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Topic: isolation temperature sensor (Read 2725 times) previous topic - next topic

natanel

Jan 10, 2012, 01:26 pm Last Edit: Jan 10, 2012, 01:42 pm by natanel Reason: 1
I need to isolate an arduino analog input connected to thermistor from all other digital and analog pins
it should be isolated for medical usage (15 kV ESD protection)
both V,GND and analog

similar to the usb isolation but between the analog pin and the thermistor
http://www.bb-elec.com/product_family.asp?FamilyId=651


any ideas where to start?

johnwasser

Sounds like you need a separate processor to read the thermistor.  Then you can add isolators between the Arduino and the secondary processor.  You will need an isolated power supply and isolated I/O lines.  Easiest would be a separate battery for power and an opto-isolator for serial communication.
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natanel

Thanks a lot
So I will look for isolated adc (looking for whole solution in a single package)
but I can not use battery
is there a device that will get the mcu power and isolate it (like the usb isolater from above)
Combinig isolation for both power and data lines
Any ideas?

robtillaart

Quote
but I can not use battery

Can you explain why not?
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

natanel

This is a request from the system not a real limit but something I can not change
is there adc isolated i/o and power (single input power for both side of the device)
or any other idea?

graynomad

#5
Jan 10, 2012, 10:40 pm Last Edit: Jan 10, 2012, 10:42 pm by Graynomad Reason: 1
You may find this in one package as an isolated Data Acquisition System but it will be very expensive I think.

Isolated DC/DC converters are pretty cheap and there are 1000s of them.

So I'd do what johnwasser said, a separate uC, DC/DC converter, and an opto coupler to send the data back to the Arduino.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

natanel

thanks you all
great help

any way if you know about an evolution board with MCU/ADC + optocopluer +Isolated DC/DC converter
please tell

dc42

#7
Jan 11, 2012, 10:40 am Last Edit: Jan 11, 2012, 11:03 am by dc42 Reason: 1
Alternatively, since the temperature measuring device is a thermistor, you could connect it across the secondary of a line isolation transformer (assuming you can find one with 15kV isolation). Then use the transformer primary as one arm of an AC bridge or voltage divider. Much simpler than using a separate MCU.

EDIT: found some 15kV isolation transformers at http://www.telbit.ch/tel/index.php?module=pagesetter&tid=4&topic=1&tpl=Products&filter=pkat06:ne:0

btw most isolated DC-DC converters won't give you 15kV isolation.

EDIT 2: just noticed you are probably not looking for 15kV isolation, since the USB unit you referred to only provides 2kV isolation. In that case, a cheap telecomms line isolation transformer would do, e.g. http://uk.rs-online.com/web/p/telecoms-high-frequency-pulse-transformers/5330160/]
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natanel

#8
Jan 11, 2012, 11:20 am Last Edit: Jan 11, 2012, 11:40 am by natanel Reason: 1
thanks

my mistake it is 1.5k dc isolation under 60601-1 spec

can you give me some link or describe the solution you suggested so I can understand it
(not realy sure what to do how to connect it to ADC of arduino, how it will get power)

EDIT: how do you get from the spec the dc isolation? http://uk.rs-online.com/web/p/telecoms-high-frequency-pulse-transformers/5330160/

dc42

#9
Jan 11, 2012, 12:03 pm Last Edit: Jan 11, 2012, 12:12 pm by dc42 Reason: 1

thanks

my mistake it is 1.5k dc isolation under 60601-1 spec

can you give me some link or describe the solution you suggested so I can understand it
(not realy sure what to do how to connect it to ADC of arduino, how it will get power)


I'd post a schematic, but the forum isn't even accepting a 4Kb image upload, so here is a description, using 2 transformers T1 and T2:

digtial output pin --- T1 primary pin 1
T1 primary pin 2 --- analog input pin --- T2 primary pin 1
T2 primary pin 2 --- capacitor +ve
capacitor -ve --- ground

Connect the thermistor across T1 secondary, and a reference resistor R across T2 secondary. Drive the digital out pin with a square wave of about 500Hz, and half way through each half cycle measure the voltage on the analog input pin (probably best done by setting up a tick interrupt every 0.5ms). The difference between the readings in alternate half cycles will give you the peak to peak AC across T2, and hence the ratio between the two resistors. I've used a second transformer to connect the reference resistor R so that the effects of transformer inductance will tend to cancel out; but you could omit it and just connect the reference resistor directly instead. These transformers are designed to work with around 600 ohm impedance, so choose a thermistor with a resistance of about that at the temperatures you want to measure, and use a similar value for the reference resistor.


EDIT: how do you get from the spec the dc isolation? http://uk.rs-online.com/web/p/telecoms-high-frequency-pulse-transformers/5330160/


If you follow the link to the data sheet, it says the isolation is 3750v RMS. Here's another one .pdf]http://www.tscomponents.com/products/ETAL_Data_Sheets/data1165%20_ETALDOC078_[1].pdf that says 5.5kv DC.

[EDIT: you'll need to cut & paste that link because the forum doesn't handle links containing square brackets properly.]
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mullenr1a

Hi natanel,

  True, 1.5KV isolation from the MAINS, earth grounds, and SELV wires, but 6KV ESD protection, at least, for 60601-1.

  You might want to look at

  Isolated power supply, VHS1-S5-Sx-SIP
  Digital isolated buffers,  Iso7520
  And an SPI thermistor interface, MAX6682

  With these you don't need another uP and you can get > 8KV of isolation.


dc42

Or MAX6682 as per your suggestion + ADUM6401 ?
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