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4-20ma loop receiver question

I need a 4-20ma loop to 0-5 volt converter to interface a sensor to a data logger. I do have a BB RCV420 chip which does just that, but it requires a dual supply of 12-15 volts (minimum), and I need to keep things simple and light because it will be used in the field away from power.

Is this the best way to go on this, possibly with a couple of small 12 volt smoke detector batteries, or is there a simpler way using an op amp that I can have a single supply....possibly a 9volt transistor radio battery?

The equipment needs to operate for 6 to 8 hours continuously, without interruption for battery changes. My data logger has a 5volt supply output if needed, and my loop sensors use 12 volt battery supplies.

Thanks

Reply to
Jim
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You could use a rail-to-rail opamp where its output can at least go to the negative rail. If you can't sense from a resistor to ground you could come down with a divider so you get into the range of a common 9V battery and build a differential amplifier.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Well, the BB chip is all nice and good, but have you considered the use of a resistor as a current-to-voltage converter? It doesn't require dual supplies, can be pretty stable, and has other advantages.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

The 4-20mA interface is for industrial environments characterized by lots of electrical noise, mechanical vibrations, long distances from the sensor to the data acquisition system, a multitude of different 'grounds',and nearly unlimited electrical power. Do any of those things pertain to your remote battery operated requirements? Are you and your sensor/data logger out in the middle of nowhere where it is electrically quiet? If so, then scrap the 4-20mA interface, use micropower components.

Reply to
Fred Bloggs

A 250 ohm resistor will convert 4-20 mA to 1-5 volts, where 1 is the low end of the range.

They did this on purpose, so that < 4 mA (0-1V) indicates a fault.

Cheers! Rich

Reply to
Rich Grise

The answer would be to use a Virtual ground with the RCV420 that you already have so that way you can use a single 12 volt supply. this will yield 6 +/- volts, minus some losses from the converter which should work out to 0..5 volts as you need. Just beware that using this kind of ground requires that your supply not be used for other devices at the time other wise, you'll get a problem where the other devices maybe using the (-) of the source for example as the ground and connecting the 2 device wouldn't work very well. There is how ever, a isolated DC-DC converter trick you could use that wouldn't give you such problems.

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Reply to
Jamie

You could also use a CD40106 running off the 9V battery as an oscillator, hook some modem transformer up to it and create +/-15V or whatever is needed on the other side. Tough to get 8 hours out of a 9V battery with that RCV chip though.

But I would think twice about the BB chip. It's expensive and Digikey is out of stock, IMHO the markings of a boutique chip.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Actually, I don't see why a basic op-amp via load R can't do the job using a 9 Volt bat, the common side from the supply to the op-amp can be past via a diode. This would thus be the common for output and would bring a non-rail op-amp closer to a rail output for the circuit at hand.

I did something like this years ago using an old 741 op-amp, I used a single isolated supply and lifted the Vee(common -) to the op-amp. of course today, we have rail to rail amps but I still like the older types for basic work.

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Reply to
Jamie

Sure it can. All you need is a simple opamp, or one wired up as a differential amp if you don't want to or can't sense to ground. Depends on whether Jim really wants to use that RCV420 or not (I wouldn't). The

741 might not be the ticket. AFAIR it can't sense to its negative rail but it's been too long since I have used one. The LM324 would be fine. 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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