Snubber question for helping with TVS across a 120V 15A relay. - Page 1
Forum section: electronics / general

author: docmur
author_rank: Regular Contributor
timestamp: September 10, 2014, 01:04:13 pm
content: HeySo I have a relay on a board I'm developing and it controls a motor that runs at 120V and 15A. The relay switches on a time base given to the board, on an On / Off cycle. If the On / Off cycle occurs at a non zero cross the relay will switch and cause and ark across the relay terminals and hence couple a Transient Voltage spike on to the board and this causes a ton of issues.To solve this problem I put a snubber circuit on the back side of the board. which you can see by my bad drawn, in paint, addition to the circuit. What I'm looking for is a more elegant solution, hopefully in one chip, that could be soldered between the relay terminals.We've tried putting TVS diodes where I put the snubbers but that never really worked, we still see a ton of TV on the board when the relay switches.I hope this was clear enough.

author: calexanian
author_rank: Super Contributor
timestamp: September 10, 2014, 05:03:25 pm
content: I would use just traditional MOV's on each side of the relay contact to common. TVS's are faster, but not as robust in repetitive service. Just size them appropriately and they should keep your arcing from getting out of hand. As for any noise , I recommend a commercially available surge protector module. They are common and available in many forms. We are driving only a 3 amp load but I am sure they have one sized for you. We use the Schaffner model FN2060 with a nice big mov across the relay contact itself. Very reliable.

author: SeanB
author_rank: Super Contributor
timestamp: September 10, 2014, 05:12:29 pm
content: Snubber is typically a capacitor of 100n to 470ries resistor from 10R to 100R in series. Capacitor is typically a 400VAC class X2 capacitor. You can also use a MOV across the incoming supply along with another on the outgoing side to clamp the transients. for 110VAC a 130V MOV will do. Power rating of the MOV is as big as you can get, or place 4 or 5 in parallel across the supply and load.

author: docmur
author_rank: Regular Contributor
timestamp: September 10, 2014, 06:16:10 pm
content: I just grabbed a screen shot of the scope.This is another setup I was using before the snub setup, the Red disc's are MOV's and we used the:VARISTOR 150V 25J 10MM RADIAL LALittlefuseV150LA5PThe TVS's in use are 128 VWM 256VC SMC.The screen capture of the 5 volt rail to the CPU on switching is attached. With the snubs the line is clean down at a time scale of 250nS.So we've tried both and I'm a little confused what to try next, sorry I should of mentioned this before.

author: SeanB
author_rank: Super Contributor
timestamp: September 10, 2014, 06:58:48 pm
content: MOv and TVS are not used in series, use one or the other. Mov is better and add a 470n Class X2 capacitor across the power lines as well.

author: docmur
author_rank: Regular Contributor
timestamp: September 10, 2014, 07:21:36 pm
content: Oh okay, didn't know that :-), I'll try that!

author: docmur
author_rank: Regular Contributor
timestamp: September 15, 2014, 08:44:37 pm
content: Okay so I grabbed a bunch of new values from the setup I'm using.The motor has a resistance of 0.516kOhms, and draws 0.075mA at 120 Volts 60Hz.I'm using a 150nF 600V rated Cap.The resistor value I through into the circuit is 10 Ohms, and I'm trying to figure out how much dissipation power I need to give to the resistor in Watts. Or what is the best way to figure this out? due to the low current I was thinking that a 1/2 Watt 10 Ohm resistor should work but I'm wondering what the group thinks.ThanksDocmur

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2014, 05:51:02 am
content: Quote from: docmur on September 15, 2014, 08:44:37 pmYou said it runs at 15A ?!!?

author: SeanB
author_rank: Super Contributor
timestamp: September 16, 2014, 07:26:55 pm
content: Relay is 15A. With that value of capacitor and the low power draw of the motor you might have issues of it running on after the relay is open. MOV is looking better.