Signal converter with analog output


Thread Starter

Juan-Carlos Furest

Hello everybody,
I am planning to develop a signal converter that works with a small liquid flowmeter that gives a frequency output signal( the frequency is proportional to the flowrate or each pulse correspond to a volume passed throught the device ). This frequency has to be converted to an analogue value. I heard of 0-10V, 4-20 mA, 0-20mA, loop powered, etc...
Which one is the commonly used as analog input to control the flowrate in the processes applications?
What are the differences between 4-20mA and a loop powered device?

Best regards

Jocko Harmet

4-20mA loop powered by a 24vdc loop power supplyis the most popular signal used in sensors, for the following reasons:
first, a current signal can traverse great distances without signal loss - voltage drop is not an issue. This type of signal is relatively immune to noise and can be passed over a twisted pair instead of expensive shielded cable.

as for loop powered, that depends on your application. if your circuit can operate on less than 4 mA, you can power your device from the loop. This simplifies things for the end user

It usually is a conductor issue. If your device is not loop powered, the user must supply power conductors and signal wiring -

Where as a loop powered device is powered by the users power supply and only one twisted pair goes out to the device.

for more on 4-20mA and loop power, check out our web page - to get right to the section you want, follow this link: LOOP SIGNALS.htm

Jocko Harmet
[email protected]

If you want to save the time and expense of developing this device yourself, several companies (including Moore Industries) offer frequency-to-dc converters. Moore's PC-programmable model FDY accepts the input you describe and provides a 4-20mA analog output. It derives its operating power from the 4-20mA loop (loop-powered). Let me know if you are interested and I will send you information on such devices.

Steve Todd
Moore Industries
[email protected]