(I'm assuming an SOV is a solenoid operated valve - correct me if I'm wrong) - do you know wether it's AC or DC?

My bad, It is a solenoid valve and works in "DC" mode

You won't be far off if you just measure the resistance, and calculate using current = volts ÷ ohms. The actual current will tend to decrease a bit if the solenoid is energised for long, as the coil resistance will rise a bit as it warms up.

Thnx john, i think this is the best possible answer available .... the offset wont be much .... i agree with it, thnx again

Welcome.

Is this a big bit of equipment? If not, another solution is to just put it on the bench, wire up a 24Vdc supply & measure the current. Solenoid valve coils are usually only a few watts, so I'd be surprised if it drew more than an amp.

It is not a big bit of equipment, actually we have to place relays for it with maximum rating of 500mA, so to be on the safe side i think i am gonna calculate with resistance without energizing it, and after that energize it and check it in series, if the difference is not that much and below 500 mA, then problem solved....

Ohm's law will let you calculate the current if you know the voltage and resistance. However, it won't tell you the rating of the solenoid; that value should be somewhat higher than the operating level.

There is a way to find out how much current the solenoid can handle: connect an ammeter and increase the voltage until there is a bright flash and smoke starts coming out of the valve.

Yes sir, I'm full of good ideas just like that. I can also tell you how to find out if wild berries are poisonous!

If the coil doesn't already have suppression, I'd strongly advise fitting a freewheel diode - say something from the 1N400x series - across it, or even better a snubber network (something like this) across the relay contacts.

It will extend the relay life, reduce the possibility of contact welding and reduce electrical interference (by reducing contact arcing).

There are two considerations, starting and running current. A.C. or D.C. In either case it's quite simple: A.C. (since you left the voltage out), place a portable ammeter (we call in an amprobe), energize the solenoid valve, record the inrush current and holding current. If it's D.C., use a D.C. amprobe but probabley a multimeter in series with the load to measure the currents.

Why don't you read the other posts before posting?????????

Hi Usman,

Most SOVs in my experience seem to draw about 1 amp, i.e. 20~25W. Depending on what you are doing with this you may want to add an economiser circuit. This is effectively the opposite of a soft start. The full 24V is supplied to draw the valve to the operated position and then a lower value, 30% ~ 50% of V_NOM, to hold the operation. This saves the valve getting hot which may be inconvenient or even sometimes dangerous. One way to achieve this which doesn't need any timers is to put a halogen lamp in series (try 24V50W). As halogens have a cold resistance of about 10% of their hot resistance the valve will operate before the element gets hot and limits the current.

With regard to the relays, why are you intending to use 500mA relays? Would it not be more sensible to overrate the relay to account for the inductive load? (even so a freewheel diode as suggested by johnDG is still worthwhile)

Regards

Chas

We don't know what type of valve it is, what the fluid is, or anything much useful re. the power.

I often use solenoid-operated valves (small pneumatic jobs like this), rated 24Vdc 0.5W.

500mA rated contacts would be ample for controlling them (they're also available with suppression, which is a bonus).

I confess I'd probably go for at least a 1A contact rating, just to ensure the thing's going to work forever.

[Edit: Not dissing your comments about reducing the current after the solenoid has pulled in - that's valid & good info.]

Hi John,

At that power, 0.5W, it isn't worth trying to keep the power down.

Most of the solenoids I'm involved with are in hydraulics or mechanical pullers and typically run at around an amp. This generates a fair bit of heat and if they are left on for a protracted period can get hot enough to burn, or damage associated or adjacent pipework or wiring.

Do these little valves start with 20mA or are they equipped with dual coils or resistive "economiser" system? I appreciate they are small (1/8") but it still seems very frugal power-wise.

Chas

Hi, Chas,

These little pneumatic jobs really do only need a few mA. They're piloted - i.e. the solenoid just gives them a bit of a shove, & air does the rest. Less than the rated operating pressure (typically 1 bar), the spool just won't move. There is no current reduction circuitry involved, & the coils are continuously rated.

Know what you're talking about, tho'. We make a solenoid-operated radiation exposure shutter (using 2 solenoids (mechanically series connected) to get enough stroke, lifting a chunk of lead or tungsten). To get them to operate needs about 10 x the continuous power rating, but they must be able to hold open for hours, so we switch in resistors or drop the voltage (or both) when the "fully open" switch is operated.

John