mA to mV conversion circuit 4

[jtcDart]

I need to make a simple circuit to convert mA to mV. I have a Fluke 789 Processmeter that will source 4-20mA for calibration. I have a need for a 0-30mV signal that the meter cannot provide. My initial thought was to place a resistor in parallel to the load, but the math works out to a 0.0006 ohm resistor.
Anyone have any ideas?

Thanks,
Todd

 

[Skogsgurra]

My idea is that 1 mA causes a 1 mV drop in a one ohm resistor.
So, if you need 30 mV at 20 mA, just use a 1.5 ohm resistor. Offset the 6 mV caused by the 4 mA. And there you are.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[DRWeig]

Like Gunnar says.

Phoenix Contact and quite a few others have ready-made converters for 4-20 mA ==> 0-1 mA. Pass it through 30Ω of resistance and you're golden.

30Ω is not a standard value except at 5 percent tolerance. You might want to work with a 50Ω pot or else grab a decade box. Or make your own.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[VEBill]

Can you accept the offset inherent in the simple solution?

4-20mA -> 6-30mV

My instinct says to aim for a much higher initial voltage, and then knock it down with a voltage divider adjacent to the input. One would need to be cognizant of the available voltage within the 4-20mA output, and be aware of the input Z for the input.

The higher intermediate voltage may allow easier use of a DVOM to help confirm correct operation at the mid point. It might also help avoid the usual millivolt problems of noise and thermal offsets.

Think about calibration, perhaps with selected values and parallel pots to tweak. One should be able to combine normal values.

 

[ScottyUK]

Many current sources capable of 4-20mA can also manage 0-20mA. The Fluke 715 certainly can - might be worth checking what the 789 can do.

 

[MikeHalloran]

Do the conversion as physically close to the millivolt input as possible.

A current signal can pass unmolested through crappy connectors and lots of unshielded wire; a tiny voltage signal cannot.



Mike Halloran
Pembroke Pines, FL, USA

 

[OperaHouse]

Whoa..that is a lot of math to do out in the field. How about two one ohm resistors in series and a three position rotary switch. One position would be Zero Reference, Two would tap the one ohm resistor for an easy to read 4-15mv and the Third would be the top of the next one ohm in series, 8-30mv an easy times two of the ma scale.

OR There are lots of ten turn precision pots on ebay with direct reading dial. Try for a pot under 500 ohms. About a 10 ohm resistor would give you 100mv for 10ma. You could adjust the current up or down to give 100mv full scale. Staying down at the bottom range of the pot would give you a readable 0-30mv

 

[jtcDart]

Hey everyone - Thanks a bunch for the suggestions. I plan on placing a pot in parallel with my leads in a small box that I can plug meter leads into each side...4-20mA in, 6-30mV out! I have jacks on order, a pot that I have adjusted and tested, and a small terminal block. Now I just have to find a small box to put it in so I can toss the whole thing in the box with the meter.

I'll say it again - THANKS FOR ALL OF YOUR SUGGESTIONS!!!

Todd