Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Post History

66%
+2 −0
Q&A How Relay inrush current limiter works?

It's a current limiter, not eliminator. The current is always supposed to flow, but be limited at startup to avoid high inrush current. All the relay does is short out R103 when activated. The r...

posted 1y ago by Olin Lathrop‭

Answer
#1: Initial revision by user avatar Olin Lathrop‭ · 2023-01-24T13:19:04Z (over 1 year ago)
It's a current <i>limiter</i>, not eliminator.  The current is always supposed to flow, but be limited at startup to avoid high inrush current.

All the relay does is short out R103 when activated.  The relay starts out off, so R103 is in series with the input power when the device is first plugged in or turned on.  The 10 &Omega; of R103 keeps the current limited.  It's not meant to allow the device to fully function, but enough to allow the input capacitors of the power supply, or whatever else might take a large initial slug of current, to get past that point.

Eventually the 12 V supply comes up, which then turns on the relay.  That shorts out R103, and the device can draw its full normal operating current without its input voltage sagging.  There may be a small current spike right when the relay closes, but presumably that has been calculated to be within tolerable limits.

The device may have additional circuitry inside that doesn't run the main device function until after the relay has been on for a few 100 ms or so.  By that time, the internal DC supplies should be up to their intended voltages.