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SSR, inductive load verse resistant load

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joshua17ss2

New Member
I recently got a relay board containing 20 SSR's (70M-OAC24). I have heard that SSR Arnt all that good for driving inductive loads like motors and solinoid vales.
I was planning on running several solinoid valves with them board, and was wondering if any one knew more about this specific relay, ive read a data sheet and couldnt locate alot about induction loads, This was removed from an inductrial machine.
I know that most SSR cant handle he induction load becuase of the insurage at startup. but if i was running a very small load, 110 at half an amp, would the insurage still be a problem, with the relay being rated at 140 and 4 amps.

If they arnt cablable ot driving an induction load, is there a small modifiction that would allow this to happen.

thanks josh
 
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crutschow

Well-Known Member
Most Helpful Member
The relay is rated for a surge of 80A whcih should handle your half amp load.

Another concern is the inductive kick when you turn off the inductive load. The inductance tries to keep the current going when the relay shuts off, generating a large voltage spike.

One way to minimize this problem is to connect a surge arrestor accross the relay. I'm not certain, but a surge arrestor, such as found in power strips you buy to protect computers, etc. should work. You simply plug the motor/solenoid valve into the surge arrestor strip and then power the strip from the relay. You can also buy such arrestors as components.
 

joshua17ss2

New Member
ssr

Thanks,
i think ill switch and just use the SSRs for the lighting controller, and switch to mechanical relays for the 110 valves and ULN2003 driver chips for the 24 volt valves.

thanks
josh
 

mneary

New Member
There's no surge at turn on. SSRs sometimes have difficulty turning on for inductive loads because the inductor takes time to draw enough current. A network inside the SSR called a snubber helps with this.

There's no inductive spike when you turn off SSRs. They can only turn off when the current is zero.

When turning off, an inductive load reaches zero current at a point in time when the supply voltage is not zero. This causes a rapidly rising edge (never more than the supply voltage). Without a snubber, the SSR could re-trigger without an input. But the snubber suppresses this voltage (dI/dT) too.

Summary: These SSRs are ideal for small motor loads. They require a power factor of 0.4 minimum, which may cause some solenoids to trigger poorly. I have used SSRs (of my own design and also purchased modules) on motors and small solenoids for over 30 years without problems. Since they require more than 24V load voltage, they are not suitable for 24V valves.
 
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joshua17ss2

New Member
thanks for the information i will keep that in mind for future use.
unfortunatly i bought all 24 volt valves for the next year, so im stuck using them
oh well, same end effect i guess

thanks for the help.
josh
 
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