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  #21  
Old 05-07-2012, 11:35 AM
t_n_k t_n_k is offline
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Quote:
Originally Posted by hunterage2000 View Post
The Howland from a book is attached
I'm still not convinced by this. What's shown is not what I understand to be a Howland pump - which is in fact capable of producing a bipolar output. The circuit shown is clearly not capable of a bi-polar output.

I'm confused by this image - perhaps you have misinterpreted the meaning of the text.
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  #22  
Old 05-07-2012, 08:17 PM
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hunterage2000 hunterage2000 is offline
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Yeah tnk it is unidirectional, Im not totally sure what is meant by the 4-20mA current output. What I imagined it to be was 4mA for No-load and 20mA for maximum load. I cant find any texts about what they are used for, just the diagram and its equations.
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Old 05-07-2012, 10:25 PM
t_n_k t_n_k is offline
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A couple of observations.

  1. It always pays off to have a design specification. Once you have that then you do the design to meet your specification.
  2. Notwithstanding the preceding point - I believe you could have produced a much simpler solution with DC excitation of the load cells. The AC excitation approach seems counter productive.
Attached is a schematic of a 5V DC excited bridge with 4-20mA output. Purely by way of example, the bridge is arbitrarily assumed to give 36mV deviation for full a scale 4-20mA output. The 4mA condition would be for -36mV bridge output.
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File Type: png DC excited Load Cell Bridge with 4-20mA output.png (11.4 KB, 11 views)

Last edited by t_n_k; 05-08-2012 at 06:53 AM.
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