Need appropriate V/mA convertor

Hello guys,
I have one problem regarding voltage to current conversion and hope you'll able to help me.
I have one device that provides analog voltage signal in range 1 to 5 V. I need to convert this
signal to signal 4...20 mA. I have searched for voltage/current converter that could do this, but in
a catalog which I have there is an optio to use input voltage 0..5V and not 1..5V. How would you solve
this? Please can you make some recommendations and save me from further wasting of time.
Thanks in advance

This could be quite easy actually...

First try to measure the input impedance (read resistance) of the device you're willing to connect to the 1..5V output device.

Most likely it's going to be 250 Ohms, but it might as well be 125 Ohms or even 500 Ohms.

1. If the input resistance is 250 Ohms you can connect input and output directly without the risk of any damage (1V / 250 Ohms = 4mA - 5V / 250 Ohms = 20mA);
2. If it is 125 Ohms you have to add an extra 125 Ohms resistor in series between the 1..5V output and the 4..20mA input.
3. If it's more than 250 Ohms, you're out of luck! You will need an active converter to overcome this problem.

BTW, the 0..5V converter could work just fine IF its output varies between 0..20mA. 1V input would still equal to 4mA output.

See site below, they have what you need:

http://www.miinet.com/products/sg_signal.shtml

Try also.


http://www.api-usa.com/signalconditioners.php

Thank you all for help.

Sparkz said:

This could be quite easy actually...
First try to measure the input impedance (read resistance) of the device you're willing to connect to the 1..5V output device.

Click to expand...

This is very interesting. I need to convert 1..5V signal to 4..20mA signal in order to transfer that signal to PLC analog card (S7-300) which is configured to expect 4..20mA, so I don't think it will be good to connect output from device that provide voltage signal to AI card input no matter what input resistance of AI card is. Is there any other solution I might use to improvizve since this is supposed to work for a short time (until device that accept this voltage signal, makes log and output current signal to AI card is back from service).?
Thanks a lot

It may be much easier than you think.
Which analog module is it exactly ?
Most, but not all, analog input modules for S7-300 can be configured in several ways.

All SM331 AI modules can handle 1..5V signals directly, without the need of a voltage/current converter! They all have 'combined' inputs. Try to dig up the specs...

Or maybe you're using ET200S modules? Current input resistance varies from 25 Ohms for 4AI-2wire/standard to 50 Ohms for 2AI-2wire/standard, 2AI-2wire/hispeed and 2AI-4wire/hispeed.

If you insist on using a current input of a SM331 module, only the SM331 AI 8 x 16bit has the ideal 250 Ohms current input resistance. So only this input module can be connected directly to your output device, for all others you'll have to add a series resistor.

The SM331 AI 8 x 14bit and the AI 8 x 13bit have an current input resistance of 50 Ohms: You would just have to add a 200 Ohms/E96 (or E192) in series between output device and analog input.

The SM331 AI 8 x 13bit has a 25 Ohms current input resistance: A 226 Ohms/E96 (or E192) resistor comes the closest to the ideal series resistor value of 225 Ohms.

Well, one way to do it could be to set up the module to handle voltage signals from 0 to 10 V, and then use scale functions, but that would require modification on existing equipment which is not smart idea (because of other channels).It is analog module SM 331 AI 8x12bit. Since that part is in operation I'm not able to test and measure resistance. It seems I'll need to order one of the convertors you suggested. I was hoping there is some quick improvized solution.
Thanks very much!

Mikas.
On the 6ES7331-7KF02-0AB0 module there are 4 "groups" of input channels. The Measuring Range Modules sets 2 analog channels each. If you are free to set 2 channels to the same then you can directly select 1-5V as the input range.
Otherwise, like you say, you have to use a converter. There are a million of these. Search for "converter" or "isolation amplifier".

mikas_m said:

... so I don't think it will be good to connect output from device that provide voltage signal to AI card input no matter what input resistance of AI card is...

Click to expand...

Analog cards use a resistor to convert current to voltage.
If the input impedance is 250 ohms, it will be 1-5 volts. There is no problem connecting a 1-5 volt source in this case.

mikas_m said:

... one way to do it could be to set up the module to handle voltage signals from 0 to 10 V, and then use scale functions, but that would require modification on existing equipment which is not smart idea (because of other channels).It is analog module SM 331 AI 8x12bit. Since that part is in operation I'm not able to test and measure resistance. It seems I'll need to order one of the convertors you suggested. I was hoping there is some quick improvized solution.

Click to expand...

So, you're unable to make any parameter changes...

However, the SM331 AI 8x12bit module also has a 25 Ohms current input resistance. If the module is already set as 4..20mA input, you could use a 226 Ohms (E192 preferred) series resistor. 1V from your output device will result in a 3.984mA input current; 5V will result in a 19.92mA current.

This would be THE quickest AND cheapest solution to your problem. Although it's not ideal, measuring error will not exceed 0,4%. If it's only temporary, it'll work just fine.