I'm fairly new to electronics, so I'm hoping I'm not so misguided that I am wasting all of your time. Basically I need to power a solenoid which in turn controls a gate valve, and the solenoid requires 24V DC. The manual for the whole thing describes very little, and I don't have any info like inrush current, which I'm worried about since any searching so far has had people providing that. The solenoid just has 2 black wires coming out of it, and is marked 24V DC.

To power the thing, I am going to use a 24V AC/DC power supply (here). I want to be able to control the whole setup remotely using a computer, so I immediately thought to use a relay controlled by USB. However, any time I've used this sort of stuff in the past, I've just been going off of Arduino tutorials or something, so I just bought what was recommended there.

I want to make sure I have a proper understanding of how to use these. What exactly is the difference between requiring 5V vs say 12V to power the relay? Can 12V handle more current passing through them, and that's the main reason to use a stronger one? If I were to say purchase this relay board, would that work? Under relay parameters it states 5V/72mA, 15A/24VDC, 10A/250VAC. The 5V I would understand as the potential I need for the relay, but does 15A/24VDC mean it can pass up to 15A/24VDC, meaning this would work for my application here? Or am I really misunderstanding this? Is there an entirely easier solution that I have no idea about? I currently envision connecting the power source to COM, then the solenoid to NO and switching it when the solenoid needs to be powered.

Additionally, bonus question, the relay will be very far away from my pc (guessing 60-70? feet of wire), should I worry about power loss by powering from USB? I just grabbed a USB powered one from a search, powering at the location and controlling via usb is perfectly fine, but usb powered would be convenient.

Thanks for your time and any help you can provide!

  • \$\begingroup\$ Thank you! The link is :lesker.com/newweb/valves/pdf/manuals/gate%20valve.pdf. \$\endgroup\$ Jan 28, 2016 at 22:21
  • \$\begingroup\$ Ok, while I don't see an inrush current spec. in the manual, it does specify 4.4W rated consumption, so a "stable" (post inrush) current of ~0.2A @ 24VDC can be assumed, so I'll start working on a diagram for a trigger-circuit with a resistor limiting inrush current to ~5A to prevent overcurrent damage to components, while not causing too significant a (~1V) voltage drop under holding current. \$\endgroup\$ Jan 28, 2016 at 22:47
  • \$\begingroup\$ Inrush is not a problem on DC powered coils because the coil MUST have series resistance and this series resistance is the dominant governor of current taken. \$\endgroup\$
    – Andy aka
    Jan 29, 2016 at 0:32

2 Answers 2


There is no real difference between the 5V and 12V relays. It's mostly a convenience depending on the supply voltage you have. A 5V relay is more convenient for USB. This voltage refers to the switching/coil voltage.

The 15A 24V DC does mean that, it's the maximum switched load can be supported. If this is good for your solenoid depends on its current. If you don't know the inrush, assume 2 to 3 times the stated current. Your solenoid is 4.5 Watts, at 24 Volts means 180 mA typical. Rule of thumb rounding, assume 1 Amp for the power supply and relay supported load voltage.

Usb has a max of 16 feet without a hub or booster. Keep it near the computer, and extend the 24 Volt side, at 60 feet (120 round trip), with typical 18/2 AWG cable, you shouldn't see any significant voltage droop. The solenoid should still trigger at a lower voltage anyway.

Alternatively, think about using a remote relay. Wifi, or RF. Or ethernet. Or if you already have the USB relay, use a small computer to drive it. Something like the Raspberry Pi with a wifi usb stick or ethernet.

  • \$\begingroup\$ Thanks! There is no stated current, though I did measure the resistance to be 141.6 Ohms. Can I just apply ohms law to get the normal current flow? \$\endgroup\$ Jan 28, 2016 at 22:46
  • \$\begingroup\$ @unknown sure. While solenoids are inductors and not resistors, a measured resistance can give a quick rule of thumb measurement. 24 / 141 is 170 mA. Their stated 4.5W divided by 24 Volts (P = V * I), is 180 mA. So let's round to 200 mA on current. Multiple by 3 for inrush. Round again, that's 1 Amp. So yeah, plenty fine. \$\endgroup\$
    – Passerby
    Jan 28, 2016 at 22:58
  • \$\begingroup\$ If the DC Solenoid/Relay's coil has a resistance of 142 ohms then the maximum current that can be taken by this coil from any "suddenly applied" voltage source is totally defined by this resistance. Inrush on DC coils is not an issue because if the core saturates, the effective DC resistance (142 ohms) is still in series and is unmodifiable by the core. It's a different story for AC coiled solenoids/relays because R can be very small and coil inductive reactance IS affected by inrush/core saturation. \$\endgroup\$
    – Andy aka
    Jan 29, 2016 at 0:27

(I'll continue to edit in content as we go)
1: We'll need to know the inrush current (or at least rated wattage) of the solenoid to be sure to spec. a sufficient relay for you.

2: You can run a usb signal buffer from your 24VDC power supply to operate the relay, thus avoiding the need for pushing usb power over 60-70 feet of wire, or using another power supply.

3: The input/coil voltage rating on a relay simply tells you the trigger voltage it's designed to run on. A 5V triggered relay will often use a higher amperage input than a 12V triggered relay that rated for switching the same load voltage & amperage (all else being equal).

4: Yes, the 15A/24VDC rating means the relay's output contacts are rated to handle up to a 15A load at 24VDC.

EDIT: Here's a circuit diagram for a potential solution circuit. Q1 buffers the input signal's voltage & current to reduce/replace losses from long transmission line. Q1 needs to be rated for >24Vce & >35mAce with sufficiently low base voltage for triggering from attenuated signal.
Switching Buffer Circuit
Design relay: G5LE-1-VDDC12 12VDC, 33mA coil w/30VDC, 8A rated contacts
(Available for about USD$1.23 from digikey.com)

  • \$\begingroup\$ Thanks for the answer! I have the solenoid, and the manual is here(lesker.com/newweb/valves/pdf/manuals/gate%20valve.pdf). Fourth page down. However, it provides very little information. The solenoid has labeled on it 24 V DC, and 4.5W. Is there a way to test for the inrush current? \$\endgroup\$ Jan 28, 2016 at 22:34
  • \$\begingroup\$ I just read through again, I can't find any information on inrush current for it in the manual, even with a search. \$\endgroup\$ Jan 28, 2016 at 22:40
  • \$\begingroup\$ I guess I should add that the measured resistance of the solenoid is 141.6 Ohms \$\endgroup\$ Jan 28, 2016 at 22:43
  • \$\begingroup\$ Updated answer with potential solution circuit & added your manual ling to OP, so all comments with that link can be deleted. \$\endgroup\$ Jan 28, 2016 at 23:45

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