Drop voltage for solenoid - Need help with obvious

[g.mccormick]

Customer has a 12VDC solenoid. I have a 24VDC DO.

Solenoid specs.
12VDC
4.7W

P=IV= 4.7 = 12 * I
I=0.39A


If I want to drop the 24V to 12V going into the solenoid, I need a resistor in series. The resistor would need to be:
(24V - 12V)/ 0.39A = 30OHMS

So I need to find a roughly 30ohm resistor with a power rating equal or greater than 5W correct?

 

[Mickey]

Your solenoid also has resistance ( 30 ohms), this cuts the amps in half, when in series with the resistor.

Will the solenoid energize at this lower value?

IMHO you should replace the 12vdc solenoid with a 24vdc solenoid. Or use a 12 power supply (to power the 12vdc solenoid) with an interposing relay controlled by the 24vdc output.

 

[bernie_carlton]

Your calculations seem ok but Mickey's suggestions are cleaner.

 

[g.mccormick]

They are ordereing 24V solenoids, but I don't have tthem here right now.

Thanks

 

[keithkyll]

Resistor is 30 Ohm, 5 Watt wirewound. In addition, you may need a capacitor across it if you have a problem with pull-in. 100 uF, 35 Volt electrolytic should be enough. Positive end at the PLC output.
I would use a 5 Watt Zener instead. 1N5349B.

 

[shooter]

yep calcs are oke, however maybe you have another 12 V solenoid, you can use it instead of a resistor.

 

[Brandon_K]

You can get around this easily by running two power supplies, with a bonded connection.

If your outputs are sinking outputs (IE, switching 0v), tie the negative of BOTH the 12v AND 24v power supplies together before they hit the common terminal on the PLC.

Feed the +12v from the 12v PSU straight to the solenoid. VERY common in my industry where it's a mix of industrial 24v and "theme park" 12v devices.

 

[mendonsy]

You should seriously consider using an interposing relay contact to actuate those solenoids. Connecting them directly to a DO can cause all sorts of problems.

 

[milmat1]

You can get around this easily by running two power supplies, with a bonded connection.

If your outputs are sinking outputs (IE, switching 0v), tie the negative of BOTH the 12v AND 24v power supplies together before they hit the common terminal on the PLC.

Feed the +12v from the 12v PSU straight to the solenoid. VERY common in my industry where it's a mix of industrial 24v and "theme park" 12v devices.

I have done this in a pinch as well. And it will work.

I would rather Use a 12VDC power supply and an interposing relay fired from the DO. Like mentioned above..

 

[GTUnit]

If you only have 1 or a few solenoids just use a relay.
Don't overcomplicate things. Resistor is NOT the way to go.

 

[Lare]

Your solenoid also has resistance ( 30 ohms), this cuts the amps in half, when in series with the resistor.

No, Amps are still same with series connection. Only Voltage is divided half between.

Amps are only divided if there is paraller resistance with coil.

 

[g.mccormick]

No, Amps are still same with series connection. Only Voltage is divided half between.

Amps are only divided if there is paraller resistance with coil.

Actually, would amps be double in this case?

The solenoid would consume .39A and the resistor would also consume .39A.



BTW. We've decided to just wait for the correct solenoid. This is not an extremely important valve.

 

[Lare]

Actually, would amps be double in this case?

The solenoid would consume .39A and the resistor would also consume .39A.



BTW. We've decided to just wait for the correct solenoid. This is not an extremely important valve.

No, amps are 0,39A everywhere on circuit and aren't divided anywhere. This goes by Ohm's law.


I= U/R -> U=I*R


-> So if you have coil and resistor on series, both 30 ohms

I = 24V/60ohm = 0,4 A on circuit.


and across one there must be 12V (because resistances are same and only series connection)

-> U=I*R

0,4 A*30 Ohms = 12V

 

[JaxGTO]

If it's temporary anyway, grab a spare 12 solenoid and wire it in series.

 

[Brandon_K]

I have done this in a pinch as well. And it will work.

I would rather Use a 12VDC power supply and an interposing relay fired from the DO. Like mentioned above..

Why only in a pinch? I drive all of my small valves directly from the DO card without issue. Always have, 15 years now. As long as you take the correct precaution of installing a shunting diode across the solenoid terminals (which you should do with a relay, also) there is no harm to the transistor output of the DO.

And there are zero issues with running a split voltage power supply as well. As there is no point of reference between +24v and +12v, both 0v rails will coexist perfectly happily with each other. Computer power supplies for example, operate in that exact manner. 3.3v, 5v and 12v are all common voltages and they all share a common 0v rail.