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

[RandyT]

Could it be the transistor I'm using? I just used what I had at hand (a PNP to 92 transistor 2N3906) which is different to the one's that Oscar uses in the LED driver circuit, but he does say that any general purpose PNP transistor should do.
What happens if I take the 1k resistor out of the equation?

The relay doesn't take much power to actuate it, so I don't think it's the transistor.  The 1k resistor should stay in place as well.

About the only thing I can think of to do at this point is swapping the A and B connections.  I know a coil doesn't care about polarity, but the diode across the contacts throws in something different that I'm not fully versed in.  The way it's wired right now, the diode appears that it would be conducting, which might be creating a short circuit.  If you wire it the other way, current should only be able to flow through the coil.

Remember, "AS IS" info

RandyT

[JoyMonkey]

I'll try flipping the diode around tonight. Thanks!

(I wish I made this thing on a breadboard first)

[JoyMonkey]

I removed the diode and the relay is working great now!
I haven't had the chance to put it back on in the opposite position, but do I really need it?

[RandyT]

I removed the diode and the relay is working great now!
I haven't had the chance to put it back on in the opposite position, but do I really need it?

No clue. You probably want to ask the guy who designed the circuit

I'm pretty sure it was there to protect the other circuitry from spikes and such.  I'd flip it around and put it back just in case.  It isn't huting anything by being there, you just had the polarity reversed..

RandyT

[ShinAce]

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.

[RandyT]

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.

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!