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Relay question (adding relay to Oscars LED driver board) - it works!

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RandyT:


--- Quote from: ShinAce on June 16, 2005, 11:07:53 pm ---Flip it around, make sure it works in that position, and leave it there.
The coil is an electro-magnet, and having a current through it "energizes" it, or creates a magnetic field. When you suddenly stop the current, the field collapses, creating a current. This back EMF consists of a big single spike. The diode just acts as a mini surge suppresor.

--- End quote ---

ShinAce,

Since you are an EE type, could you please tell me how that diode in parallel with the coil prevents back feed?  I would have thought that to do that, it would have to have been in series, not parallel (which is why I said to ask the guy who designed the circuit)?

Just hoping to learn something from this one myself.  :)

Thanks,
RandyT

ShinAce:

If the diode is in parallel, we have 2 possible cases.
1) Current is flowing through the relay coil in the proper direction(with respect to the diode, since the relay has no sense of direction), and the diode is basically not even connected.
2) Power is interrupted to the coil, yet it wants to get rid of its stored energy really quickly. In this case, the diode acts as a resistor(nearly a short circuit, actually), absorbing the load.

If the diode were in series, it would allow the relay to work properly. But when the power is turned off, I don't know what will happen. I'm guessing here, but my gut feeling is that it will PREVENT surges, instead of absorbing them. Which sounds great, but that energy still needs to go somewhere, and you're not letting it do that.

BTW, in case 1), if the diode is wired backwards, the relay will not engage, and you are basically faced with a short circuit scenario. The diode will not enjoy this.
However, I've used diodes as temperature sensors, and took a lighter to them when testing. At 200 degress celsius(internal temp of the diode), the diodes will still work. I'm sure you've heard that passing power through a diode will cause about a 1/2 volt loss. That loss is temperature dependant. At higher temperatures, the voltage drop becomes lower. That's right, diodes work better when they're hot.

JoyMonkey:

I just flipped the diode around and sure enough it works perfectly now! Even with the resistor and LED included! Yaaayyy!

Thanks Randy, Popcorrin and ShinAce for holding my hand through this- couldn't have done a thing without you!

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