Author Topic: Snubber Circuit Suggestion  (Read 8996 times)

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Offline rocky79Topic starter

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Re: Snubber Circuit Suggestion
« Reply #25 on: January 04, 2017, 01:00:27 am »
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.

Hello,

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.
 

Offline Ian.M

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Re: Snubber Circuit Suggestion
« Reply #26 on: January 04, 2017, 03:41:57 am »
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.
« Last Edit: January 04, 2017, 05:08:15 am by Ian.M »
 
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Offline rocky79Topic starter

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Re: Snubber Circuit Suggestion
« Reply #27 on: January 06, 2017, 05:44:09 am »
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.
 

Offline Ian.M

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Re: Snubber Circuit Suggestion
« Reply #28 on: January 06, 2017, 08:40:57 am »
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.
 
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