Need to make 4 - 20mA loop isolators

Thread Starter

bowlingo

Joined Jun 29, 2011
162
Hi all,

I have a combined PH and CF transmitter of which can be found here

http://www.omega.co.uk/ppt/pptsc.asp?ref=CDTX300

I need to make 2 x 4 - 20mA loop isolators that I can connect to each output. The unit runs on a 24V power supply

Someone has pointed me to some of the components

http://docs-europe.electrocomponents.com/webdocs/0028/0900766b800289f3.pdf

http://www.rapidonline.com/Electrical-Power/1W-Dual-output-SMD-DC-to-DC-converters-RSD-series-36106/?sid=003c3b70-f289-488d-b4a3-bb276c21a821

This will be the fist time I have made something like this and would like to do it on small prototype boards with chip housings etc.

Can anyone provide me with the correct parts from farnell or RS and possibly an easy to understand diagram?

Thanks
 

MrChips

Joined Oct 2, 2009
21,364
I would assume that if two readings are transmitted via a two wire interface, then this is a digital output. You will need a digital processor such as a microcomputer to interrogate the data.
 

Thread Starter

bowlingo

Joined Jun 29, 2011
162
I would assume that if two readings are transmitted via a two wire interface, then this is a digital output. You will need a digital processor such as a microcomputer to interrogate the data.
This is the unit I have used on it in the past

http://uk.rs-online.com/web/p/products/4662214/?searchTerm=4662214&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B367D247C5E5C647B377D247C5E5C647B31307D2426706F3D3126736E3D592673743D52535F53544F434B5F4E554D424552267573743D343636323231342677633D4E4F4E4526

As you can see its disgustingly expensive £134.40 including VAT

I am sure these can be made easily using basic components and a bit of soldering..trouble is what components from RS or Farnells, chip housing etc and a wiring diagram...I would like to use the existing 24V DC supply

Thanks
 
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MrChips

Joined Oct 2, 2009
21,364
I am sure these can be made easily using basic components and a bit of soldering..trouble is what components from RS or Farnells, chip housing etc
I can never figure why people say it must be easy to do and they come running to us for help.
 

Thread Starter

bowlingo

Joined Jun 29, 2011
162
I can never figure why people say it must be easy to do and they come running to us for help.
That is a very good point..can anyone who knows how to do this help me please as quite rightly as mrchips has pointed out its not easy to do hence the reason I am asking
 
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Thread Starter

bowlingo

Joined Jun 29, 2011
162
I believe that's the same device my first reference op note discusses.

If you want true isolation that circuit won't do. You need an added isolator as shown in the op note. Otherwise you can just power it from a DC supply within the circuit's voltage limits of operation.

Thanks for your help...can you help me out with the exact components?....I really would like to make this and test it
 

crutschow

Joined Mar 14, 2008
25,128
If you want a cookbook design here's a circuit shown in Figure 19 of this ap note.

Edit: The circuit parts are readily available and low cost. It uses an optocoupler for high isolation between input and output.
 
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