# Voltage divider for hydraullic valve

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#### chrisaus2009

##### New Member
I want to be able to control a hydraullic valve I have where I have two sources, one is from my PLC (variable voltage -10 to +10V), one is a constant +24V DC supply from a switch mode PSU I have. The hydraullic valve sits at rest with +12V, spins one way at +7V and the other at +17V, I have made this circuit http://web.aanet.com.au/chrisaus/cct.JPG

the circuit does exactly what I want, the only problem is I want to be able to isolate the PLC from the +24V supply so either terminal (common/supply) on the PLC doesn't see more than 10V, otherwise it will cook the PLC. Is there any way to do this without using transistors and without using a POT (even though there's one in there for simulation purposes)? So that the circuit is ONLY controlled by the variable PLC voltage +/-10V (linearly).

Thanks

#### user_88

##### Member
possible solution ...

Just a rough concept at this point .... don't have a schematic ....:

Consider a two input unity feedback summing amplifier.
One of the two inputs is held constant at +12 Vdc... a voltage divider from the 24 V psu.
The other input to the amplifier is connected to the variable voltage output of the PLC, and sees a range of voltage from + 5 Vdc for one direction, to 0 Vdc for the stop condition, to - 5 Vdc for the opposite rotation direction.
If the output of the summing amplifier yields a negative voltage, then send the resultant signal through a unity feedback amplifier, to achieve the correct polarity.

If this idea meets your basic expectations, you would want to get a circuit simulation, or a breadboard confirmation, including op amp selection, appropriate components, power supplies, and layout.

Additional considerations:
.... estimate the max current necessary to operate the valve. From your previous diagram, that value would be 20 mA.
.... Is the zero voltage control position on the valve critical? Do you need to have absolute zero volts, or is approximately zero close enough, to prevent problems such as dithering, in the stop position.

#### chrisaus2009

##### New Member
A few problems there:
1. The logistics of this project make it hard for me to go and get an operational amplifier or transistors, the components I described in the circuit are components that I currently have lying around in the work shed.
2. Most analog components such as op amps or microprocessors are only good for 20mA output. If you draw anymore from them then you damage the chips. I can tell you know this hydraulic valve draws more than 20mA, I'd need a current amplifier (ie. using BJT's or something) to boost this current up.
3. I could do this easily with some transistors, refer to step 1.
4. +12V is stop position, not 0V. Approximately 12V is okay, the PLC can obtain feedback to adjust the input voltage to ensure the valve is receiving a voltage of +12V when stopped.

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#### chrisaus2009

##### New Member
I've come up with this:

http:\\web.aanet.com.au\chrisaus\cct1.JPG

Just using the PLC to shift the circuits reference voltage from -10 to +10 to give V8 +7 to +17V.

That shouldn't harm the PLC will it? As the entire source voltage (+24V) will be dissipated over the 3 resistors.

#### user_88

##### Member
Your new circuit has an element of ingenuity to it. However, with the resistor values that are shown, I don't think it will work, unless the valve itself has a high impedance .... which is not the case if it draws any significant current.

The result will be that the potentiometer, R7, and the shunt valve circuit resistance will form a parallel resistor pair of a relatively low value. This will abrogate your voltage divider scheme ... as it is presently drawn.

Maybe you could set up the voltage divider using lower resistor values ...
20Ω pot., 10Ω, and 10Ω.
You would have to check the current source and sink capabilities of the power supply, and in particular, the PLC ....
With a set of lower value resistors, you might have current levels
on the order of 1 A.
Also, it would useful to know the current required for the valve itself.

#### chrisaus2009

##### New Member
yeah i realize that, ill wait for the guy to get back and take it from there. thanks for your help

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