I am designing a circuit to power a solenoid valve, to control the flow of compressed air through a device at my work. The specification sheet for the solenoid valve says that it is a 12V DC solenoid with a maximum current draw of 0.17A. The power supply that I got is the kind that plugs into a wall outlet and is rated to output 12V DC, 1A.
If a 1A-rated power supply is connected to a component that is said to "draw" 0.17A of current, does that mean that 1A of current will flow through the circuit regardless, and overload the component? I thought that in order to avoid overpowering the solenoid valve, I need to make a current divider (see attached circuit diagram): divert 0.8A through a parallel resistor, so only 0.2A of the power supply output actually goes through the solenoid. Is this necessary, or am I making a logical error?
My other question about the design concerns using the RC network to make the solenoid valve open gradually. I don't want the equipment downstream of the solenoid valve to get damaged by an instantaneous swing in pressure, so I want the valve to open gradually. That's why I planned to put another node parallel to the solenoid: a 70 ohm resistor to divert 0.17A current into charging the capacitor (for example a 8300 uF capacitor for a time constant of 0.5 seconds). I figure that as the capacitor charges, less and less current will get diverted through this node, and therefore more and more current will go to the solenoid. Will this work to make the solenoid valve open gradually over 0.5 seconds, instead of instantaneously, or do I need a different design?
Thank you.

 

You do not need the 15 ohm resistor to consume the extra current. The solenoid will only draw the current it needs. Most solenoids require less voltage to keep them energized than the initial voltage to energize them. Knowing this holding voltage may help with the operation you want. Connecting the solenoid directly in parallel with C1 of your circuit, and possibly changing the value of the 70 ohm resistor could do two things.
I would cause the solenoid to energize slower and denergize slower also. The capacitor C1 may need to to be changed to another value also.

 

If a 1A-rated power supply is connected to a component that is said to "draw" 0.17A of current, does that mean that 1A of current will flow through the circuit regardless, and overload the component?

No.

It means that up to 1 A of current is available from the power supply while it maintains a constant 12 V output voltage. The solenoid "draws" what it needs. The coil's internal resistance limits this to 170 mA at 12 V. That's the good news.

The bad news is that the power supply almost certainly does not have constant current limiting set to exactly 1 A. If it did, your R-C method might work. Try again, this time assuming that the power supply output always is 12 V and can supply infinite current, meaning that you can't shunt away excess current. What you can do is make a voltage ramp and buffer it with a transistor to drive the solenoid with a steadily increasing voltage. I'm being intentionally vague; take a swing at a solution and post the schematic.

ak

 

What you can do is make a voltage ramp and buffer it with a transistor to drive the solenoid with a steadily increasing voltage.

Possibly using the miller effect. Ask Mr Google.

 

I doubt wether a solenoid valve can be controlled in this manner Mostly solenoids valves are open/shut and this will occur at a particular voltage/current.If you want controlled opening you may need proper control valve.

 

For gradual opening Google "proportional valve"... I'm sure he knows more about it than I do...


edit: There's a thread here on that topic.https://forum.allaboutcircuits.com/threads/proportional-valve-control-circuit-help.142458/

 

For gradual opening Google "proportional valve"... I'm sure he knows more about it than I do...


edit: There's a thread here on that topic.https://forum.allaboutcircuits.com/threads/proportional-valve-control-circuit-help.142458/

There are special solenoid valves intended for proportional use.

Typical solenoid valves have their internal geometry optimized for snappy on-off behavior. My attempts at proportional control of standard solenoid valves have failed miserably. Once I got proportional solenoids, it was super easy.

Admittedly, I didn't try a huge number of non-proportional solenoids before breaking down and buying proportional valves. Maybe some standard valves also take proportional control well. It would just be a question of random luck whether or not it was possible with whatever valve you've got.

 

ASCO comes to mind but other companies also market slow opening and slow closing solenoid valves including variations of slow and fast, for example slow open and fast close. I doubt you will get the performance you are looking for with a standard solenoid valve for the reasons mentioned.

Ron

 

You might be able to use something other than slow valve action to protect downstream devices from sudden pressure changes.

Depending on how much flow you need in normal operation, it might be as simple as using a valve with a small orifice so that it takes longer for air to flow through and fill the tubes/devices downstream (or using a separate jet, nozzle, drilled plug, etc. just past the valve.)

Another thing that comes to mind is hammer-arrestors used in commercial plumbing. I imagine the cushioning effect of a typical water one might not work the same with air, but there may be a comparable equivalent for air systems.

 

Solenoid valves work by equalizing pressure on two sides of a diaphragm. A small spring is enough to close the valve and keep water (or air) from flowing. The attached link has an illustration that shows the basic function of how such a valve can be used to control high volumes and pressures with a small needle valve that either vents one side of the diaphragm to the output stream or closes it off to cause equalization on both sides of the valve.

https://en.wikipedia.org/wiki/Solenoid_valve

If your goal is to gradually open and close such a valve know this: It won't work. As long as the needle valve is open the diaphragm will not close. But when the needle valve is closed then the pressure will equalize on both sides of the valve and the spring alone will be sufficient to prevent further fluid (or air) from flowing. Your water valve in your toilet works this way. That's why a small input can stop water pressure typically 40 to 60 PSI. If you were to try and close a valve directly you'd have to apply equal pressure or greater to close the valve. And the amount of movement (throw) would be much greater than the small movement your toilet fill valve sees.

 

With the right valves, you should be able to do it with air. You might need a valve with an air pilot.

Picture a double acting air cylinder. They normally have valves where you can restrict the exhaust ports. This changes how fast the valve moves. You can change both directions. I used this for double-acting air cylinders, not single acting ones.

I also did a few mechanical shutters. One design used a rotary solenoid that needed a high kick and then a low voltage an lots of wasted heat, but there was plenty of air cooling available.

The other shutter design was just the opposite. I wanted to slow the opening of a shutter, because really close by was a $800.00 USD 1 mm thin filter and this was activated every 3 minutes for an 8 hour day. I was able to use the inherent slow start of a Veleman PWM module for that.

There are solenoid and injector drivers that can be used now.