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Please excuse my ignorance - I am first and foremost a software person, and I only fool around occasionally with hardware...

I want to get 5-6 solenoid valves working in parallel. The solenoids are 12v DC, 500mA. I have a single 12v DC, 1A wall adapter. I would like to wire up the solenoids in parallel. To my understanding, with respect to voltage, it doesn't matter how many solenoids I have in parallel: Kirchhoff's law states that the voltage will always be 12v over each solenoid, even if I turn them all on at once. However, it is also my understanding that the current (amps) will be 'distributed' across each of the solenoids in parallel. That is, the 1A will be distributed across the N solenoids such that each has a current of 1A/N on it.

If I am understanding correctly, then, I would need a new 12v DC power supply with N * 500mA to adequately power all my solenoids?

Thanks!

EDIT: And as a more general question... What should I "watch out for"? Would too much current, or too much voltage damage the solenoids?

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  • \$\begingroup\$ How frequent all your solenoids are energizd simultanuselly? I'm asking this because at holding position the current requirement reduced to about a third. One more suggestion is: don't use a SMPS to drive multiple inductive loads, because of the inrush current requirement at startup. \$\endgroup\$ – GR Tech Jan 27 '15 at 5:05
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You need a power supply that can supply 12 volts at 3 Amps or more. Each solenoid will attempt to draw 500 mA. If the power supply cannot provide the total current required, depending on the supply the voltage may drop, reducing the current the solenoids draw, and possibly damaging the supply, or getting to a point where the solenoids will not operate.

If the supply is capable of providing a larger current than the solenoids require, there is no problem - the supply will not force the excess current it is capable of through the solenoids.

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Each solenoid want 500ma, and you need a supply that provides 500N. Watch for really hot supplies, but more likely solenoids that don't open.

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Yes, You will need a much more robust supply with current rating to supply the N*500mA plus some additional capacity to support the surge current when you first turn on the solenoids.

Some of the current surge demand can be met by using capacitors right near the solenoid switching point.

The solenoids will sink current they require according to the voltage you provide. Even if your power supply was able to supply 100A the solenoid, using your example, would still draw the 500mA (plus the previously mentioned surge current).

Watch out for the high voltage spike that can happen when the solenoid coil is switched off. This demands that you put a reversed biased diode across the solenoid coil. I would put one diode right at the terminals of each solenoid. This goes along with my next suggestion.

I would put one transistor or FET driver per solenoid. This will isolate each solenoid from the others so that variations of the DC resistance of each so not cause uneven actuation as if they were all directly wired in parallel.

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  • \$\begingroup\$ I guess I'm not sure how I would use a transistor here? My understanding is that they are used to 'switch' circuits on and off (I am using a mechanical relay to do this). \$\endgroup\$ – arcologies Jan 27 '15 at 1:08
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    \$\begingroup\$ Placing a capacitor in parallel with the coil will cause a current surge on turn-on, but there will be none without the capacitor because the coil's inductance will cause the current to ramp up as the magnetic field that builds up around the coil fights the change in current from the supply. If the solenoids are wired in parallel and all switched on at once - or not - from a single voltage source, there will be no interaction between solenoids as long as their magnetic fields don't comingle. \$\endgroup\$ – EM Fields Jan 27 '15 at 1:29
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    \$\begingroup\$ @EMFields - My suggestion was not to put the capacitor across the coil. I said at the switching point meaning that it would be on the supply rail to GND across the coil and what ever switching device is used in series with the coil. \$\endgroup\$ – Michael Karas Jan 27 '15 at 3:19
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    \$\begingroup\$ @guywhoneedsahand - If you are using a relay to switch all the solenoids then just wire them all in parallel. Just check that your relay contacts have enough current carrying capability to handle the total current. It is still a good idea to use the reversed bias diodes at the coil terminals because the spikes that occur at turn off can be harmful to relay contacts. \$\endgroup\$ – Michael Karas Jan 27 '15 at 3:23
  • \$\begingroup\$ Good save on the location of the switching point, but if the solenoids don't cause a turn-on surge, then there's no need for the capacitors at all regardless of whether the solenoids are high or low-side switched. The catch diode, however, is always a good idea and should be sized to allow it to harmlessly dissipate the power in the turn-off spike. \$\endgroup\$ – EM Fields Jan 27 '15 at 8:09
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If the distance between your solenoids and power supply is considerable you also need to take the wire gauge into account as well.

Example: 16AWG or 1.5mm2 metric copper wire has a resistance 0.0128Ω/m. @3A this equals a voltage drop of 0.038V/m. At a distance of 50ft or 15m for instance that would be a voltage drop of 1.15V. The result is a reduced force on the armament or even malfunction of the valves.

  • If such is the case you can either move the PS closer to the solenoids or, if that's not an option, use ticker wire gauge.
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