All About Relays
erix
Posts: 2
Howdy,
I've been lurking around here for a while and finally decided to jump in. I've got a board and a BS2 and I'm ready to build things!
The first thing I've been asked to build is a timed relay system to control three 120V/15A motors. I will be running them sequentially for 15 minutes each. Easy stuff - connect relays to the BS2 via the darlington array on my board.
Unfortunately, when researching relays on the Crydom website, I've hit a snag - which to choose?
There are MOSFET output, SCR output, and Triac output....
There is Zero crossing and random....
Can some body give me a recommendation for a relay to use?
Much Abliged!
erix
I've been lurking around here for a while and finally decided to jump in. I've got a board and a BS2 and I'm ready to build things!
The first thing I've been asked to build is a timed relay system to control three 120V/15A motors. I will be running them sequentially for 15 minutes each. Easy stuff - connect relays to the BS2 via the darlington array on my board.
Unfortunately, when researching relays on the Crydom website, I've hit a snag - which to choose?
There are MOSFET output, SCR output, and Triac output....
There is Zero crossing and random....
Can some body give me a recommendation for a relay to use?
Much Abliged!
erix
Comments
you probably want zero-crossing. A zero crossing ssr will not turn on until the voltage differential is zero. basically on the AC sine wave it waits until the sine wave hits zero before turning on. That way you don't get a huge spike.Also zero crossing/switching means no appreciable noise.
the snubber "snubs" inductive load kick-back when you switch off. I have seen snubberless triacs meaning that the triac doesn't need to have the snubber circuit to protect it.
If its mosfet or triac or whatever, that is the switching device inside the SSR. I have no idea honestly which one is better for inductive loads or what-have-you although I would like to know too as my next project will control resistive heaters.
I think for those, we'll both have to wait for someone else to answer.. (also, anyone, if I'm wrong on this stuff, please let us both know
just went and did some reading at omega, according to them twin SCR types are more reliable and are capable of higher overloads before failure than triacs.
Also, everything I have read emphatically states that you protect the SSR with an appropriate fast-blowing I2T fuse, as SSRs can fail in the closed position, thereby creating a short. Also make sure that you choose an SSR with the proper switching signal. some are set for 4-20ma whilst some are set for 0-5v. If you are using the stamp to control a darlington or something then use the darlington on the ssr, I would think you'd be fine.
Post Edited (Dunnsept) : 4/6/2006 8:52:50 PM GMT
" the main rule of thumb is that for inductive loads such as motors, use the Random turn-on style, and for resistive heaters you can use either the random or zero cross styles. "
This was from Pete B. the "Applications Engineering Manager" at Crydom. He also said to go to support, then "view all tech tips" for help on selecting the best relay for an app.
hth
paul...
They offer DPST and have systems of plugs and fusing that are very appealing when messing around with 115vac. Even chassis. You can have circuit breakers mounted along side the relays for ease of maintence and safety.
The main advantage here is a DPST completely shuts down the AC when off -- both sides. And even in the even of a failure, usually one side will shut down.
Mechanical relays also tend to fail in an OFF condition whereas I am led to believe that SSRs often fail in an ON condition.
And, they do offer DPDT, SPDT, etc. for even more convienence. Of course they don't have the speed, they do make noise.
But then, the SSR generally create 1 watt of heat for each AMP [noparse][[/noparse] 15amps = 15watts of heat that you have to dump]
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
ah ha!· zero crossing with highly inductive loads can produce high inductive 'kick' voltages...·
also read that SCRs are favored above triacs for highly inductive loads like motors and for anything above 12A, as SCRS aren't prone to latch-up like triacs are.
thanks orion! I totally forgot about all that inductive/capacitive current lag/lead stuff.· sheesh.. I guess I'd better go read my basic electrics books again...
Post Edited (Dunnsept) : 4/7/2006 1:28:16 PM GMT
Kramer,
I would use a mechanical relay for this app, but I'm still would like to hear if anyone has had good luck using SSRs for this type of load. The manufacture claims they will SSRs will hold up, but I'm skeptical and as you have stated there are many other reasons not to use SSRs for good sized motors.
Post Edited (Orion) : 4/7/2006 1:31:00 PM GMT
For 15 minuite cycles, you just don't seem to need to work that hard. Of course, you can get 40amp rated devices and the 'derating' will help. But nobody really knows how much.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
The reason current still flows in an inductive circuit is because the magnetic field collapses and tries to push electron through the wires. In Capacitors the charged plates are discharging of course.
We had a case of our (on topic) Stamp resetting ocasionally when the breaker was pulled because of the inductive kick from a small fan(0.3 amp). We resolved it by putting a .01/500V cap across the breaker. We never had a problem with our SSR (zero-crossing) 25 amp relays in the same circuit.
--Paul
(still on topic) my first project is going to be to control my coffee roaster either with my BS2 to start or with an SX. Of course, for that project, my relay will be controlling a resistive heater load, and crydom says random or zero crossing. depending on cost, I might just go with a mechanical since that's a lot of heat to contend with on an SSR and I would like to keep things semi-portable.
paul...
Post Edited (Dunnsept) : 4/7/2006 7:34:38 PM GMT
Are there any SSRs that would turn off at zero crossing for the current ???
Guess the voltage would spike but it might be easier to deal with than current spikes...
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Living on the planet Earth might be expensive but it includes a free trip around the sun every year...
/Bamse
I think I shall investigate the mechanical relays section on McMaster-Carr's website...