Relay Contact Bounce question - Page 1
Forum section: electronics / projects

author: vincemulhollon
author_rank: Newbie
timestamp: December 22, 2021, 09:53:17 pm
content: I'm probably gonna get roasted for forgetting something obvious...So I hook up a relay NO contact to +5 and NC to gnd and turn the coil on and off and look on the dual trace oscope watching the coil, and the contact voltage.No surprise at all I see hideous contact bounce when the relay turns on for about 1.5 milliseconds. As I should. Relays be relays, they do that contact bounce thing.Then I turn off the relay and see no contact bounce. This is beautiful like a work of debounced art. Why? Shouldn't it arc or poorly conduct for a ms just like when it slams into place and turns on? Its the same on all 4 relays I have access to. I have to admit I've never tried this before and strongly expected contact bounce on both "make" and "break" but am somewhat surprised to see no contact bounce on "break".The background of this project, if any of this matters, is I wrote a Node-RED software driver to turn a relay on and off using one of those $9 keyestudio relay hats on a raspberry pi while under computer control.https://flows.nodered.org/node/node-red-contrib-keyestudio-relay-shield-ks0212Then I tested the heck out of the relays using my LCR bridge to verify contact resistance and open capacitance and shorted inductance, high voltage insultation tester to see if the relay contacts really are kilovolt rated and if the insulation between contacts and coil is good enough, hooked it up to my scope to verify switching speed vs the datasheet (faster!) and contact bounce (my puzzler as mentioned above) and I shoved RF thru using a TG and spectrum analyzer on various Amateur Radio bands (turns out to be a halfway decent RF relay... not bad for something only rated up to 60 hz by mfgr). I put all those test results in a documentary video for the software/board/project athttps://youtu.be/5Al1RFTqQQMPlz don't watch the video if you're driving a car because I don't want you to fall asleep and crash. My day job is programming not being Hollywood actor, and this is one of those side-projects-for-fun.Anyway, in summary, to restate, my question is, shouldn't a relay bounce as much on 'break' as it does on 'make'? My actual oscope traces say, "no" my thinking is its a bulk mechanical device so the answer should be "yes"? Or I'm missing something painfully obvious? The springs or if I took this bad boy to the southern hemisphere it would be reverse or ? Yeah I know I'm probably missing something obvious so 'roast away' I'm ready for it...

author: wraper
author_rank: Unknown
timestamp: December 22, 2021, 10:01:28 pm
content: When the opposite contacts hit each other, they bounce back. When released, they don't hit anything. For arching, you need something more than 5V at low current.

author: fourfathom
author_rank: Super Contributor
timestamp: December 22, 2021, 10:11:08 pm
content: When you power off the relay, the common contact hits the grounded NC contact, pulling the measured voltage to zero. I assume you are measuring it with a scope probe (10M Ohm to ground). If it bounces, there's nothing to pull the contact voltage positive and so there is no visible bounce. Swap the NO and NC connections and see if the "on" bounce disappears (now switching to ground), and if you now have "off" bounce.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2021, 01:43:54 am
content: Imagine the dynamics when the contacts move. The magnetic field builds fairly quickly (within a ms or so -- depends on the L/R time constant, and how the coil is driven), applying a force to the armature (typically a steel plate in a relay). The plate pulls, dragging along with it the springs holding the contacts. The spring deflects some, due to the contact mass. At the same time, the contacts torque and bounce a little, depending on how much the spring flexes, how everything is centered, and all that. But that's okay because they're in flight between end stops. Fast forward 10ms or so, the contacts smash together; the plate and spring mass continue to move, the stopping wave propagates up and reflects back down them -- meanwhile the contacts twist and torque again from all this flex going on, but some ms later they may bounce up (entirely off the fixed contact -- contact bounce as such), and so on until the mechanical energy dissipates and the contacts return to a static resting position.This all happens in a blink, so it's basically impossible to see by eye; it's visible on high speed camera, or with a scale model that's big enough (the delays are essentially proportional to the length scale -- which is to say, the materials have a fixed speed of sound). Also doesn't help that the movements may be microscopic -- if the contacts lift by mere nanometers, that's enough to measure electrically.Conversely when power is released, the spring force takes over, and the contact swings slowly over to its initial position. The dynamic force is much less in this case; note that force increases as the armature closes on the coil, acceleration increases as it travels, so it really slams around in that case. (Which is also why holding current is many times lower than pull-in current.) The opposite isn't true here though. So it may happen that turn-off bounce is actually none.But you can still imagine there might be cases, like if the contacts are fairly square/flat shaped, and that little amount of flex/rotation/torque that occurs due to the spring motion -- maybe that's enough to break the contact briefly. Or because the contact surfaces are rough (they always are), maybe the sliding motion that comes with the spring flexing, causes the contact to skip. Lots of possibilities.Turn-off could actually be better under load. At high current, the contacts weld, microscopically; this will take some force to pull apart, not more than the spring force of course -- within nominal ratings anyway, or so we'd hope(!), but perhaps enough to hold the contacts together before snapping the little metal bridge, and then it's clear to the other side.Or maybe not, because the sliding motion shears the weld point on its way out. Lots of possibilities.In any case, yep, bounce is generally worse on "make" than "break", and not particularly great in general. If you can avoid making assumptions about either, you'll be better off. For example, a typical debounce circuit uses an SPDT switch and flip-flop, so that, on the first "make" after the switch is thrown in either direction, that state is registered at the output, and no amount of per-contact bounce will disturb it. (The contact would have to bounce completely to the other side -- a fairly absurd condition, or at least, we'd hope so..)Tim