Being a relay controller manufacturer means we have to answer to customers when things go wrong.  When it comes to power, there are plenty of opportunities for things to go wrong.  Powering a USB relay controller has to be met more caution than you might realize.

USB Powered Relay Boards

High-power mechanical USB powered relay boards are absolutely prohibited as per USB specification.  We lose a tremendous amount of revenue each year because we refuse to compete in the USB powered relay board segment of the market.  We are frequently contacted to be a supplier in this area.  The USB specification has clearly provided guidelines as to how power should be utilized from the USB port, and surging currents produced by high-current mechanical relays is strictly prohibited.  While a few small signal relays can be safely powered from the USB Port, the market for such relays is relatively limited.  Solid state relays may be safely powered from the USB Port, and we have considered manufacturing such devices, but in 21 years of business, not a single customer has ever asked us for a USB powered solid state relay controller.

Power Load-Sharing USB Relay Controllers

A load sharing USB relay board is a controller that has a power supply that is expected to control the relay board and the external load being switched by the relay.  This is only safe when working with resistive loads (lights, resistors, heating elements, or other non-inductive loads).  Sharing the power supply of the relay controller with a motor, solenoid, valve, pump, or other inductive loads will cause serious problems in terms of reliability, and can cause damage to the USB port of your computer.  Ideally, inductive loads and the USB relay board will each have their own power supply.  If only one power supply is possible in your installation, we do have a work-around available, so please consult with us if you MUST load-share your power supply with an inductive load.  Load-Sharing inductive loads with a USB relay controller should be avoided whenever possible.