problems with howland current source

[t_n_k]

Originally Posted by hunterage2000

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.

[hunterage2000]

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.

[t_n_k]

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.