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I want to control a hydraulic valve which accepts a variable voltage from 0V to 12V. That's easy enough with a DAC, but I'm not sure what load I can drive with a DAC. Unfortunately it's not easy getting your hands on data sheets for hydraulic valves so I'm not sure what current will be required by the solenoid. I've sent a question to the maker and hopefully they'll provide the specs.

Mean time I thought I'd try to find out more. How do you drive a load from a DAC? I've looked over a few DAC data sheets and they don't seem to specify the load that they can drive. If they DAC only drives a limited load how do you get a higher load? I know there is a Current mirror but is there a similar mechanism for Voltage?

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    \$\begingroup\$ A power transistor in emitter follower configuration might be a good starting point. Or a unity-gain operational amplifier buffer. Search [transistors] and [amplifiers]... \$\endgroup\$ – MarkU Jul 19 '14 at 22:58
  • \$\begingroup\$ in this context, I'm assuming DAC doesn't stand for Digital to Analog Converter....what does it mean in this context? \$\endgroup\$ – Funkyguy Jul 19 '14 at 23:41
  • \$\begingroup\$ Is it possible that your valve has built-in drive circuitry, which accepts 0v to 12V control signal? I was going to ask for a link to the valve's datasheet. But you've addressed that preemptively. If the valve requires a driver with a power stage, and the manufacturer expects you to build it, then you should ask the manufacturer for a reference design or some such insight. \$\endgroup\$ – Nick Alexeev Jul 20 '14 at 3:03
  • \$\begingroup\$ It's likely you don't want/need analogue control but on-off control. \$\endgroup\$ – Andy aka Jul 20 '14 at 9:34
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    \$\begingroup\$ @ Andy: Since he's already stated that the valve accepts a variable-voltage control input and that the source of that variable input is a DAC, it seems most likely that the valve is to be used for proportional - rather than bang-bang - fluid flow control. \$\endgroup\$ – EM Fields Jul 20 '14 at 20:31
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If you Google "Programmable DC power supply" you'll get a lot of hits for power supplies with output voltages and currents adequate to run your valve which can be controlled by low-level DC signals to the supply.

Like the buffered output of your DAC or, perhaps, unbuffered and straight into the supply.

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Here's a quick and easy method, if it turns out you need to push some current over the entire 0-12V range.

schematic

simulate this circuit – Schematic created using CircuitLab

The op amp should be a rail-to-rail input and output op amp, typically CMOS. The MOSFETs need to have relatively low thresholds. It may also be necessary to heat sink them.

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In all likelihood a proportional hydraulic valve of the type that you describe can be directly controlled via a switched pulse width modulation (PWM) waveform. The concept would be to use two pairs of high power MOSFETs in a H-bridge configuration. The bridge would switch each end of the control coil from 0 to 12V. Then each side of the control coil would be switched the opposite polarity as the other. This meaning of course that one end would switch to GND while the other went to +12V and switch to +12V as the other end went to GND.

The frequency of the PMW waveform would want to be in a frequency range that was higher than the mechanical resonance of the control valve but not so high as to make it difficult for the H-bridge to fully switch the coil ends between the +12 and GND levels.

By varying the duty cycle of the PWM waveform over some range (could be 0 to 100% or even a lesser range of 5% to 95%) the valve position would change in proportion to the duty cycle.

I believe (although I have not looked recently) that some industrial hydraulic valves already have the H-bridge drivers built in and all that is necessary is to connect up the +12V, GND and a suitable PWM signal.

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