HP 3488A mains outlet switching is my design ok?  - Page 1
Forum section: electronics / projects

author: PascalNE
author_rank: Regular Contributor
timestamp: May 05, 2025, 01:02:40 pm
content: Hello,I've recently acquired an HP 3488A switching unit and a 44471A 10 channel relay card along with it. My idea is to use the 44471A to switch on and off other measurement devices as part of an automated system.The design is very simple but since I have limited mains wiring experience I thought I would post it here in case I'm making some big design mistake.I've attached a schematic and a (very crude) wiring diagram. Note that in both of them the switch symbols represent the 44471AI was thinking of using inline fuse holders like thishttps://www.aliexpress.com/item/1005007846817140.htmlAnd sockets like thishttps://www.aliexpress.com/item/1005005582836996.htmlAnd then a 3d printed plastic casing and of course no mains wiring will be exposed.Does this look right? And what kind of fuses would best suit this application?Any help would be appreciated.Thanks, Pascal.

author: PascalNE
author_rank: Regular Contributor
timestamp: May 08, 2025, 08:20:01 pm
content: Can anybody recommend what kind of fuse would be best?

author: inse
author_rank: Super Contributor
timestamp: May 08, 2025, 09:33:32 pm
content: I don’t think this is good for mains switching, please check the manual thoroughly before attempting to do so

author: PascalNE
author_rank: Regular Contributor
timestamp: May 08, 2025, 09:48:13 pm
content: Look at the fourth picture I attached to the first post the manual says "Since the relays on the 44471a can handle up to 250V rms at 2 amps it is ideally suited for switching AC line voltages such as air solenoids, lights and motor starters"

author: themadhippy
author_rank: Super Contributor
timestamp: May 08, 2025, 10:00:50 pm
content: Personally i wouldn't trust those sockets,something likeshttps://www.canford.co.uk/Products/2030000/42-137_CANFORD-REVERSE-MOUNT-13A-SOCKET-White?gQT=1would be my choice, same goes for the fuse holder,if your building a panel why not use some decent panel mount fuse holders.,or better still a 2A mcb for each socket.

author: PascalNE
author_rank: Regular Contributor
timestamp: May 08, 2025, 10:08:00 pm
content: Good point thanks those definitely look better than the Aliexpress ones I was just having trouble finding them elsewhere. And I'll definitely use some panel mount fuse holders don't know why I didn't think of that

author: srb1954
author_rank: Super Contributor
timestamp: May 09, 2025, 02:34:15 am
content: Quote from: PascalNE on May 08, 2025, 09:48:13 pmNote the manual's recommendation for the installation of contact protection networks. These are essential to reduce damage to the relay contacts from excessive arcing when switching inductive loads.Also note the maximum 500 VA switching capacity of these relays; this may be exceeded with loads that have high inrush currents even if the normal operating load is under 2A RMS. These high inrush currents may damage the relay contacts or even lead to contact welding.

author: calzap
author_rank: Frequent Contributor
timestamp: May 09, 2025, 05:00:47 am
content: Fuse choice would be between fast vs. slow blow, and the decision would depend on time tolerance of plugged-in equipment to overload condition. Suggest you use “listed” receptacles. Second srb1954’s suggestion that you consider inrush currents if powering inductive loads.Mike

author: PascalNE
author_rank: Regular Contributor
timestamp: May 09, 2025, 08:08:30 am
content: Quote from: srb1954 on May 09, 2025, 02:34:15 amYeah I misread the manual here I thought it said that they were already installed.Shouldn’t most normal multimeters and sources have more resistive properties? Im only intending to plug in lower power devices under 100W but mostly lower thats 0.4A at 230V so there’d be some headroom for inrush current. Please correct me if I am wrong though this is not my area of expertise.Thanks for your responses

author: inse
author_rank: Super Contributor
timestamp: May 09, 2025, 09:29:15 am
content: If the load to be switched contains a switch mode supply, the inrush can be nasty.A motion detector that activated a notebook power brick failed after some time due to welded contacts.

author: PascalNE
author_rank: Regular Contributor
timestamp: May 09, 2025, 09:48:09 am
content: I cant imagine ill be switching any loads greater than 50VA would the inrush current really be nearly 10x?Still it's probably best I add relay protection for more freedom just didn't want to add another layer to the project, I suppose I'll get started on design.

author: inse
author_rank: Super Contributor
timestamp: May 09, 2025, 10:08:46 am
content: Even 100x, if there is no inrush limitation, especially for SMPS

author: PascalNE
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:37:35 am
content: I'd assume most SMPS would have some form of inrush current limiter no?I've taken a look at the 44471A board it's interesting they let you modify it like this but yes I'll do some calculations and find out what values will be best.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 10, 2025, 02:56:41 pm
content: Quote from: PascalNE on May 09, 2025, 10:37:35 amA lot of notebook powers supply bricks are made so small nowadays that thay don't have a room for NTC resistor.