Is usb relay board safe for high powers?

Thread Starter

John Wilgraber

Joined Jun 4, 2019
2
I'd like to use this usb device:

https://www.aliexpress.com/item/32956365824.html?spm=a2g0s.8937460.0.0.64552e0ezng2GS

to control 220-240V/AC appliances, drawing around 5A. Specifically, to control my central heating unit, which is not programmable itself.

The device seems easy to program and to wire, however, I am concerned because I found this warning:

"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."
(https://ncd.io/choosing-the-right-usb-relay-controller/#document-5)

I am wondering whether that warning applies to the device above (whose relay's specs can be found here: http://www.circuitbasics.com/wp-content/uploads/2015/11/SRD-05VDC-SL-C-Datasheet.pdf).

Thanks!
 

mvas

Joined Jun 19, 2017
539
The coil appears to use 71ma
The device appears to use "> 300 ma", which is not a exact spec.
It does not appear to be "high current".
 

dendad

Joined Feb 20, 2016
4,452
I would like to see the isolation slot on that USB relay board extended a bit for better protection. But at least it has a slot. A lot do not.
What are you intending to run as a controller?

Have a look instead at..
https://sonoff.itead.cc/en/products/sonoff/sonoff-basic
And this may be useful too...
https://sonoff.itead.cc/en/products/sonoff/sonoff-th
Temperature setting and monitoring.
Connect over WiFi. They have a range of products, very good prices, and extensively hackable with quite a large community doing so.
 

Hymie

Joined Mar 30, 2018
1,277
If you were to attempt to use this USB device to control a piece of mains powered equipment, then it would need to be housed in a suitable enclosure to prevent access to the live parts.

The relay has contacts rated at 10A, and so should be OK for your proposed application of around 5A. However the relay has no inductive current rating, which could be a problem if the load is predominantly inductive (e.g. mostly motor current).

The biggest issue with this unit is the separation between the mains circuits and the USB low voltage circuit. As a good rule of thumb, these circuits (mains and low voltage) should be separated by a minimum distance of 5mm for 240Vac mains and 2.5mm for 120Vac mains.

Based on the image of the track side of the board, the distance from the mains COM PCB track to the two relay coil solder pads (close to the centre of the PCB) does not appear to be at least 5mm – and so would make this unsuitable for switching mains at 240Vac. The distance may be greater than 2.5mm, then being OK to switch mains at 120Vac.
 

Thread Starter

John Wilgraber

Joined Jun 4, 2019
2
Thanks everyone, your answers've been quite useful in deciding not to use that cheapo, mainly due to insulation shortcomings. The sonoff units suggested look amazingly cheap, so I'd try to go for them, instead!
 
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