# Convert 4-20mA at 125 VDC to 4-20mA at 24 VDC

Y

#### Yap Chih Hsiung

How can I convert a 4-20mA signal at 125 VDC coming from a device to 4-20mA
at 24 VDC?

Thank you.

R

#### Rufus

AFAIK, There is no conversion. Amps is amps. The voltage required to drivie
the amperage will be relative to the load
of the current path.

E

#### Ericka F. Calton

My question is, what are the necessary parameters for establishing the
stability of the signal
after it is converted?

Ericka F. Calton ([email protected])
Master's student, Biomedical Engineering

J

#### Johan Bengtsson

What the voltage behind the 4-20mA signal is should normally not be of any concern. the "reading" device normally let the current pass a resistor and read the voltage drop. This voltage drop don't have anything to do with the voltage of the source as long as the signal is within it's limits (ie between 4mA and 20mA). If there is another concern such as some common ground or something similar and this is the problem: insulating I/I repeaters exist and would do the work.

/Johan Bengtsson

----------------------------------------
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/

N

#### Nachiket Sahasrabudhe

Hello Yap,
You may use an isolator module that isolates input signal, output signal and
supply voltage.The isolation modules available are DIN rail mounting
type.The module should withstand at least 500 Volts DC between input and
output.
Regards.
Shrikant

N

#### Neil Jepsen

A 4-20mA "receiver" won't know what the voltage behind the 4-20mA source is. i.e. A far as the load is convcerned, it does't matter a damn what the voltage behind the source is. (Provided the isolation to ground is adequate). A 4-20mA
source can be though of as a "variable" constant current source, and the transmitter simply cranks the volatge up, until the correct current is flowing. Hence, a 24volt 4-20mA souce can only drive 20mA into a 1200 ohm load, but a
125volt source can supply 20mA into a 6250 Ohm load. If your load was say 500 ohms, then the signal generator would only crank the voltage up to 10volts, irrespective of whether the total available voltage was 24v or 125 volts. If you were worried about protecting your receiving device, you could put a 10w 3.3k resistor in series with your loop, and a 24v zener accross your receiving device. This would limit the maximum voltage seen by the receiver, if the load
resistor was to go open, to 24volts.
neil.

W

#### William L. Mostia, Jr

>A 4-20mA "receiver" won't know what the voltage behind the 4-20mA source is. i.e. A far as the load is convcerned, it does't matter a damn what
>the voltage behind the source is. (Provided the isolation to ground is adequate).........<

While this may be semantics, the correct specification in this case for the receiver unit that involves the voltage at the receiver unit's terminals is the common mode voltage(CMV) spec and not the isolation from ground spec. This spec may also be called the maximum input voltage. This represents the maximum voltage to ground that can appear on the input terminals. For example, the Allen-Bradley analog input module 1771-IFE has a CMV spec of +/- 14.25 volts and the Modicon ADU206 analog input module has a maximum input voltage of +/- 30 volts. This spec is not always published on spec sheets and may have to be asked for. Exceeding this voltage can cause mis-operation and possibly damage to the receiver unit.

William(Bill) L. Mostia, Jr. PE
Principal Engineer
WLM Engineering Co
Independent I&E Consultant
P.O. Box 1129
Kemah, TX 77565
E-Mail: [email protected]

These opinions are my own and are offered on the basis of Caveat Emptor.

V

#### Vitor Finkel

Johan Bengtsson wrote:

>What the voltage behind the 4-20mA signal is should normally not be of
>any concern. the "reading" device normally let the current pass a resistor
>and read the voltage drop. This voltage drop don't have anything to do with
>the voltage of the source as long as the signal is within it's limits (ie
>between 4mA and 20mA).

There is the catch ! If the resistor is calculated to provide a voltage
drop larger than 24 VDC it will surelly limit the current to lower values
than 4-20 mA.

>If there is another concern such as some common ground or something similar
>and this is the problem: insulating I/I repeaters exist and would do the
work.

I am sorry to point this out to you Johan. There is no way you can make a
resistor to generate a voltage drop larger than the voltage supplied by the
power supply that feeds it.

Otherwise we are attributing to a mere resistor the capability of generating
electricity. This could lead to a very efficient and free energy generating
system based on perpetual motion. Let's patent it and we'll soon be richer
than Bill Gates.

As a matter of fact, as you increase the resistor value trying to increase
voltage drop above a certain value, the resistor will limit the current to
a value lower than the 20 mA you taught it would be "pumped" into it by the
24 Vdc supply. There is no way a 4-20 mA circuit, generating this current
from a 24 Vdc can generate a higher voltage than the 24 Vdc, except with
a sort of converter circuit ( for instance: transform Vdc into AC, pass it
through a transformer, rectify it back to DC, but then current will be
limited, since the power drawn will never be greater than 20mA X 24VDC
= 480 mW ), or using any other externally higher voltage power supply and
any sort of amplifier circuit.

Just remember: Resistors are not allowed to generate voltage, but only to
cause voltage drops, never larger than available voltage.

Vitor Finkel [email protected]
P.O. Box 16061 tel (+55) 21 285-5641
22222.970 Rio de Janeiro Brazil fax (+55) 21 205-3339

J

#### Johan Bengtsson

Sorry, did probably not read the question properly. I missed the direction you wanted for the conversion....

Ok, so what you need is a I/I converter with a voltage source on the output side greater than 24V.

I don't know if they are availiable in a off-the-shelf manner. Of course building one will not be too hard to do, but that may not be what you want. I could design a circuit for you if you don't find any other solution and want to build it yourself. Write me back directly in that case and we'll discuss the spec.

Just out of curiosity, what kind of device is it this signal is feed into?

/Johan Bengtsson

----------------------------------------
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

J

#### Jose Tolentino

Hi Listmembers,

There ain't to catch on what Johan had written due to the fact that it is very common for all PLC analog input modules or signal processors using method of successive approximations that the "resistor" in question is
only 250 Ohms thus will cause a voltage drop ranging from 1V-5V DC.

I cannot think of any definite answer on a 4-20mA @ 125VDC to 4-20mA @ 24VDC conversion here unless I am sure on whether the 125VDC refers to the excitation voltage ( or power supply) of a current transmitter and the 24VDC refers to the external power supply for the "reader" i.e. PLC single-ended 4-20mA analog input module. If I am right with this presumption then indeed a conversion is needed as all PLC analog 4-20mA analog input modules' common-mode voltage do not exceed 40VDC!

This subject interests me a lot and it would be nice if Mr. Yap would give as the application in details!!! Only right there and then that we can all pool our talents to answer the question of conversion.