I am using an Arduino to switch a 240VAC 10 amp relay off and on.
How do I stop/minimise arcing of the relay contacts to preserve the relay?
You can try a resistor/capacitor snubber. Lots of information is available on line on how to choose the components, which depend on the rest of your circuitry, or you can buy the proper combinations as modules, e.g. http://www.paktron.com/pdf/Quencharc_QRL.pdf
Google "RC snubbers" for more info.
Note: the capacitors have to be high voltage AC types and are not generally available in hobby outlets.
Specifically capacitors for mains have to be rated for mains use, basically not
start fires when a 2kV spike hits them (or whatever the worst-case mains transient
is in the relevant standard).
For a snubber circuit you also have to calculate the maximum voltage the load
inductance will generate with that particular snubber circuit, the capacitor has
to withstand that, not just mains itself. Snubbers have to be matched to their load.
Most large current AC mains relay contacts are designed for many operations at or less then their ratings, the wear on the contacts is a function of how many switching operations (under load) are made over time. The normal practice is to use contacts rated well above the actual load current, say 20-30 amps thus there will be a lot more contact material for longer life. AC switching takes advantage of suppressing contact arc because of the automatic zero voltage crossing every 8.3 milliseconds. DC circuits of the same current rating will arc much worst then with AC.
lemming:
I am using an Arduino to switch a 240VAC 10 amp relay off and on.How do I stop/minimise arcing of the relay contacts to preserve the relay?
Buy a relay that's designed for the job...
Thanks people. I'll look up RC snubbers.
@fungus who wote
Buy a relay that's designed for the job...
I have bought a relay thats designed for the job. The loads will be between 2 to 5 amps on the relay contacts @240VAC.
No matter what the rating of the relay and the load upon it, arcing is detrimental to the contacts and shorten their life. The relay is working fine now and probably will be in a year or two, but I would like it to last as long as possible and therefore the reason for this post.
lemming:
I have bought a relay thats designed for the job. The loads will be between 2 to 5 amps on the relay contacts @240VAC.
"Amps" is only one parameter. If you're getting a lot of arcing then your load might have big transient currents when you switch it on/off.
We don't know what the load is though, so it's pointless trying to give a sensible answer...
The load is a small 240 Vac pump motor.
I've been using solid state relays on 10 amp, 240VAC well pumps for years with no obvious problems. The relay input terminals connect to an optoisolator that can be driven directly by an Arduino output.
They are getting to be quite cheap on Ebay: http://www.ebay.com/itm/2-TWO-Crydom-D2440-3851-Solid-State-Relays-240VAC-40A-3-5-32VDC-/131094607851?pt=LH_DefaultDomain_0&hash=item1e85d8f7eb
Thanks jremington.
I notice that the solid state relays have a heat sink backing.
Do they put out a lot of heat when driving a pump motor (say, at 10 amp; the worst case scenario). This is not an issue with standard relays.
Do they need any extra components to dampen inductive flyback like FETs and Triacs?
My SSRs do not heat up significantly when operating, but I have read that some of the newer, cheaper ones do get quite warm. I believe that all have snubber circuitry built in and have been operating on that assumption for some time without problems. So, my connections on the high power end are very simple, just 220VAC from a local dual circuit breaker, relay and pump in series. The relay inputs connect directly to the microprocessor output pin and ground.
lemming:
Thanks jremington.I notice that the solid state relays have a heat sink backing.
Do they put out a lot of heat when driving a pump motor (say, at 10 amp; the worst case scenario). This is not an issue with standard relays.
Thyristor devices (SCR and triac) can have up to 2 volts voltage drop across them when conducting, so at 10 amp you could have up to 20 watts of heat dissipation to deal with. So yes, heat sinks are required when operating these devices at higher current levels.
Do they need any extra components to dampen inductive flyback like FETs and Triacs?
Inductive flyback is not a issue with AC voltage, but driving inductive loads like motors often does require a 'snubber circuit' across the SSR output terminals as the resulting phase shift between voltage and current can cause problems for the SSR to turn off reliably.