Snubber Circuit Suggestion - Page 1
Forum section: electronics / projects

author: rocky79
author_rank: Contributor
timestamp: December 01, 2016, 06:32:58 pm
content: I am seeking some feedback in improving the reed switch snubber protection circuit shown below.The switch will drive a coil of an AC relay. Please note that the AC relay coil is not accessible. Any protection will have to be across the reed contact.Here is the relay part#Spec sheet:- Relay coil operating current is 24mA, 115AC.- The reed switch SW1 is Mk23-90C-2Existing RC values. R=100Ohms. C=5nFAny value close to 10nF will make the capacitor conduct and causes the relay to activate.Here is a screenshot showing 3 waveforms. On the common reed terminal I placed the probe ( blue waveform) 115 AC 400Hz . On the NO terminal I placed the yellow probe.The purple waveform represents the voltage difference across the reed switch contacts ( the potential difference of the blue and yellow)Here is a scope shot showing what happens when I activate the reed switch “SW1" with the snubber circuit above R=100, C=5nF. The voltage spike is 180V. The max contact rating on the reed switch is 175V.

author: tatus1969
author_rank: Super Contributor
timestamp: December 01, 2016, 08:08:30 pm
content: how about thus: a 470r in series with the coil, a 150v suppressor diode across it. no snubber.

author: rocky79
author_rank: Contributor
timestamp: December 01, 2016, 08:41:54 pm
content: Quote from: tatus1969 on December 01, 2016, 08:08:30 pmThank you. You mean a bi-directional TVS diode across the relay contact?I am going to use a resistor R in series. How critical is the value of the resistor. I have seen 300 ohms, I could use 470 or 500.

author: tatus1969
author_rank: Super Contributor
timestamp: December 01, 2016, 08:47:37 pm
content: Quote from: rocky79 on December 01, 2016, 08:41:54 pmexactly. i calculated the resistor to 10v drop at rated coil current, in order not to "steal" too much.

author: rocky79
author_rank: Contributor
timestamp: December 01, 2016, 09:48:02 pm
content: I see, If I want to design to the worst coil current max of 40mA at 25C then I think a 300 ohms will give a 12VAC drop which is ok I think.Regarding a snubber the reason I think it’s not working because a larger capacitor than 5nF will start conducting at 400Hz. A smaller cap like this will not limit the contact voltage to 175, So like you mentioned a TVS will be a better solution.Wish TVS would you recommend? what clamping voltage would you pick?

author: tatus1969
author_rank: Super Contributor
timestamp: December 01, 2016, 10:52:00 pm
content: i would prioritize contact rating and choose one with max 175 clamp voltage, lets say a 150v type. choose a series and check the specs. you'll have some residual voltage/current in OFF position, make sure that it stays below 30v rms at max line voltage.

author: rocky79
author_rank: Contributor
timestamp: December 01, 2016, 11:28:49 pm
content: With a 175 clamping voltage and since normal voltage 115 AC -165 peak is present across the reed switch in off position, I am afraid I am going to have an issue with the TVS conducting. The 165V being well over the breakdown voltage of a typical TVS of around 108vExample is little fuse part# 1KSMB120CA

author: Ian.M
author_rank: Super Contributor
timestamp: December 02, 2016, 01:04:03 am
content: There simply isn't enough margin between the peak AC voltage and the reed switch rating to handle anything other than a purely resistive load. Without access to the other side of the AC supply (which would let you use the reed switch in the gate circuit of a TRIAC without exposing it to the back EMF from the relay coil), its totally FUBARed.

author: rocky79
author_rank: Contributor
timestamp: December 02, 2016, 01:32:40 am
content: Quote from: Ian.M on December 02, 2016, 01:04:03 amIan, how would use a TRIAC in this configuration, do you have an example schematic?

author: Ian.M
author_rank: Super Contributor
timestamp: December 02, 2016, 02:57:06 am
content: Actually, thinking about it a little further, you don't need access to Neutral. After the reed switch opens, the TRIAC will hold conduction till the load current falls to zero, so there will be very little back-EMF when it switches off. Therefore the switch and a suitable series gate resistor can be simply wired between G and MT2, A simple RC snubber across MT1 and MT2 will take care of any residual switching spike.

author: rocky79
author_rank: Contributor
timestamp: December 02, 2016, 04:02:41 am
content: Ian, thank you for the reply. You mean something like this?

author: Ian.M
author_rank: Super Contributor
timestamp: December 02, 2016, 08:55:41 am
content: Not quite:

author: rocky79
author_rank: Contributor
timestamp: December 05, 2016, 10:27:29 pm
content: I am trying to size up a current probe to measure the inrush current inline with the switch.Based on the info above what do you think the inrush current will go up to?The Tektronics current probe A622 has a minimum Current measurement of 50mA. I wonder if that's low enough. Any advice is appreciate it.If the inrush current is low( lower than the Max power rating on the switch) then I will add a current limiting resistor in series with the switch and I should be ok.

author: Benta
author_rank: Super Contributor
timestamp: December 05, 2016, 10:39:54 pm
content: What is the purpose of R4 and C5 ?

author: rocky79
author_rank: Contributor
timestamp: December 05, 2016, 11:36:44 pm
content: Quote from: Benta on December 05, 2016, 10:39:54 pmThat's a snubber circuit that reduces voltage spike across the relay contacts.

author: rocky79
author_rank: Contributor
timestamp: December 22, 2016, 07:16:28 pm
content: The Triac Circuit works nicely for AC application but what if I want to use it for DC as well. The DC application is also interfaced with a DC Coil.Any ideas on a design that could be used for both AC &DC?

author: Ian.M
author_rank: Super Contributor
timestamp: December 22, 2016, 08:29:44 pm
content: Normally, a DC relay has a fairly low coil voltage, typically under 48V. That gives you a lot more headroom for the snubber circuit to work without exceeding the reed switch's peak voltage rating.

author: rocky79
author_rank: Contributor
timestamp: December 22, 2016, 09:28:48 pm
content: Ian, you are right however I am just thinking of a single design that can work for both AC & DC coil application.

author: Ian.M
author_rank: Super Contributor
timestamp: December 23, 2016, 12:31:44 am
content: A generic AC/DC design for a two wire switch circuit is going to be very difficult to do and rather complex. The reed switch alone wont have the same ratings for AC and DC, and any snubber or TVS must have component value(s) chosen for the expected circuit voltage and current.Thinking further about the DC case, its highly likely that it will be essential to off-load the relay coil current from the switch, e.g. with a transistor, as otherwise it may suffer for uncontrolled arcing when the contacts open.

author: voltsandjolts
author_rank: Unknown
timestamp: December 23, 2016, 08:47:34 am
content: Quote from: rocky79 on December 05, 2016, 10:27:29 pmRemember you can place multiple turns of the current carrying wire through the current probe. e.g. 1 turn == 50mA sensitivity, then 10 turns == 5mA sensitivity.

author: tatus1969
author_rank: Super Contributor
timestamp: December 23, 2016, 09:14:05 am
content: Quote from: rocky79 on December 22, 2016, 07:16:28 pmIs it really necessary to use a relay? Otherwise I would definitely go for an electronic solution, that will not suffer from arcing, be more flexible to handle AC and DC and wide input voltage ranges and frequencies, etc etc

author: rocky79
author_rank: Contributor
timestamp: December 24, 2016, 04:08:51 am
content: Quote from: Ian.M on December 23, 2016, 12:31:44 amIan, you mean voltage spike due to coil inductive kick back?I am going to try a PNP diode connected either a BJT or Mosfet and have a snubber across the transistor terminal to slow down that voltage spikes from hitting the VCEmax or VDSmax

author: rocky79
author_rank: Contributor
timestamp: December 24, 2016, 04:11:07 am
content: Quote from: voltsandjolts on December 23, 2016, 08:47:34 amThat’s a great tip. Thank you for the insight.I have already rented a tectronix Current probe that did it for me and found that the current spikes were several hundred mAs up to couple amps in certain instances. Way more that I anticipated.

author: rocky79
author_rank: Contributor
timestamp: December 24, 2016, 04:12:45 am
content: Quote from: tatus1969 on December 23, 2016, 09:14:05 amI know but It is necessary for this application.

author: voltsandjolts
author_rank: Unknown
timestamp: December 24, 2016, 10:15:55 am
content: QuoteIf you really need the physical isolation offered by a relay and the snubber circuit proves problematic, maybe the answer is to use both a relay and a triac. Use the triac to switch off the load then switch the relay.

author: rocky79
author_rank: Contributor
timestamp: January 04, 2017, 01:00:27 am
content: Quote from: Ian.M on December 23, 2016, 12:31:44 amHello,I would like your feedback on this schematics. We would like to protect the reed switch contacts from the inductive voltage spike cause by the 28vDC coil. Which path the current will take when the inductive voltage spike happens? will it be through the Collector emitter of Q1 or the Emitter base? If it's emitter base then that will fry the transistor immediately.

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2017, 03:41:57 am
content: Better put an anti-parallel diode across B-E to prevent the B-E junction possbly Zenering as the relay coil rings with C2 (if its underdamped).Alternatively try the attached circuit, which even with an absolutely horrible relay (150R coil with 1H inductance, can suppress the spike. The Miller capacitor C1 slows down turnoff to reduce back EMF. If the relay coil inductance is changed, C1 will need to be adjusted for the desired turnoff speed to limit the max transient voltage across the switching circuit.N.B. Relay coil inductance is usually higher when energised. If possible measure it with the armature held in contact with the pole piece.

author: rocky79
author_rank: Contributor
timestamp: January 06, 2017, 05:44:09 am
content: Thank you IAN. I appreciate your feedback.I measured the voltage spike which was 110v from the 28v coil. It is well within the Max voltage that the reed switch can handle.So no need for the transistor circuitry anymore.

author: Ian.M
author_rank: Super Contributor
timestamp: January 06, 2017, 08:40:57 am
content: For DC, you may find it advantageous to use a Diode+RC snubber, as it can offer far better spike suppression than a simple RC snubber without increasing the initial switch closure current significantly. Its like a normal RC snubber except you add a fast diode (e.g. Schottky) across the resistor so when the switch opens the load current transiently flows through the capacitor and the diode with minimal initial extra voltage drop. The resistor provides a discharge path for the capacitor when the switch closes, limiting the current. The resistor can typically be increased to several K, and because the extra current through the switch is limited by the resistor, you can use a much larger capacitor to suppress the spike to the desired limiting voltage - typically double the supply voltage.