Looking for a signal conditioner

I've got a level transducer that I want to use on a project. The transducer
has a 4/20 mA output equivalent to 0 to 5 metres. Unfortunately the range of
the transducer is too large for what I need. I only need about 4 to 9.8 mA =
0 to 1.8 metres. The receiving instrument is 4/20 mA input = 0 to 100%. My
100% is 1.8 metres and not the 5 metres of the tx.
Is there a signal conditioner available where I could input the 4 to 9.8 mA
and get 4 - 20 mA output, or is there another way (apart from buying a new
tx to the range I require).
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You don't indicate what sort of receiving instrument you are using. or what sort of control you have on your level transducer. You basically need to change the slope parameter on one of them. If your receiver is a dcs, an sldc, or a plc, there is a decent chance that you could redefine the bandwidth in the receiver without a lot of trouble.
There are some signal available which convert between various protocols. One of these might have a range which works for you. Setting your initial signal up as a voltage (0 to 10 volts) using a resistor might let you change from 0 to 10 v to 4 to 20 mA, which would give you your range. Unless you absolutely need the extra precision or absolutely must have the full 16mA bandwidth, I'd either get a transducer which fits the right range, or just go with the 9.8 mA maximum signal, rather than modify it.
I'm not so concerned about your ability to fool with it. It's avoiding the headache for the next guy who has to deal with non standard equipment.
Reply to
Herman Family
It is an analogue readout 0 - 100%
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You sure it doesnt have a zero and a span adjustments to let you set the 0 to 100 percent to twhatver loop current you need? Most I've seen have, rather than rigid 4-20 = 0 to 100.
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Draw a new card for the meter. Your intended use is nearly full scale; that ought to leave you with enough accuracy.
Reply to
Jerry Avins
Without hesitation, I'd reccomend to anyone who plays with 4-20mA loops, +/-10V command signals, valve drivers and so on to have one of these in their toolbox...
Weidmuller WAS4 PRO DC/DC part no. 8560740000
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(go to the website for Weidmuller, Downloads, Catalogues & Brochures, Product Information, Electronics, Analogue signal conditioning, page 28 onwards). Sorry, its big at about 3MB of PDF, but it is well worth having for reference.
OK, why do I think they are so good? Apart from pulling me out of the brown smelly just recently, these things will take more or less any voltage or current signal and linear scale it to more or less any other voltage or current signal with true three port isolation. They do standard calibrated ranges like 4->20mA or -10->+10V. They do bipolar current like a +/-10mA command signal. They run off just about any auxillary supply to hand either AC or DC. They have span and zero pots for weird sub-ranges of either input or output. And a killer feature... 10kHz bandwidth with a switchable filter to slug that to 10Hz bandwidth.
For signal conditioners they are quite pricey (£130 or so each in the UK). But the beauty is they bodge anything to anything.
As for the brown smelly I mentioned? I'm using some to actually drive hydraulic servo valves directly. The old controller I have wants to send +/-50mA PWM current with 500Hz dither and my modern replacement valves with on-board electronics expect +/-10mA. Yes, I could try setting up a current divider with parallel resistors across the input to the new valve, but it was easier just to put one of those conditioners in and set the appropriate input and output ranges, 10kHz bandwidth and off I went. Got a ground loop isolation which turned out to be beneficial as a lucky side effect too.
Hope this saves you trawling through many, many catalogues.
Best Regards, Dave Slee
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Dave Slee
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Thanks, these look very useful.
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