# How do I decide the resistance of a DC solenoid valve?

I am designing a circuit for a solenoid valve from BIO-CHEM. My product number is 075P2N024-02SQ. I want to know the resistance of the valve in order to decide how big a resistance I should use in series. Here is the printscreen of electrical chart.

1. I use "Power:2.8w" and "Current:0.1 amps"to get one value of resistance: 280 ohm.

2. I use "Voltage:24VDC" and " Current:0.1 amps" to get one value of resistance: 240 ohm.

3. I use "Power:2.8w" and "Voltage:24VDC" to get one value of resistance: 205.714 ohm.

4. I use a ohmmeter to get the value: 213.6 ohm

Which one should I believe?

• You won't actually want a series resistor at all. It's the coil resistance to give an idea of the current that will be drawn. They don't need current limiting like say a LED. Just apply the rated voltage and make sure your supply can handle the current. – PeterJ May 27 '13 at 2:55
• @PeterJ,I use a photomos to control this valve and photomos has its on resistance. Does it has any influence on solenoid valve? and photomos can endure at least 100mA – oilpig May 27 '13 at 3:26
• I've not familiar with photomos in particular but they appear to be MOSFET devices, wouldn't the resistance be really low like less than an ohm? But you'll want something above 100mA, even at 24V above that is right on the limit and that's at room temperature it may go higher than that. – PeterJ May 27 '13 at 3:37
• @PeterJ,the on resistance of phtomos is about 40 ohm, so how can i calculate the current if i connect photomos and solenoid valve in series? – oilpig May 27 '13 at 6:53
• You can just add them, so take say the lowest value of 205 + 40 = 245R which as per Ohm's Law is 98mA at 24V. When the coil is colder than 21C the resistance may decrease and put it above the 100mA limit though. – PeterJ May 27 '13 at 7:33

You don't wish a series resistor. You want a supply that can provide 24 VDC @ 0.1 A. You can think of the device as having resistance in the 206-280 Ohm range as you wish. But that's what it is, not what it requires. See PeterJ's comment.

I might guess the difference in your calculations is rounding. Or there may be a maximum power the part can take, to allow for voltages a little higher than 24 VDC, but it doesn't need that much to operate.

• thank you for your suggestions. can you tell me the electical theory behind this phenomenon that i can get four different resistances? – oilpig May 27 '13 at 8:26

You do need a series R as well as a parallel C. There is a difference between the initial current to drive the solenoid to its new position and the current to hold it there.

The Power through the coil sets the max current you want to run through the coil. so Power/Voltage=Current. Voltage/Current=Resistance. I had a solenoid that measured 9.6mH, at 10KHz and 1V. It is listed as a 2W, 12V solenoid. 2W/12V=0.1667A or 72 ohms for the coil DCR. Usually you want a series R at about 80% of the DCR so in my case that would be 57 ohms. To get about 150mA and for it to decay to a 90mA hold current, I paralleled a 100uF cap with the 57 ohm series R. This gives me about 150mA current pulse that decays out in about 5ms. Now put in the solenoid and verify that everything works as expected. Don't be afraid (or surprised) that you have to tune this a bit but overall this is the correct way to drive a solenoid. By doing it this way, you eliminate issues with power supply voltage tolerances and overheating the coil.

I hope that helps.

Pease/Rakor have a discussion here:

• Good link. Bob Pease was amazing. – tcrosley Sep 4 '14 at 0:09