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relay contacts
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relay contacts

by pm6041141 on Mon Mar 22, 2010 5:24 pm

I have been thinking about a circuit for operating a sprinkler solenoid. It's a 24v AC solenoid. I am going to use a 12v (coil) relay because I have it. I will activate the relay coil with a 2n3904 transistor and a reversed biased diode across for flyback protection for the uC so, this question is not about protection for the uC.

Q: on the contact side of a relay.... This is exactly the setup on distributor ignition otto engines. That is, an inductor switched by "points". Points are relay contacts. The points have a 'condenser' across them. A condenser is just a capacitor that is there to help the points from arching and 'pitting'. I know I am over thinking this but who here does/doesn't use spark suppression on the contact side of their relays? General thoughts are also welcome.
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Re: relay contacts

by zener on Mon Mar 22, 2010 10:37 pm

With AC you are always switching at a random point in the wave, so there is much less wear on the contacts. The wear tends to "even out". In a car it is switching DC always of the same polarity (you know you are dating yourself by talking about parts that cars haven't had for 20 years). So I don't think it is typically used. BTW, the diode is not to protect the cpu, it is to protect the transistor. Although I suppose a big spike could take out the transistor and then the cpu.

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Re: relay contacts

by pstemari on Thu Apr 01, 2010 2:20 am

In a traditional auto ignition the spark coil has a lot more inductance than your sprinkler solenoid. The circuit's designed to take the 12v from the battery and generate 10,000-20,000 volts for the spark plug by interrupting the inductor. The capacitor helps with that by providing a rebound mechanism for the current in the primary of the spark coil
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Re: relay contacts

by minerva on Mon Apr 05, 2010 4:21 am

pstemari wrote:In a traditional auto ignition the spark coil has a lot more inductance than your sprinkler solenoid. The circuit's designed to take the 12v from the battery and generate 10,000-20,000 volts for the spark plug by interrupting the inductor. The capacitor helps with that by providing a rebound mechanism for the current in the primary of the spark coil


The ignition coil isn't just an inductor generating that high voltage by back EMF, it is a transformer with a high-voltage many-turns secondary.

My understanding of the purpose of that capacitor across the distributor points was that it's intended to stop EMI coming through on the radio, but maybe I'm wrong.
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Re: relay contacts

by zener on Tue Apr 06, 2010 12:07 am

The "condenser" existed way before the car radio! (about 20 years before, 1910 vs 1930).

From Wikipedia:

"The points allow the coil to charge magnetically and then, when they are opened by a cam arrangement, the magnetic field collapses and a large (20 kV or greater) voltage is produced. The capacitor is used to absorb the back EMF from the magnetic field in the coil to minimize point contact burning and maximize point life. The Kettering system became the primary ignition system for many years in the automotive industry due to its lower cost, higher reliability and relative simplicity."

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Re: relay contacts

by pstemari on Sat Apr 10, 2010 12:04 pm

It's also forming an LC loop with the primary of the coil. When the points open you get a oscillation between the capacitor and the primary that continues until it either dissipates from resistance or the energy is sucked out by the spark firing.

It would be interesting to hook a scope up to that--I don't know how many times it's going to oscillate--but I'd be leery of hooking expensive electronics up to a circuit that generates that sort of high voltage transients.
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Re: relay contacts

by pm6041141 on Sat Apr 10, 2010 12:21 pm

You just want to use a high voltage probe to attenuate the signal. I have used a 50Kv probe hooked up to a scary expensive HP scope at work. Allen engine analyzers are big rolling scopes for the most part. Depends on the model. But you can get them pretty cheap.
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