# Thread: Using resistor to convert to 4-20ma to 1-5v

1. Regular Guest
Join Date
Jun 2008
Location
NC
Posts
87
Post Likes

## Using resistor to convert to 4-20ma to 1-5v

When placing a resistor in the 4-20ma to convert it to voltage is it placed in series or parallel?

When placing it in parallel i get no voltage at the inputs. I run the 4-20ma signal into the analog input, with the common of the 24vdc to the common of the controller, then place the resistor between the common and analog input of the controller. This results in nothing

When placing the 4-20ma signal into one end of the resistor and out the other end i got 7 volts which makes no sense since its a 250ohm resistor. I should be reading around 1.5-2vlts (ma output is 6.21) Either the resistor is bad or ??

Also, how do you figure out the wattage of the resistor to use??

Also had to just flip dipswitches to convert prior to this

thanks!

2. Ive never used a resistor for a AI usually only on AO. It seems like you had it installed correctly.

3. Professional Member
Join Date
Mar 2005
Location
Georgia
Posts
402
Post Likes
Originally Posted by Shockwave
When placing a resistor in the 4-20ma to convert it to voltage is it placed in series or parallel?

When placing it in parallel i get no voltage at the inputs. I run the 4-20ma signal into the analog input, with the common of the 24vdc to the common of the controller, then place the resistor between the common and analog input of the controller. This results in nothing

When placing the 4-20ma signal into one end of the resistor and out the other end i got 7 volts which makes no sense since its a 250ohm resistor. I should be reading around 1.5-2vlts (ma output is 6.21) Either the resistor is bad or ??

Also, how do you figure out the wattage of the resistor to use??

Also had to just flip dipswitches to convert prior to this

thanks!
Is this 4-20 with a two wire connection?

4. Regular Guest
Join Date
Jun 2008
Location
NC
Posts
87
Post Likes
Is this 4-20 with a two wire connection?
Yes a two wire

wattage i figured out, max volts squared divided by resistor value = watts

5. Professional Member
Join Date
Feb 2013
Location
Phoenix, AZ
Posts
198
Post Likes
Here is the normal hookup for a 2-wire 4-20ma instrument converted to 1-5V at the controller (resistor is 250 ohm):

6. Regular Guest
Join Date
Mar 2012
Location
WV
Posts
21
Post Likes
I use a 500ohm and just go across the two wires at the device that is controlling the device.One side on positive one side on negative. If that makes sense. Works everytime. Also this will not work the other way around

7. Regular Guest
Join Date
Mar 2012
Location
WV
Posts
21
Post Likes
Oh sorry buddy.. that for 4-20 to 0-10VDC I read your post wrong

8. 500 ohm resistor will work, however most manufactures I believe use 570. This will give 2-10VDC not 0-10. Wattage shouldn't matter, as amp draw is almost nonexistent. It's a resistive circuit, so ohms law applies.

9. Professional Member
Join Date
Feb 2013
Location
Phoenix, AZ
Posts
198
Post Likes
Originally Posted by LKJoel
500 ohm resistor will work, however most manufactures I believe use 570. This will give 2-10VDC not 0-10. Wattage shouldn't matter, as amp draw is almost nonexistent. It's a resistive circuit, so ohms law applies.
If Ohm's law applies, (and it does), then 570 ohms will not give the expected results:

At 4ma:

E = I R
E = 0.004 X 570
E = 2.28V

At 20ma:

E = 0.02 X 570
E = 11.4V

The load resistance needs to be 500 ohms for 2-10V and 250 ohms for 1-5V.

wattage is:

P = I^2 X R

P = 0.02^2 X 250
P = 0.1 W

P = 0.02^2 X 500
P = 0.2 W

In both cases wattage is less than 1/4W, but the 500 ohm resistor is close, and will get warm (at 20ma).

10. Originally Posted by DDC_Dan
If Ohm's law applies, (and it does), then 570 ohms will not give the expected results:

At 4ma:

E = I R
E = 0.004 X 570
E = 2.28V

At 20ma:

E = 0.02 X 570
E = 11.4V

The load resistance needs to be 500 ohms for 2-10V and 250 ohms for 1-5V.
(at 20ma).
I am we'll aware of how ohms law works. And of course the "correct" answer is 500 ohms. I do however know that I have seen in several applications where manufacturers have used a 570 ohm resistor to preform the same function. Don't know why, if anybody does know I would appreciate an answer

11. Professional Member
Join Date
Feb 2013
Location
Phoenix, AZ
Posts
198
Post Likes
Originally Posted by LKJoel
I do however know that I have seen in several applications where manufacturers have used a 570 ohm resistor to preform the same function.
Could you give an example of one? I have not run across this, would be interesting to examine.

Page 1 of 2 12 Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.