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I have done a bit of research but cannot find anything hard and fast describing what I am trying to do.

I have a solenoid in the steering rack of my racing car. This solenoid is controlled by the ECU and varies the amount of power steering the rack receives using a solenoid. As the car goes faster it gets less power steering. This is unnerving on track and not optimal.

I would like to manually control the voltage input to the solenoid from inside the car. Solenoid input voltage specs below

  • Maximum (highest power steering) 5.5V @ 1.1A
  • Minimum (least power steering) 2.1V @ 0.4A

The car supplies 14.9V with the engine running.

I figure I can wire in a toggle switch. When off the ECU circuit is connected and the car runs as intended. When switch on the circuit runs through the potentiometer.

  • Potentiometer specs are 20 ohm 0.125W @ 350V AC
  • 12V switching voltage @ 1.5A

enter image description here

I have drawn up a small schematic but I do not think it is correct. With the specified potentiometer and a 30 ohm resistor before it I can vary the output voltage between 6V and 1.2V which should be sufficient.

Is this correct or am I way off?

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    \$\begingroup\$ If the solenoid requires up to 1.1A then your resistive divider of 30K and 20K is completely incapable of providing this by a factor of about 2,000. Even omitting the 20K pot, you could only get about 0.5mA at the most via a 30K resistor. If you did build a resistive divider then the upper resistor would need to be around 8 ohms (again ignoring the pot) and be capable of dissipating 10W. It gets a lot worse once you factor in the pot. At one point you do say it's a 30 ohm resistor and 20 ohm pot but rated at 0.125W which is far too low. I suggest you look at a different method. \$\endgroup\$ Commented Jan 26, 2023 at 16:16
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    \$\begingroup\$ Using resistor divider for such current is not suitable. You should explore other possibilities like using an adjustable voltage regulator or a buck converter. \$\endgroup\$ Commented Jan 26, 2023 at 17:16
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    \$\begingroup\$ are you certain that the device is a solenoid? ... from the description, it seems to be a linear motor \$\endgroup\$
    – jsotola
    Commented Jan 26, 2023 at 18:11
  • \$\begingroup\$ @jsotola Yeah, solenoids are either ON or OFF, which means he'd only need a manual override switch if he that were the case. This is NOT a solenoid! We need a part # on the device he's trying to control. \$\endgroup\$
    – LarryBud
    Commented Jan 26, 2023 at 23:39

1 Answer 1

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Solenoid force is related to current, so you can get by with a resistance in series. The resistor has to handle some power, so I wouldn't use a potentiometer because it might fail with unpredictable results. It could theoretically work if you found one that could handle the power. You can treat your solenoid as a 5-ohm resistor.

If you can determine what current you like, I think your best bet would be a fixed resistor with a value between 8 and 32 ohms. It will have to handle the power, at least 15 watts. I put in a catch diode which should be rated at 2 amps. I also added a second switch to allow you to disconnect the ECU prior to changing states, but this might not be necessary if you have a break-before-make switch. I would never try to switch states when you're in motion.

schematic

simulate this circuit – Schematic created using CircuitLab

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