Snubber Diodes on Solenoid Coil - Page 1
Forum section: electronics / projects

author: purplechip
author_rank: Newbie
timestamp: November 11, 2016, 10:15:40 am
content: Hello all,I have been following EEVBlog channels on youtube for a long time now, however building a little project I am after some advice.The project I am building is a control box for a solenoid valve. I have wired a switch for the function through a Standard Automotive 4 Pin 30A Relay, then from the relay out to the solenoid coil on the valve.The only spec's I have on the coil at the moment are 12V DC and 2.5A.My question is; Is it completely necessary to run a snubber protection diode to protect the relay contacts from the high voltage when the coil is de-energised or will the relay contacts be robust enough to withstand voltage spikes. I also do not want any voltage spikes feeding back into the power supply to the circuit. Also please be aware that the entire control box is built inside an electrical enclosure and I am not making any PCB's so everything is mounted and fed with wires. So if i can get away without using any feed back protection it will make it a lot easier and neater.Apologize if this belongs in the beginner section but being new to this forum I thought I would give it a go here.

author: tautech
author_rank: Super Contributor
timestamp: November 11, 2016, 10:27:20 am
content: Welcome to the forum.It usually is not necessary to provide snubbers for relays......for the semiconductors driving the relays, well that's another matter.A schematic will help with the best advice.

author: Gyro
author_rank: Super Contributor
timestamp: November 11, 2016, 10:34:59 am
content: That said, a series R-C snubber wired across the solenoid will probably make the contacts last longer and reduce RF noise getting into other bits of the circuit.

author: danadak
author_rank: Super Contributor
timestamp: November 11, 2016, 11:02:18 am
content: Use protection, that equals reliability, life, repeat-ability.http://cp.literature.agilent.com/litweb/pdf/5988-6917EN.pdfRegards, Dana.

author: purplechip
author_rank: Newbie
timestamp: November 11, 2016, 11:22:24 am
content: Thanks for the replies and the suggestions.In regards to adding a diode across the coil, how would i go in calculating the required size for the diode required?

author: macboy
author_rank: Super Contributor
timestamp: November 11, 2016, 02:03:00 pm
content: Absolutely use the snubber. A snubber diode is actually kind of a cheap and easy cheat used because it requires no real calculations or thought. An properly tuned RC snubber will perform better, both in terms of radiated EMI/RFI and in release time. If minimizing the release time isn't a concern then you can just use the diode. The diode needs to conduct the rated current of the coil (2.5 A) and its reverse voltage must be greater than the working voltage of the coil (12 V). In your case you can use 1N540x diodes which are rated at 3 A. You could probably use a smaller one instead since the current is just a short pulse, but why risk the failure down the road? Ideally this diode is placed at the load (your solenoid coil) not at the switch (the relay output contacts). Obviously you also want a snubber across the relay coil as well, or your switch won't last very long.

author: danadak
author_rank: Super Contributor
timestamp: November 11, 2016, 03:39:10 pm
content: These might help -http://www.cde.com/resources/technical-papers/design.pdfhttps://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtmlRegards, Dana.

author: calexanian
author_rank: Super Contributor
timestamp: November 11, 2016, 09:32:42 pm
content: In its most basic form a free wheeling diode across the relay coil is good. Plain old 1N4004 or so is fine. For the solenoid itself I recommend a 47 or 24 volt MOV for a 12 volt solenoid. The solenoid will create quite a spike when it is disengaged. Mov's will go a long way to preserve your contacts.