HVAC Tech School: The Easy Way to Convert 0-135 ohms to 4-20 ma

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What is the easiest way to convert 0-135 ohm signals to 4-20ma signals?

One of the most common questions we receive  at Stromquist & Company  is how to convert zero to 135 ohms signal into a 4 to 20 milliamp signal.


Think about how many times you have been in the field and your control signals don’t match. Maybe you have an older damper or valve actuator that operates off of a 135-ohm signal and the new controls system ends out a 4-20 ma signal. 

One option is to replace the actuator that takes a 0-135 ohm signal with an actuator that accepts a 4-20 ma signal. Your local control vendor will love you for this, and it will work but this is the expensive way to solve the control signal conflict.

A less expensive way is to use a device that will convert a 0-135 ohm signal to a 4-20 ma signal. 

ACI offers a plug and play interface board for this exact application. Watch as  ACI’s Tech Support Manager, Matt Buchholz, demonstrates how these devices work.

These are typically used in retrofit applications like the situation we mentioned before,  where you have a zero to 135 ohm actuator and your controller is sending out a 4-20 ma control signal. You will see these on control applications controlling ducks, tanks, boilers, pipes, and other heat exchangers.

The  ACI analog rescaling module DRM can be factory calibrated for a zero to 135-ohm input and a 4 to 20 milliamp output.

Another common question we receive is how to convert a 4 to 20 milliamp signal into a zero to 135-ohm signal.

Another product  we handle at Stromquist & Company is an interface from ACI called the DRM 3.1. The DRM series accepts analog pulse or floating-point input signals converts them into a proportional resistive output. it directly replaces a variable resistance controller and simulates the action of a slide wire or rotary potentially actuator..

The DRN 3.1 must be ordered with a resistance network, the resistance network would be ordered to match your required resistance output. The most common is a zero to 135 but other common values are zero to 270 ohms, zero to 1000 ohms and zero to 10,000 ohms.

The DRN3.1 is an interface that allows microprocessor control of a variable resistance. The DRN3.1’s isolated resistor network can be controlled by several different DDC signal types. It directly replaces a variable resistance controller and simulates the action of a slide wire or rotary potentiometer. All connections of the simulated potentiometer, the wiper and both ends of the resistance range, are available on the DRN3.1terminal strip.  The DRN3.1 accepts an ANALOG, PULSE, or FLOATING POINT input signal (including triac) and converts it into a proportional resistive output. The output resistance does not wrap around if the input signal exceeds the highest or lowest selected input value.  The DRN3.1 has on-board fail-back relays that lock out the original resistive signal during operation.  However, if the supply power is lost, control of the circuit will revert back to the original controller signal.  An easy local override can be made by placing a fixed (or variable) resistor between W and R Fail-safe terminals.  The DRN3.1 is covered by ACI’s Two (2) Year Limited Warranty.

The DRN4 is a resistive output motor actuator interface that accepts several types of DDC system signals. The DRN4 output is 0 to 135 ohms. The input signal types are field selectable by an 8-position DIP switch.  The floating point input accepts two digital signals, one for increase and the other for decrease.  The floating point full scale rate of change is 55 seconds.  Some triac input signals require an accessory.  The DRN4 is supplied in an enclosure that can be directly mounted to a 1/2 inch knockout on the motor acuator.  Color coded wire leads with spade connectors are provided for electrical connections.  The DRN4 is covered by ACI’s Two (2) Year Limited Warranty.

Thanks to Matt and the team any ACI for great products and great training resources.

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