If you can measure the coil resistance of the 12V solenoid coils with a DMM you can use the same value of resistor or one that is close, connected in series with the 12 solenoid to work on 24V. The wattage of the resistor will depend upon the current flow. If you post the resistance there are forum members that can help you calculate the wattage for the resistors.

Ohmmeters measure resistance exactly as you are proposing to do it. The inductance only changes how long the current takes to reach final value.
The solenoid will switch (reach final current value) faster with 24V, because the L/R time constant gets cut in half when you add a series resistor equal to the resistance of the solenoid.

Originally Posted by DSGarcia

K7elp60,
Thanks. My main question was if I could/should do it. It appears that you think that it is OK.

With a coil, I didn't think I could directly measure the resistance. My thought was to measure the current while the coil were energized, then calculate the resistance based on current flow and voltage (or get the wattage specification from the manufacturer once I find out the mfg. model number and then calculate current and resistance).

From there I can calculate wattage, but I will oversize (derate) the resistors to allow for a higher ambient temperature (worst case is in a closed box in the summer). Each coil will only fire about 5 seconds per minute.

Does that sound reasonable (i.e., do all the normal calculations hold true with an inductive load)?

Thanks,
Dale

That plan should work fine. Lets say the 12v solenoid draws 1 amp. That would represent a 12 ohm load. If you added a 12 ohm resistor is series with the solenoid wired to 24v, then the solenoid would draw the same one amp.

However the 12 ohm 'dropping' resistor would disspate 12 watts of power and have to be size to say 20 or higher watts depending on where and how it's mounted.

Do first find out the current requirements of the 12v solenoid, the rest should be easy.

Lefty

Originally Posted by DSGarcia

K7elp60,
Thanks. My main question was if I could/should do it. It appears that you think that it is OK.

From there I can calculate wattage, but I will oversize (derate) the resistors to allow for a higher ambient temperature (worst case is in a closed box in the summer). Each coil will only fire about 5 seconds per minute.

Does that sound reasonable (i.e., do all the normal calculations hold true with an inductive load)?

Thanks,
Dale

Since the coils will only be energized for 5 seconds the resistors normally would not need to be oversize. In fact they could be undersize, but I think this is a judgement call.
As a side note I just did as recommended. Have a 12V relay. Measured resistance of coil....110 ohms. Put 120 ohm resistor in series, relay energized on 24 volts with very close to same current draw on 12V.

Originally Posted by DSGarcia

Thanks everyone. I wasn't sure how inductive loads affected the calculations and solution. My plan is to find out the manufacturer and model number. Their specification would be given as voltage and wattage and then I can calculate the rest. However, when I had someone go and look at the solenoid coils, there were no visible markings. I will try to get the information from the other vendor.
Thanks,
Dale

You gave the impression that you have an ohmmeter. That and some arithmetic are all you need.

Originally Posted by SixBullets

Hello every one, adding series resistance is a wrong plan as it delays the valve actuation time<snip>

That is wrong. As I said in my previous post, adding a series resistor, along with doubling the supply voltage, actually speeds up valve actuation time, because it reduces the L/R time constant. The simulation below (in LTspice) illustrates this.

Originally Posted by SixBullets

Hello every one, adding series resistance is a wrong plan as it delays the valve actuation time and also wattage of the resistance needed will be large. This is wrong, as Roff showed in his post. The time constant is L/R.
for any inductive load voltage is not that critical as is the current
so adding additional circuit (PWM control) to control the current in the valve is the best solution.

I would not use the PWM solution since the soleniod is designed to operate with a 12 Volt supply, if you apply 24 V pulses, it will reduce the operating life since the solenoid will move faster and hit the stop harder.

Originally Posted by ljcox

I would not use the PWM solution since the soleniod is designed to operate with a 12 Volt supply, if you apply 24 V pulses, it will reduce the operating life since the solenoid will move faster and hit the stop harder.

You could PWM it with a period much shorter than the time constant of the solenoid, but it seems to me like that might slow it down somewhat. With the OP's low duty cycle (5 sec/min), a resistor seems like the simplest solution to me.

Originally Posted by SixBullets

Hi Roff,
your analysis is correct.

Hi ljcox,
only limiting factor for selecting the voltage will be coil insulation.
solenoid hitting harder can be controlled by PWM.

I will never go for series resistance even if it is simpler(not permanent) solution as the latching and holding current are different for solenoid. And also the additional load on the supply.

Are you saying that the PWM pulses would be tapered, ie. short at the start and gradually increased in width?

If so, it should work if there is a diode across the solenoid as the current would be a type of sawtooth waveform with a gradually increasing average.

I not sure what he meant, but here's a sim of the solenoid running of 12V, and the same one PWM'ed at 50% on 24V. Looks OK to me. But I'm no solenoid expert.
I included the schematic of compb (behavioral comparator), in case anyone wonders what it is. I just used it as a quick and dirty PWM gate, or modulator, or whatever the hell you want to call it.