Flyback Diode Necessity

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RJS100
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Flyback Diode Necessity

Post by RJS100 »

I read that if the output relays on the Acorn board are used to control a direct current solenoid valve (for example to turn on mist coolant) then it is good practice to wire a flyback diode across the coils of the solenoid valve. This is said to prevent voltage spikes created when power to the solenoid valve is shut off.

I currently use an Acorn output to turn on a 12 V DC mill light. I would like to add a 120 V AC enclosure light that will turn on with the mill light. To avoid using another output relay on the Acorn board, I installed a small SPST relay with a 12 VDC coil. This allows me to turn on the 120 V AC lights when the 12-volt mill light is turned on.

Do I need to wire a Flyback diode across the coil of this SPST relay to prevent voltage spikes from damaging the Acorn output relays?
Are Flyback diodes necessary to protect the output relays on the Acorn board?

Thanks... Richard
rk9268vc
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Re: Flyback Diode Necessity

Post by rk9268vc »

Use a solenoid with a built in relay, dont power the coils directly.

example:


This way all Acorn is doing is powering a relay. No flyback diode needed.

"Do I need to wire a Flyback diode across the coil of this SPST relay"
no
ShawnM
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Re: Flyback Diode Necessity

Post by ShawnM »

I would add a diode to any DC relay and a snubber circuit to any AC relay coil or contactor. It's just cheap insurance.
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Re: Flyback Diode Necessity

Post by Gary Campbell »

If I were going to power a low power 120vac device I would use a solid state relay, which are made for that exact task
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RJS100
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Re: Flyback Diode Necessity

Post by RJS100 »

Thanks for your thoughts. Very much appreciated. The solenoid shown is exactly what I have been using. I have been directed to use snubbers and flyback diodes exactly as Shawn has noted.

The idea of a solid-state relay is also something I never thought of. No coil, no inductance... No voltage spike.

Thanks.
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Re: Flyback Diode Necessity

Post by Upnorth »

How reliable are the solid state relays?
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Re: Flyback Diode Necessity

Post by Richards »

Solid State Relays are dependable. I've used hundreds of them in photo printers without a failure. They do have one drawback which is that some of them allow leakage voltage when they are turned off. A normal mechanical relay completely opens the circuit when OFF An SSR doesn't always totally shut off, depending on the SSR type and its load. The lighter the load, the more apt the SSR is to have some leakage. The leakage seems to be more common in SSRs that turn ON/OFF using a zero-voltage detection circuit. It's not something to worry about, but you need to be aware that there might be a shock hazard when you would assume that the power is off.
-Mike Richards
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Re: Flyback Diode Necessity

Post by suntravel »

On the production machines we change all small relays to SSR, because they are extreme reliable. I did not know about a fail the last 20 years.

Voltage leak is there make it irritating to check with some Multimeters if they are off.

Uwe
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