converting 0-10 vdc signal to 4-20 ma signal

[mujtaba12]

Hi,

I need to ask if some one has ever converted 0-10vdc signal to 4-20 ma signal.. has anybody tried this my sensor is giving 0-10 vdc output but my controller accepts 4-20 ma signal...

[BACnet]

What is the impedance of the input?

Can it accept a 0-20ma signal, or only a 4-20ma signal?

[simux]

On the Johnson AHU controllers they use
1. You use 250ohm for 5 VDC and 500ohm for 10 VDC
2. This gives you a 0 to 20 ma signal

[mujtaba12]

only 4-20 ma signal

What is the impedance of the input?

Can it accept a 0-20ma signal, or only a 4-20ma signal?

[mujtaba12]

how we have to connect parralel to the volatage terminal coming out from the transducer..normally we have a 4-20 ma signal and if we need to convert it to voltage we use 500 ohm resistance in parallel to the current terminals to convert it inot 2-10 vdc

On the Johnson AHU controllers they use
1. You use 250ohm for 5 VDC and 500ohm for 10 VDC
2. This gives you a 0 to 20 ma signal

[BACnet]

how we have to connect parralel to the volatage terminal coming out from the transducer..normally we have a 4-20 ma signal and if we need to convert it to voltage we use 500 ohm resistance in parallel to the current terminals to convert it inot 2-10 vdc

It's a whole lot easier to convert a current to a voltage than it is the other way around.

It would seem that you are saying that you need to scale the signal (0-100% to 20-100%) in addition to changing it from voltage to current. If that truly is the case, you can't just fix everything with a resistor- you're in need of a op-amp circuit of some sort.

[mujtaba12]

cant we achieve this by using a simple resistance??in series or in parallel??

It's a whole lot easier to convert a current to a voltage than it is the other way around.

It would seem that you are saying that you need to scale the signal (0-100% to 20-100%) in addition to changing it from voltage to current. If that truly is the case, you can't just fix everything with a resistor- you're in need of a op-amp circuit of some sort.

[mujtaba12]

ok suppose if I need 0-10 vdc to 0-20ma then what can I do??[QUOTE=BACnet;11271172]It's a whole lot easier to convert a current to a voltage than it is the other way around.

It would seem that you are saying that you need to scale the signal (0-100% to 20-100%)

[BACnet]

In a word- No.

You still haven't given us the impedance information which is the most important thing in this entire thread. Not having seen it though, I can still say that the answer is "No." A resistor network is not going to scale/condition your signal and also boost up the power of the signal enough to achieve 20ma across the input.

Are you absolutely sure that the board will only read in a 4-20ma signal? What brand makes an analog input that can't be configured?

If that really is the case, you could always tear out the input resistors (which convert the current signal to a voltage signal that the ADC reads) and replace them with a set that will look at the voltage directly.

But that would violate all of the certifications for the device and most likely the warranty.

[BACnet]

ok suppose if I need 0-10 vdc to 0-20ma then what can I do??

We still need you to tell us the impedance.

[mujtaba12]

Its a tac humidity sensor.. I dont know its impedance

We still need you to tell us the impedance.

[BACnet]

Its a tac humidity sensor.. I dont know its impedance

The input impedance. Of the controller's analog input. That's the information we need to continue this discussion.

Of course if it's a BACnet controller you can attach the sensor to a better controller (one that can handle other sorts of AI's) and then map the data over to this controller's AI via network writes. Of course you have to set the "Out of Service" property to "true" on the overridden AI to allow it to take the write.

[BACnet]

According to this PDF, TAC humidity sensors can be set to put out 0-10 or 4-20.

Looks like this is a non-issue...