Voltage drop question

[iant]

What is the terminal voltage of the old transmitter
I calculate the resistive loop @ 4mA to 10.5 K Ohm.
But for a 6 V drop I am missing something

 

[JeffKiper]

I don't have the old transmitter or the old alarms. They tossed them out.

 

[iant]

@ 40Vdc - 10.5k = 0.0038 mA -- (10.5k @ 48 Vdc 4mA)
@ 40Vdc - 2.5k = 0.016 mA -- (2.5K loop @ 48 Vdc 20mA)
not sure still thinking - theses are mental notes

 

[JeffKiper]

Ian I did a small spreadsheet that does the calculations for me.
you can change the numbers in the tan box.

I did the numbers and it looks like I need a 43 volt system to be mid-range. I am hoping the power supply has some voltage regulation and I can turn it down to at least 46 volts.

 

[iant]

the supply is 48 Vdc
- the Transmitter Voltage is 40V max 10V min

your calculated Voltage drop is 6 V @ 4mA ~ 30V @ 20mA @ 48V
Is this calculated or did you measure this originally?
where did you measure the V drop from

6 Devices in a series circuit from your transmitter
@ 48V 4mA loop Resistance = 6v / 4mA = 1.5k or 250 ohm each
@ 40V 4ma x 1.5k = 6v 20mA x 1.5k = 30v max v = 34 min = 10
using a Additional resistor to drop the loop supply from
48 v to 40 V @ 4 mA = 8v / 4 mA = 2 k ohm

My original Calculations still work

 

[JeffKiper]

Those numbers are calculated not measured numbers. I will be playing with the system in the morning. I will let you know what I did to b-gger it up.

 

[iant]

worst case senario - add an additional power supply

 

[JeffKiper]

I have a problem with the system that is kicking my backside.

I have a system with 5 devices on it and it works fine. Same transmitter, same power supply. 1 less device on the loop.
I had a transmitter shipped in today. Went to the site used a 24 VDC power supply and verified all the devices worked they do. When I put them in a series loop on the 48 volt system I get 2.8mA. The transmitter has a minimum minimum 3.8mA so I am lost. I will be calling technical support Thursday for help.

 

[iant]

is that any five out of six or only 1 doesnt work

 

[rdrast]

Can't you just split some of the devices out, and retransmit the loop signal to them with an ActionPak or similar device?
Or replace one of the process meters (I imagine there are some) with one capable of retransmission?

 

[JeffKiper]

Sorry I wasn't clear. The loop with 6 devices doesn't work. I can remove 1 device and it still does not work. I maybe able to retransmit the customer wants to do it as cheap as possible. So we all know how much fun that is.

 

[Mickey]

customer wants to do it as cheap as possible

Cheap and possible don't always work.

 

[iant]

it is not a case of a Hart system that needs to be configured is it.

 

[JeffKiper]

Sorry I just got this job up and going the pipe fitters took a looong time. I replaced the power supply wiith an adjustable one. All of my problems went away. I set it up for 43 vdc and everything is good now.

Thanks for your help.

 

[Lancie1]

Jeff,

I liked your spreadsheet, so updated it so that it works for many cases by changing the 4 top yellow variables, and also calculates the desired power supply Base Voltage.