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schematic

simulate this circuit – Schematic created using CircuitLab

I am designing a control system for a Hydraulic Valve Amplifier Card (Bosch VT-VSPA2-1) and I need to send an analog input signal to the card, which the card translates into valve spool movement in the proportional valve. The analog input on the card ranges from -10 VDC (100% reverse flow) to +10 VDC (100% forward flow) and 0V being no flow. It also requires Re > 100kOhms (which I'm not sure what that means and is a questions in itself).

I am using a 24VDC/3.75A power supply to power the card. Can i use (2) 5 Ohm Rheostats (one (+) voltage, one (-) voltage) to vary the voltage to the analog input on the card? The input voltage needs to be field adjustable.

JCR
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    Add a schematic of your idea (because it isn't clear from the text). There's a schematic button on the editor toolbar and it saves an inline editable schematic in your post. Double-click to set parameters, 'V', 'H' and 'R' to flip and rotate. – Transistor Mar 29 '19 at 14:39
  • Right, schematic added. It appears that you want to switch in and out two preset voltage levels for two preset speeds. Is that correct? – Transistor Mar 29 '19 at 14:52
  • That is correct. The switches are to bypass a PLC (other inputs on the card) to operate the hydraulic card if needed. – JCR Mar 29 '19 at 14:55
  • As a matter of interest, why did you chose 5 Ω for your resistor values? – Transistor Mar 29 '19 at 17:28

1 Answers1

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Your circuit has several problems:

  1. A 5 Ω resistance on a 24 V supply could supply almost 5 A and that's at maximum resistance. The card will only require mA. The 5 Ω resistor will not limit the voltage so the 24 V will be passed through and may destroy the card.
  2. There is no limit on your adjustment.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A possible solution.

How it works:

  • R1 and R2 form a 24:10 divider reducing the 24 V to 10 V at the top of R2. This means that R2, when wired as a potentiometer, can be adjusted so that its wiper voltage varies from 0 to +10 V and can't be adjusted above +10 V by the operator.
  • R3 and R4 perform the same function for the negative voltages.
  • With both FWD and REV relays off (as shown) the control input is grounded ensuring that it is held at zero volts and can't take off due to interference or stray voltages on a floating input.
  • When FWD is energised the voltage set by R2 is fed into the input.
  • When REV is energised the voltage set by R4 is fed into the input.
  • FWD will override REV if both are switched on simultaneously due to PLC error. You can reverse this logic, if required.
Transistor
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  • You're still not specifying correctly. I think you mean "The forward voltage needs to vary between 0 and 10 V and the reverse voltage needs to vary between 0 and -10 V. I have shown this with independent adjustment of the forward and reverse speeds. If that's not what you require then a further edit of your question is required. – Transistor Mar 29 '19 at 15:21
  • Yes, I apologize, you are correct. – JCR Mar 29 '19 at 15:36