RC snubber - Page 1
Forum section: electronics / beginners

author: imperyal
author_rank: Newbie
timestamp: January 02, 2012, 09:26:10 pm
content: Hello to all! I'm watching the video blog for almoust a year regularly and I have to say, it's the best you can find!Now, and after lots of trays on other blogs, I will try to get an answer here...I have a control circuit based on Arduino that is controlling a water pump, the arduino activates one of two relays that are connected to large contactors (to the respective coils). But when the coils are activated they cause interfearence on the circuit and cause the LCD to go mad, the characters are all corrupted and the LCD needs a restart to go back to normal. I think an rc snubber will resolve the problem... But I don't know what values to use on the thing, resistor and cap. The coil is drawing less the 500 mA at 230v. On other forms I have a suggestion to use 220Ohm resistor and 0.5 microF cap. Is that right? And what power rating should I use for the resistor?Thanks a lot!

author: amspire
author_rank: Super Contributor
timestamp: January 02, 2012, 11:48:18 pm
content: Sounds like there are two things - the relays, and then the contactors that the relays are powering.First up, the relays need reverse biased diodes across the coils.Second is the contactors. That snubber will dissipate a lot or power - over 7Watts.If the contactor coils can be powered by 240 DC instead of AC, you can connect the coil to a bridge rectifier with a reverse biased diode across the coil. You probably cannot though, so the alternative is to have a 1uF capacitor connected via a bridge rectifier so it has DC across it. The point about this arrangement is that after the capacitor is charged, no continuous power is being dissipated. Add a slow bleed resistor across it - say 10MOhms. Then connect the AC inputs of the diode bridge in series with a 100 ohm snubber resistor. The power rating of the snubber resistor will depend on how often the contactor is switched.The relays contacts will have to be rated to handle the contactor current + the peak charging current of the snubber. With a 100 ohm resistor, the peak snubber current will be over 3A peak.Richard

author: imperyal
author_rank: Newbie
timestamp: January 03, 2012, 02:05:45 am
content: Thank you!I already have the reverse diode on the relays coils, otherwise the transistors I'm using would be failing shortly. And the snubber, I'm thinking in installing them in parallel with the contactor coils (I sow that on a practical examples), that way when the coil is in use, the snubber will not be handling the current, it will flow on the coil, right? Only on power on/off the snubber could help smooth things a bit... I think...

author: amspire
author_rank: Super Contributor
timestamp: January 03, 2012, 02:54:19 am
content: Quote from: imperyal on January 03, 2012, 02:05:45 amIt will be dissipating power, unless you add the rectifier bridge across the capacitor as I suggested. But I did get my calculations wrong - with a 220 ohms resistor, it will be under 0.5W rather then 7W as I suggested. So I think the snubber you suggested is good. It seems to be hard to find the coil data of contactors. If the current is 100mA peak, then the maximum spike across the 220 ohm resistors would be 22 volts which is excellentRichard.

author: imperyal
author_rank: Newbie
timestamp: January 03, 2012, 10:14:19 am
content: umm... Yes, the data for the coils is very hard to find. On the coil i will be using I did measure around 200 mA of current. The rectifier bridge across the capacitor, you mean in parallel? Thank you!