4 to 20 mA into 0 to 750 ohms

Max4507

Member
Join Date
Feb 2016
Location
Québec
Posts
35
Hi everyone!

I'm not sure this is a fully PLC question, but I do not know a better place to ask this question!

I am looking to retrofit an old burner controlled with a Barber Colman 560 into a PLC solution. This temperature controller have an output which is described as it : 4 to 20 mA into 0 to 750 ohms. This output controls SCR, and documentations about those SCR is not available.

So my question is, is this 4 to 20mA output a standard 4-20mA output? I do not understand why they wrote .. into 0 to 750 ohms.

Best regards,
 
I agree with labeledas.

It actually says 4-20ma into 0-750 ohm load or 0-10ma into 0-1500 ohm load.
 
that's the Max output from the controller
750 ohms at 20ma = 15VDC
1500 ohms at 10 = 15VDC
that's what it can drive
the switched output is a triac rated at 1 A Max Voltage of 240VAC only
any 4-20 ma input into a PLC will work as it states it is isolated from the inputs
the 0-750 ohms means that it is current limited or short protected
a lot of 0-20ma drivers are short protected
 
a lot of 0-20ma drivers are short protected
All are. A correctly wired mA current output can drive across a short circuit - the current is regulated by the driver to the its intended value.

I had run into a poorly designed loop powered device that failed when the loop power supply voltage was connected to the wrong terminal, but that happened 7-8 years ago and the product is no longer on the market.
 
those are dangerous words in our trade "all, none, always and never"
while it may be true the newer controls are short circuit protected but
some of the older ma outputs are not short circuit protected and if you do make the mistake of shorting the output it will destroy the diver so I try to avoid that.
if you are wrong and it is not protected and you blow a $2K sensor who pays for it.
 

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