Need Help -Replacement parts for 4-20 ma loop controller

Thread Starter

Tarmizi Hamid

Joined Jun 7, 2015
5
Hi there, im repairing one of my client's equipment. Please refer to picture below. my problem is i need to replace the black components. the components is combination of relay which is uses 4-20ma as control current to complete the secondary circuit which is 15vac converting to 5vdc. can you guys help me out here. im stucked. i cant find any replacement component such as ssr or conventional relay to replace this item. this is custom made from China.
 

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MaxHeadRoom

Joined Jul 18, 2013
28,619
If you know the function of the original, then you should be able to duplicate it, if it is a custom made component then you need to show a little more of the circuit, that particular picture of the board shows very little without knowing what and where the connecting pins do.
Max.
 

wayneh

Joined Sep 9, 2010
17,496
I think you're stuck with working out the schematic from the PCB tracing on the back of that board, which looks like it shouldn't be too hard. Do you know the pinouts of those connectors?
 

panic mode

Joined Oct 10, 2011
2,715
I am with wayneh, you need to know functionality of the unit and
replicate it using whatever you have at disposal.
4..20mA transducers/signal converters are very common.

what I find odd is:
1. you refer to it as 'relay'
2. it uses analog 4..20mA loop signal to 'convert 15VAC to 5VDC'

my guess is that loop signal is just converted to PWM for example. 15VAC is odd too...
 

Thread Starter

Tarmizi Hamid

Joined Jun 7, 2015
5
here is the diagram.


thats how the all six pins connected. 2 pins for the 15vac. 2 pins are for signal a signal b, both are 4-20ma, the last 2 pins is the ouput which will produce 0-5vdc to activate the ssr. The 4-20ma also have voltage around 1-5v.
 
Last edited:

ian field

Joined Oct 27, 2012
6,536
4 to 20mA...Sounds like a car's oxygen sensor.
Its an ancient serial protocol that dates back nearly to the telegraph stations in the wild west before they had wireless.

I think it was used on some teleprinters and a few early serial I/F ASCII RTTY printers.

May even have been used for the old tikka-tape machines.
 

ian field

Joined Oct 27, 2012
6,536
I just implemented one for a Hydraulic (4-20ma) pressure sensor!
Max.
I suppose there's nothing to stop you using it for analogue level transmission, if you don't mind the possibility of drift.

My first introduction to 4-20mA loops was from computer books - I did start reading up on it all those many years ago, but it quickly became apparent that by the time I'd learned enough, it would be entirely ousted by RS232.

Desperate as I was all those years ago for a printer, I refused a number of offers of 4-20mA loop printers because by then RS232 was the only game in town.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
But we are not talking about digital transmission, the PDF gives an excellent description of the 4-20ma loop which is still popular 60yrs hence for all kinds of transducers, Omega (et-al) , one of the biggest manuf of transducers still offer their products in this medium, and if there were reliability issues due to drift etc, I doubt they would be continuing with this type for critical measuring, weigh scales etc.;)
Max,
 
Guys:

1) For serial transmission, there is no 4 in Current loop. A 20 maybe. See, http://www.google.com/url?sa=t&rct=...=8ZCKiNW7OBbtr-u2J9fERQ&bvm=bv.96041959,d.b2w

DECwriters and ASr33 Teletypes were inherently current loop with cards to change to RS232.

2) 4-20 mA is an analog loop where 4 mA is used to power the sensor through the signal wires. Voltage drops due to wiring resistance is therefore not an issue.
It's very common in process control.
 

ian field

Joined Oct 27, 2012
6,536
Guys:

1) For serial transmission, there is no 4 in Current loop. A 20 maybe. See, http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CCsQFjAC&url=http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf&ei=_DCDVeTELoScsAXspICADA&usg=AFQjCNFo7-PfaSP86Ki1KKHqCmf8dBtnMA&sig2=8ZCKiNW7OBbtr-u2J9fERQ&bvm=bv.96041959,d.b2w

DECwriters and ASr33 Teletypes were inherently current loop with cards to change to RS232.

2) 4-20 mA is an analog loop where 4 mA is used to power the sensor through the signal wires. Voltage drops due to wiring resistance is therefore not an issue.
It's very common in process control.
I was vaguely aware of analogue applications for current loop communication, but my own interest in it was purely to get a printer on my IBM XT. By that point in time, RS232 was rapidly taking over and any interest in current loops rapidly evaporated.
 

Thread Starter

Tarmizi Hamid

Joined Jun 7, 2015
5
Just got back from client's office, what I need basically something from off the shelf, 15vac convert to 0-5vdc, the gate will be control by 4-20ma current. So the output will proportionate with the control current. Can come body help me what product that I can buy.
 
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