# 4-20mA circuit issue

#### sidexter

Feb 23, 2023
2
Hi,

I have a circuit which gives 4-20mA current, however the as the sensor drift the current also drift below 4mA (which is ideally correct ).

however, i want to restrict the current to 4mA , i.e. the current should increase from 4mA to 20mA but should not go below 4mA incase of drift in sensor.

Request you to kindly assist on how that can be achieved.

Thanks & Regards,
Siddhesh

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,179
Show us a schematic diagram of your circuit. Without it we have no chance of giving you a meaningful answer.

#### kellys_eye

Jun 25, 2010
5,684
The principle of the 4-20mA is that when your sensor outputs 0 the circuit 'sends' 4mA....... if it's dropping below 4mA then there's something fundamentally wrong.

#### Kabelsalat

Jul 5, 2011
178
Agree, seems it's a scaling issue.

#### sidexter

Feb 23, 2023
2
Hi,

Thank you for the prompt reply, the scaling is working fine, infact there is no issue with the circuit.
my issue is, I dont want the current to fall below 4mA.
according to the circuit, i get 4mA when TP4 is at 0mV, but when sensor drifts, TP4 goes into negative voltage usually -20mV to 60mV. i want to restrict the TP4 voltage to 0mV in this case.

Thanks,
Siddhesh

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Jun 25, 2010
5,684

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,179
Klick on it, rotate by 90 °.
@sidexter : what part is U2? Wouldn't this part offer a setting that ignored negative inputs?

#### kellys_eye

Jun 25, 2010
5,684
but when sensor drifts, TP4 goes into negative voltage usually -20mV to 60mV.
So what? The fact that:
according to the circuit, i get 4mA when TP4 is at 0mV,
Which means it's working as it should.

What relevance has the inner workings of the 4-20mA circuit to do with what you want to read? If you are getting 4mA when the (external) sensor reads 0 and 20mA when the sensor is reading (whatever) maximum then the system is working as it should be.

Or am I missing something here?

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