 5-2411WI dc/dc converter chip with a Vin Gnd and a Vout Gnd. 

 

On the input side of the dc/dc Vin is a 12V battery and on the output side is a 5V micro. 

 

I would like the micro on the Vout side of the dc/dc to read the battery voltage level on Vin, but I have stumbled across a problem that they share different grounds.  I know how to turn things on and off using an opto but I do not know how to read something.  

 

Could somebody offer me a circuit idea or isolated comparator or the best practice for reading and isolated voltage with separate grounds?

 

 

Vin Gnd ------------- TEN 5-2411WI ---------- Vout Gnd

 

Many thanks.

 

Tuurbo46

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It's quite possible to use opto-isolators to isolate an analogue signal. The trick is to use two of them so that one is in a feedback loop to linearise things.

 

As an example, to help your search engine terms...

 

http://www.electroschematics.com...

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#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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why use an isolated dc2dc converter with a battery? 

 

Jim

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ki0bk wrote:
why use an isolated dc2dc converter with a battery?

Hmmm--12V ... automotive?  One way to start an automotive design?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Brian Fairchild wrote:
It's quite possible to use opto-isolators to isolate an analogue signal.

Indeed.

 

There are also specifically-designed devices; eg, http://www.analog.com/en/products/linear-products/isolation-amplifiers.html

 

Another option is to put the ADC on one side of the isolation barrier - so that you only have to pass digital stuff through the isolator.

 

Do you really need isolation anyhow?

 

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 offer me a circuit idea

I tend to over complicate things, but:

 

Although Brian mentioned a linearized opto-isolator, I never had good results with that when I played with a few years ago.

(Admittedly, I didn't spend long on it...) 

 

My project required isolation between the signal and the real world, and I had a micro on both sides of the isolation.

The project ADC converted the analog data, put it in a "packet", (start of packet, data, checksum), and fed it through an opto-isolator to the second micro.

The data transmission / reception was easy, it simply used the USARTS.

 

So, applying this concept to your project, I'd consider putting a Tiny with an ADC on the 12V battery side, read the voltage through a resistor divider, (with a cap across the lower leg...), and pump the data out the USART to an opto-isolator.

The "down-side" of this, of course, is that you add the complexity of an additional micro, and hence also an additional 5V power supply running off the battery side of things.

The up side is that a continuous ADC to packet to USART project is pretty simple if you have much experience with micros.

 

If you simply wanted a 3 level indication, (Vbat > 14V, 10-14V, < 10V) type indication, then a couple op-amps configured as comparators, (and a "window comparator"), could feed three separate opto-isolators, like dash board idiot lights, to give data to the isolated side.  Once again, a power supply for the op-amps would be required.  One might even use something like the old LM324 op-amps, Vcc 0/32Vdc, but some filter / load dump protection would still be required.

 

That said, this is starting to sound like the old "What good is a 555 timer chip these days?" Threads.  Before I'd go back to analog window comparators, I'd use a small micro with an ADC, especially given the low update data rate needed for a battery monitor.

 

The old project below shows a PCB with a Mega168 on the isolated side, a Tiny-whatever it was on the real world side, and two, 6-pin dip chips which were the opto-isolators for bi-directional comm's between the two sides.

 

JC

 

 

Edit:  Both a typo or two, and to note that I must be typing awfully slowly today, as several other comments beat me to the punch. 

 

 

 

Last Edited: Tue. Jan 23, 2018 - 09:01 PM
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DocJC wrote:
My project required isolation between the signal and the real world, and I had a micro on both sides of the isolation. The project ADC converted the analog data, put it in a "packet", (start of packet, data, checksum), and fed it through an opto-isolator to the second micro. The data transmission / reception was easy, it simply used the USARTS.

 

You beat me to it, that was what I was going to suggest after the OP explained why the iso was chosen....

 

 

Jim

 

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DocJC wrote:
Although Brian mentioned a linearized opto-isolator, I never had good results with that when I played with a few years ago.

Have you ever tried with a proper analogue isolator - such as I linked.

 

I've never actually tried them - so don't know how well (or otherwise) they work out in practice...

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Nope, I've not use one of those.

IIRC I used external compensation networks, several types, one with a transistor and an op-amp, etc.

It was far more trouble than it was worth.

I spent hours playing with circuits and scoping & plotting results...

 

It took about 10 minutes to add the ADC setup, packetization, and USART data streaming to the M168 that was already in use.

 

Pick your battles, and designing a linear opto-isolator just wasn't on my list, (for long, anyway).

 

BTW, gotta love the price range on the link of parts, $1 USD to > $100 USD, (each, for quantity 1000) !

 

JC

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I guess that's why they make purpose-built parts!

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Might use an isolated system in a battery stack, like an E.V.? Would lead to unbalanced battery charge, but I can imagine someone trying to do it.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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How about a voltage to PWM converter on the isolated side, send that signal across the optocoupler to a timer input on the avr (I'm assuming that the microcontroller is an AVR. Figuring out the changes needed for different microcontrollers is an exercise left for the reader :-)?

 

- S

 

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With the low prices of uC's nowadays there is hardly any excuse to not throw in an extra uC.

Because of the battery powered nature the current consumption / battery life is probably a concern.

An uC can sleep most of the time and every now and then wake up and push the battery voltage through an optocoupler.

With a 2nd opto coupler & two direction communication you can even use that to wake the uC up on the moments you ant info from it.

 

What kind of battery size are we talking about here?

How big of a consern is battery life?

These modules have an efficiency of 80% or lower.

 tracopower.com/products/ten5wi.pdf

Which means you have the opportunity to extend battery life by pushing less energy through them.

 

How much functionality can you move from the 5V side to the 12V side?

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Design your own DC-DC converter that switch with a freq that is linear to voltage on batt., and then make a AC tap of the 5V that go to a counter on the uC. 

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mnehpets wrote:
How about a voltage to PWM converter on the isolated side

or voltage to frequency?

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Ross McKenzie ValuSoft Melbourne Australia

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Hi All,

 

Thanks for all your help.

 

I have opted to go for awneil solution.  I have selected the ADUM3190. OK so far so good, I have decided to use figure 29 example circuit on page 13 of the datasheet, but I have a couple of questions.

 

1) I was hoping I could make VDD1 = 5V and VDD2 = 12V, but looking hard at the datasheet at the top of page 3 just below specifications it quotes "VDD1 = VDD2 = 5 V, unless otherwise noted"  Does this mean I cannot make VDD2 a different voltage to VDD1?

 

2) To measure the battery voltage I have dropped the voltage down using a resistor network (see below), and i am feeding this voltage directly into pin 12 (+IN) on figure 29 example.  The output of figure 29 pin 7 (EA Out) I am feeding straight into the ADC pin on the 5V micro.  

 

Vin (9V - 14.4V) ------ 9k3 Res ------- Vout (3V - 4.8V) ------- 4k7 Res ------ GND

 

3) Am I correct in thinking the output (EA Out) swings between 0.4 V and 2.4 V, when my input (+IN) swings between 3V and 4.8V.

 

4) Can you see any big mistakes I am making?

 

Many Thanks,

 

Tuurbo46

 

 

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mnehpets wrote:
How about a voltage to PWM converter on the isolated side, send that signal across the optocoupler to a timer input on the avr (I'm assuming that the microcontroller is an AVR.

via http://cds.linear.com/docs/en/datasheet/69921234fc.pdf (page 25)

Linear Technology

LTC6992 - TimerBlox: Voltage-Controlled Pulse Width Modulator (PWM)

http://www.linear.com/product/LTC6992-1

https://www.mouser.com/Semiconductors/Power-Management-ICs/_/N-wnwh?Keyword=ltc6992&FS=True

 

"Dare to be naïve." - Buckminster Fuller

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4) Can you see any big mistakes I am making?

Given your avatar, and the reference to 12V, one might assume you are working on a vehicle.

 

Hopefully you are already fully aware of "Load Dump" issues, and the high voltage positive and negative spikes that can be found on vehicle power busses.

 

Failure to protect your circuitry, including all inputs such as the voltage divider, can result in premature failure of the electronics.

 

JC 

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Tuurbo46 wrote:
I have selected the ADUM3190. OK so far so good, I have decided to use figure 29 example circuit on page 13 of the datasheet, but I have a couple of questions

For detailed questions about the specific chracteristics of particular components, you really should be asking the manufacturer - Analog Devices, in this case.

 

The Product Page has a section on 'Discussions' - there is a link to Ask the Analog Community at the end of it: http://www.analog.com/en/products/interface-isolation/isolation/isolation-amplifiers/adum3190.html#product-discussions

 

 

Does this mean I cannot make VDD2 a different voltage to VDD1?

I very much doubt it - but the only people who can really confirm it are AnalogDevices themselves.

 

 

 

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