I need to control 3 devices, via three relays. I wish to use a 4-relay/usb controller to do this.

I currently use this device but I now need three relays:


My Power supply is operating at 24 VDC, with the ability to supply 2A and 120W. This device powers the three devices without any problems.

The devices draw less than 8 Watts each as far as I can ascertain.

What should I look out for when choosing suitable relays?

Any tips on suitable devices? I need to control the device from my Python code.

  • 2
    \$\begingroup\$ The characteristics of the relay has nothing to do with what language the code will be in that will control the hardware that actuates the relay. Sounds like you need a relay with 24V coil, of which there are many. Otherwise, the switch specs rely on information you haven't given. \$\endgroup\$ Feb 9, 2012 at 16:16
  • 1
    \$\begingroup\$ It might be worth telling us something about the devices you want to switch (voltage/current ratings, etc) - I wasn't sure whether the 24V was for the devices or the Barionet 100 (or both) \$\endgroup\$
    – Oli Glaser
    Feb 9, 2012 at 21:32
  • 1
    \$\begingroup\$ 24V @ 2A is 48W. If your supply can go to 60V, then you can get the full 120W. Otherwise you're (potentially) current limited. \$\endgroup\$
    – W5VO
    Feb 10, 2012 at 3:06
  • \$\begingroup\$ For general tips on relay design: cp.literature.agilent.com/litweb/pdf/5988-6917EN.pdf \$\endgroup\$
    – Alex L
    Feb 10, 2012 at 17:21

4 Answers 4


The Barionet 100 has 4 open drain digital outputs capable of sinking up to 100mA at up to 24V.

Looking at the manual gives you an example of using them:

Barionet outputs

Something like this 24VDC 15mA coil current (250V 10A contact rating) G5LA-1 24DC relay would be compatible with these outputs.


On the contact side of the relay you'll need to be able to switch at least 24V and at least i = p/v = 8W / 24V = 1/3A, if I understand you correctly. Are the three load devices inductive? Is there a significant inrush current when you turn them on? We need this information to really fully determine their current requirements, but over-sizing this relay shouldn't be too costly anyway. If you want to be really really safe, maybe just get 5A relays like the Barix device has, then you'll extend its capabilities in a consistent way, which could be useful later on.

If the power supply is outputting 24VDC at a maximum of 2A, it should be 48W, not 120W, so we're probably missing something as far as the power supply goes. Does it have other output voltages, and 120W is the total?

You need to think about current on the coil side of the relay as well. Will you be using the digital outputs of the Barix device for this? These are open-collector which is very suitable for turning on relay coils, but you'll need to find out what the current and voltage limits of these outputs are, and make sure your chosen relay doesn't exceed these.

To do this right you also need to consider that the coil is an inductive load, and put a snubber network of some type across the coil. A simple rectifier diode will probably suffice, but putting a zener in series with the rectifier is better since it will allow the contacts to get moving faster upon release, which means the relay will last longer.


As each of your devices draws 1/3A at 24V, I would seriously consider using a MOSFET, such as the IRLM series available at Digi-Key. If you are not able to have a common return, there are plenty of optically isolated transistor arrays available.


Could you use a board like this connected to 3/4 digital outputs from your Barionet?

This has the advantage of being optically isolated as well.

Futurlec Opto-isolated Relay


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