E = IR: It’s Not Just a Good Idea, it’s the Law

I recently had a conversation with a customer that resulted in one of those forehead-slapping “duh” moments for me, and I thought it might be worth passing along. Here’s the story:

The customer had an application that required an analog linear feedback sensor that provided an output of 1 volt to 5 volts over the linear stroke range. Now, a 1-5V output is not very common, and the particular sensor he was interested in was only available with either a 0-10V or a 4-20 mA output. What to do? Perhaps the answer should have been obvious to me, but it was the customer who provided the solution this time: “couldn’t I use a 4-20 mA output and 250 ohm resistor to get my 1-5V output?” Why, yes….yes you could (smack…..duh!). And I know it will work, because we have the law on our side. Ohm’s Law, that is: E = IR, or voltage equals current x resistance.

Let’s check it:

4 (mA) x 250 (ohms) = 1 (volt)

20 (mA) x 250 (ohms) = 5 (volts)

So there you have it. Take a very common 4-20 mA output and drop it across a 250 ohm resistor and, lo and behold, you have your less common 1-5V signal. And, if you do this conversion right at the input to the controller, you get the added benefit of increased noise immunity of the 4-20 mA signal.

And, yes, I’m sure I knew of this little trick at one time. Maybe the part of my brain where this information was stored got overwritten by the names of the contestants on The Amazing Race or by the rollout plans for my million dollar consumer product idea: Dehydrated Water (just add water). But let’s keep that just between us, ok?

To learn more about analog feedback sensors visit www.balluff.us

3 Replies to “E = IR: It’s Not Just a Good Idea, it’s the Law”

  1. Hi
    I am facing a problem now with 4-20mA current signal (Level Transmitter) to Siemens S7-1200 PLC. Now the Open circuit voltage is 20.37V. But if we connected with PLC the Voltage is 2.3V. Also if I measure the ampere in series with a 100K resister then it is 7.3mA. But the PLC is indicating Overflow bit 32627 always without any changes.
    So can I use 250Ohm resister right at the PLC terminal..?

    1. Scott Rosenberger says:

      Hello Ajay, I’d rather not presume to offer troubleshooting advice for a specific product. Probably better to contact the manufacturer of your specific level transmitter in this case.

  2. Tony Barnett says:

    Classic, I’ve often forgotten about this simple mod, twice with same customer (still can’t figure out why he’s still talking to me, perhaps I make him feel smart). You’ve been brave to publish it though. 🙂

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