if so,could you explain its operation ? and what is the max current will this npn hold?

 

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What is this valve to be used for? If it's anything that demands even a modicum of accuracy, then you have a lot more to think about then just the driving current. Flow control valves have a lot of hysteresis, meaning if you inject 5A coming up from zero amps, the valve spool will stop in a different position than had you injected 5A coming down from 10A. You can't just blindly inject 5A and expect a predictable amount of flow to result. For this reason, some valves have LVDT position sensor feedback to the control circuitry to make sure that a given signal always results in a repeatable spool position; this is known as internal loop feedback. But even having a repeatable spool position does not equate to having a repeatable flow output; bernoulli forces, venturi forces, star wars forces, (the effects of nonlinearity and other pressures and flows in the the system and the amount of demand of fluid from the valve, etc.) all come into play and if you want a repeatable flow output you need to have external loop feedback as well, taken from a flow sensor. You need to have a dither function to eliminate hysteresis. Commercial valve controls come with all these functions. Designing thus is not trivial if you need accuracy; you might consider buying something off the shelf if so.

 

i will use it in a plastic injection machine

 

Like injection molding machine? Ive worked on a few injection molding machines and never encountered a proportional valve. Is this meant to be an improvement? How?

 

the proportional valve control pressure and flow of the oil in the machine . if high pressure needed to push or pull .it is hydraulic or servo valve

 

i found this circuit http://electronicdesign.com/analog/maximize-pwm-driver-resolution-matching-output-application
http://electronicdesign.com/site-fi...nicdesign.com/content/content/62966/62966.jpg
will the MJ122 (npn darlington tr) get hot?will it need heatsink?
but there are another block diagrams for companies http://www.bucherhydraulics.com/bausteine.net/f/9265/SAN-527_400-P-512101-E-01.pdf?fd=3 and it contains ramp generator on the card which i don't know its job.