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Does solid state relay needs a protecting diode for the pin 3 out of a 555 monostable? Answered

recently i need some timer for my capacitor charger to be used in a coilgun.
i just finished a 555 one shot monostable pcb design.  my charger require about 24 watt of power which is unsufficient for the 555 pin 3 out,. I know that a protecting diode can be put with a relay. but
a solid state for a solid state realay. does it need a diode. i just want to know because i will put it in my design 

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steveastroukBest Answer (author)2010-10-09

No. The diode is there to protect your circuit from the spike induced by the collapsing magnetic field in an ordinary relay.

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ARJOON (author)steveastrouk2010-10-09

so will it be necessary for a solid state relay. and yes i want to protect the circuit

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seandogue (author)ARJOON2010-10-09

Um, not if you're replying to what Steve said it isn't. That's the input side. and no, a 555 trailing edge is not going to take out an SSR's input. The output side is a separate issue, and although they can be used in a pinch, a diode may not be the best solution. Also, consider that to be effective, a protection diode must be able to pass the surge current, so it should be selected well above the quiescent (DC) load current

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ARJOON (author)seandogue2010-10-09

you are not understanding. when we use a normal relay.in it there is a coil that must be on so that the contact leads of the switch to touch in the othe side. so when current stop flowing. there is a spike that goes back which will affect my 555 circuit. so a protecting diode needs to be used. and now for a solidsate relay. does it use something like a normal relay do which will cause a spike and a diode will be used to protect the 555 circuit.

therefore is a diode necessary? sorry if you had mentioned it or not above because i has some difficulties in understanding english well.

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seandogue (author)ARJOON2010-10-09

I am understanding very well, having been a professional engineer for over twenty years.

The inputs to an SSR do not need protection from a 555 output, nor does the output of the 555 require protection from a negative edge at the input to the SSR

The only time one would consider such protection at the input side is when using a coli drive relay. English is an excuse.

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steveastrouk (author)ARJOON2010-10-09

No. The diode is there to protect your circuit from the spike induced by the collapsing magnetic field in an ordinary relay. Since you don't have an ordinary relay you don't need the diode.

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seandogue (author)2010-10-09

No, however, the output side of the SSR (solid state relay) should be protected if it is rated at or near to the output side operating voltage, since kick-back is several times the operating voltage in most cases, UNLESS the SSR is already protected by internal kickback protection. (Usually these protected SSRs are considered to be rated for use with motors)

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omowright (author)seandogue2018-02-11

All fast state-change devices must be protected at the output. There is the element of time during which there are collapsing fields after the SSR has switched off. No harm in adding a diode ...

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