Controlling Solenoid Valves with ULN2003A, Inrush Current, Arduino

Question

I want to control a total of five solenoid valves with my Arduino. The solenoid walves operate at 12 volts and have a power consumption of 5 watts per solenoid valve, which equals a current of roughly 420 milliamps.

My idea is to use a ULN2003A transistor array to do this. The ULN2003A datasheet says it supports voltages of up to 50 volts and a maximum current of 500 milliamps per output.

My question is: Is the ULN2003A with its 500 milliamps maximum current rating per output sufficient to reliably switch the solenoid valves? I'm asking because solenoid valves are inductive loads and if I'm not mistaken, they are subject to inrush current that far exceeds the normal current.

Answer 1

You need to read the data sheet carefully- the maximum current per output can be closer to 50mA than 500mA, depending on the conditions. See figures 4 and 5 in the datasheet.

Surge is not an issue with DC solenoids, but you do need to connect the COM to the supply voltage.

For such high current I would recommend discrete MOSFETs (logic level) and diodes across the solenoids. For example, AO3400A and 1N40005/M5.

Answer 2

The current won't surge as you expect. Because the solenoids are intended for constant application of the rated 12VDC, they are built with enough internal resistance to limit the current; the rated current is the largest that (with 12V) they will ever have.

In addition to inductance (which can be subject to nonlinear effects), the solenoid has internal resistance, by design.
Solenoids for AC application with less internal resistance, would also limit current according to AC frequency and their inductance.

As a practical matter, the ULN2003A also has thermal limits and (at the ground pin) a total-of-all-outputs current that cannot exceed 2.5A; at 420 mA per section, six solenoids will be too much to drive simultaneously.

Answer 3

A few things on solenoids (either relays or valves), since you are using a ULN2003 I'll assume you are going to drive it with a voltage step:

1. When apply a voltage step, the current will rise until the electro-magnetic force exceeds static friction.
2. At this point the plunger (for a valve) or the armature (for a relay) will start moving and create back-EMF which will reducing the current as movement accelerates.
3. Once the movement ends (usually abruptly), back-EMF goes to zero and the current will raise to stall value.
4. Usually at this point the voltage is reduced (by PWM) to ~40% of initial value as less force is needed to maintain position as opposed to initiate motion (keep in mind that force is linked to current, not voltage).

Datasheets often specs both values: boost (initiate movement) and hold (maintain position).

A good rule of thumb is that hold is ~40% of boost.

By default, I'd assume the power is given in static (where the valve will be most of the time) so the 420mA would be the hold current then you'd need overhead to be able to activate it properly (~800mA).